How many chromosomes do people have? - Genetics Home Reference How many chromosomes do people have? How many chromosomes do people have? In humans, each cell normally contains 23 pairs of chromosomes, for a total of 46. Twenty-two of these pairs, called autosomes, look the same in both males and females. The 23rd pair, the sex chromosomes, differ between males and females. Females have two copies of the X chromosome, while males have one X and one Y chromosome. The 22 autosomes are numbered by size. The other two chromosomes, X and Y, are the sex chromosomes. This picture of the human chromosomes lined up in pairs is called a karyotype. Credit: U.S. National Library of Medicine For more information about the 23 pairs of human chromosomes: Chromosome Abnormalities Fact Sheet - National Human Genome Research Institute (NHGRI) National Human Genome Research Institute Enter word(s) to search Glossary of Terms What are chromosomes? Chromosomes are the structures that hold genes. Genes are the individual instructions that tell our bodies how to develop and function; they govern physical and medical characteristics, such as hair color, blood type and susceptibility to disease.

Many chromosomes have two segments, called "arms," separated by a pinched region known as the centromere. The shorter arm is called the "p" arm. The longer arm is called the "q" arm. Top of page Where are chromosomes found in the body? The body is made up of individual units called cells. Your body has many different kinds of cells, such as skin cells, liver cells and blood cells. In the center of most cells is a structure called the nucleus. This is where chromosomes are located. Top of page How many chromosomes do humans have? The typical number of chromosomes in a human cell is 46: 23 pairs, holding an estimated total of 20,000 to 25,000 genes. One set of 23 chromosomes is inherited from the biological mother (from the egg), and the other set is inherited from the biological father (from the sperm). Of the 23 pairs of chromosomes, the first 22 pairs are called "autosomes." The final pair is called the "sex chromosomes." Sex chromosomes determine an individual's sex: females have two X chromosomes (XX), and males have an X and a Y chromosome (XY).

The mother and father each contribute one set of 22 autosomes and one sex chromosome. Top of page How do scientists study chromosomes? For a century, scientists studied chromosomes by looking at them under a microscope. In order for chromosomes to be seen this way, they need to be stained. Once stained, the chromosomes look like strings with light and dark "bands," and their picture can be taken. A picture, or chromosome map, of all 46 chromosomes is called a karyotype. The karyotype can help identify abnormalities in the structure or the number of chromosomes. To help identify chromosomes, the pairs have been numbered from 1 to 22, with the 23rd pair labeled "X" and "Y." In addition, the bands that appear after staining are numbered; the higher the number, the farther that area is from the centromere. In the past decade, newer techniques have been developed that allow scientists and doctors to screen for chromosomal abnormalities without using a microscope. These newer methods compare the patient's DNA to a normal DNA sample. The comparison can be used to find chromosomal abnormalities where the two samples differ. One such method is called noninvasive prenatal testing.

This is a test to screen a pregnancy to determine whether a baby has an increased chance of having specific chromosome disorders. The test examines the baby's DNA in the mother's blood. Top of page What are chromosome abnormalities? There are many types of chromosome abnormalities. However, they can be organized into two basic groups: numerical abnormalities and structural abnormalities. Numerical Abnormalities: When an individual is missing one of the chromosomes from a pair, the condition is called monosomy. When an individual has more than two chromosomes instead of a pair, the condition is called trisomy. An example of a condition caused by numerical abnormalities is Down syndrome, which is marked by mental retardation, learning difficulties, a characteristic facial appearance and poor muscle tone (hypotonia) in infancy. An individual with Down syndrome has three copies of chromosome 21 rather than two; for that reason, the condition is also known as Trisomy 21. An example of monosomy, in which an individual lacks a chromosome, is Turner syndrome. In Turner syndrome, a female is born with only one sex chromosome, an X, and is usually shorter than average and unable to have children, among other difficulties.

Structural Abnormalities: A chromosome's structure can be altered in several ways. Deletions: A portion of the chromosome is missing or deleted. Duplications: A portion of the chromosome is duplicated, resulting in extra genetic material. Translocations: A portion of one chromosome is transferred to another chromosome. There are two main types of translocation. In a reciprocal translocation, segments from two different chromosomes have been exchanged. In a Robertsonian translocation, an entire chromosome has attached to another at the centromere. Inversions: A portion of the chromosome has broken off, turned upside down, and reattached. As a result, the genetic material is inverted. Rings: A portion of a chromosome has broken off and formed a circle or ring. This can happen with or without loss of genetic material. Most chromosome abnormalities occur as an accident in the egg or sperm. In these cases, the abnormality is present in every cell of the body. Some abnormalities, however, happen after conception; then some cells have the abnormality and some do not. Chromosome abnormalities can be inherited from a parent (such as a translocation) or be "de novo" (new to the individual).

This is why, when a child is found to have an abnormality, chromosome studies are often performed on the parents. Top of page How do chromosome abnormalities happen? Chromosome abnormalities usually occur when there is an error in cell division. There are two kinds of cell division, mitosis and meiosis. Mitosis results in two cells that are duplicates of the original cell. One cell with 46 chromosomes divides and becomes two cells with 46 chromosomes each. This kind of cell division occurs throughout the body, except in the reproductive organs. This is the way most of the cells that make up our body are made and replaced. Meiosis results in cells with half the number of chromosomes, 23, instead of the normal 46. This is the type of cell division that occurs in the reproductive organs, resulting in the eggs and sperm. In both processes, the correct number of chromosomes is supposed to end up in the resulting cells. However, errors in cell division can result in cells with too few or too many copies of a chromosome. Errors can also occur when the chromosomes are being duplicated. Other factors that can increase the risk of chromosome abnormalities are: Maternal Age: Women are born with all the eggs they will ever have.

Some researchers believe that errors can crop up in the eggs' genetic material as they age. Older women are at higher risk of giving birth to babies with chromosome abnormalities than younger women. Because men produce new sperm throughout their lives, paternal age does not increase risk of chromosome abnormalities. Environment: Although there is no conclusive evidence that specific environmental factors cause chromosome abnormalities, it is still possible that the environment may play a role in the occurrence of genetic errors. Top of page Glossary of Terms In descriptions of chromosomes and chromosome abnormalities, some words may be unfamiliar. The meanings of many of these words may be found in the Talking Glossary of Genetic Terms . A few additional terms are listed in the glossary below. Chromatin: The complex of nucleic acids (DNA or RNA) and proteins that make up chromosomes; located in the cell nucleus. De novo: A chromosome abnormality that occurred in the individual and was not inherited from the parents. Inversion: A portion of the chromosome has broken off, turned upside down and reattached; the genetic material is inverted.

Meiosis: Cell division resulting in gametes (eggs and sperm cells), each containing one set of chromosomes-that is, 23 single chromosomes-rather than the 23 paired sets of chromosomes in nonreproductive cells. Mitosis: Cell division resulting in cells that have paired sets of chromosomes. Mosaicism: Abnormal chromosome division resulting in two or more kinds of cells, each containing different numbers of chromosomes (chromosome mosaicism). Reciprocal translocation: Segments from two different chromosomes have changed places. Ring chromosome: A portion of a chromosome has broken off and formed a circle or ring. This can happen with or without loss of genetic material. Robertsonian translocation: Two chromosomes have fused, usually at the centromere, creating a translocation. Only certain chromosomes are capable of participating in this kind of translocation. Telomere: Specialized DNA sequences located at the end of chromosomes. Free Genetics Lesson Twelve : Sex Determination, Chromosomes and Mosaicism Principles of Genetics is copyright protected, is the sole property of the author (Dr Jamie Love © 2002 - 2010) and is sold exclusively by Merlin Science . Any form of reproduction by any media is strictly forbidden.

In this sample, only about a quarter of the course is available. The remaining sections are included in the complete hypertextbook, which does not have the advertisements displayed here in this sample. To learn more about the course and hypertextbook, visit the Principles of Genetics website. Lesson Twelve Sex Determination, Chromosomes and Mosaicism by Dr Jamie Love Genetics Course So, let's talk about sex. Genes located on a particular chromosome are said to be linked to it. Genes linked to (that is, on) the sex chromosomes have unusual modes of inheritance that every biologist must understand. However, before we go into that material in the next lesson, we must first understand the sex chromosomes and how sex is determined. Sex is determined in different ways depending upon the taxa. [&QUOT;Taxa&QUOT; means the unit of taxonomy. (Taxonomy is the science of classification of organisms.) Taxa could be the species, genus, family, etc. level. It's a general term. Sex determination methods vary among different phylum and classes.

Those are taxa.] Many plants (peas, maize, etc.) and some animals (earthworms, hydra) have both male and female sex organs in the same individual and produce both male and female gametes - sperm and egg, respectively. These organisms are monoecious. [In Greek that means &QUOT;one house&QUOT; so this means both sexes live in the same &QUOT;house&QUOT; or individual.] Occasionally the word hermaphrodite is used to mean the same thing. Regardless, among these taxa there are no differences among the sexes because each individual is both sexes! To them, sex is just an extra organ system. Most animals and some plants (such as, holly and date palms) are dioecious. [In Greek that means &QUOT;two houses&QUOT; so this means each sex lives in a different &QUOT;house&QUOT; or individual.] Dioecious organisms come in two sexes, male and female, and each individual will produce only one type of gamete. People are dioecious.

Among some dioecious taxa - such as some species of fish, alligators and sea turtles - sex is determined not by genetics but by the environment! Concentrations of hormones or differences in temperature will cause the developing embryo to develop as either a male or a female. However, the sex of most dioecious individuals is determined not by the environment but by genetics. Insects have evolved some unusual sex determining systems. For example, the sex of a honeybee is determined by whether the egg is fertilized or not. That is, a haploid zygote develops into a male (drone) but a diploid zygote (produced by fertilization) produces a female bee (either worker or queen, depending upon what the larva is fed during its development). This kind of &QUOT;ploidy-based&QUOT; sex determination is not common. In most dioecious taxa the sex is determined by specific sex chromosomes. Insects, again, have found variations on this theme (involving the ratios and numbers of sex chromosomes and non-sex chromosomes) but we will not go into those complications.

Most animals have a pair of sex chromosomes and, together or individually (depending on the taxa) these determine the sex of the developing embryo and thus the sex of the fully-grown individual. I won't go further into the &QUOT;unusual&QUOT; sex determining mechanisms that have evolved because you are probably keen to learn about sex determination in humans. I will now describe what is known as the &QUOT;XY system&QUOT; of sex determination. It applies to all mammals and several other taxa, but does not apply to all taxa. Birds, butterflies, some reptiles and some fish use a sex determination system called the &QUOT;ZW system&QUOT; in which everything is backwards. (The males are ZZ and the females are ZW.) I don't expect you to know that system but you should be aware that not all taxa follow the XY system. You will recall that we have 46 chromosomes or 23 pairs of chromosomes. You will also recall that chromosomes are often grouped as homologous pairs because these pairs are composed of chromosomes that share the same genes at the same loci. That is true of 22 pairs of our chromosomes.

Those chromosomes are not involved in sex determination and are called autosomes. They form the familiar tetrads at meiosis I (composed of one maternal and one paternal chromosome) and they are numbered from the largest (#1) to the smallest (#22). The 23rd pair of human chromosomes is not really homologous, at least not in men. Male mammals have one X chromosome and one Y chromosome. These are the sex chromosomes and they don't look at all alike. They do not share any loci or genes. Female mammals do not have a Y chromosome. Most women have two X chromosomes. [Don't let the word &QUOT;most&QUOT; bother you. I will teach you about the exceptions shortly.] Summary: We have 23 pairs of chromosomes. We have 22 pairs of autosomes and one pair of sex chromosomes. Among the sex chromosomes, men have XY and women have XX. On the Y chromosome, at a particular locus which is not found on the X chromosome, there is a gene called the &QUOT;sex-determining region of the Y&QUOT; or simply &QUOT;SRY&QUOT;.

It is responsible for turning a mammalian embryo into a male mammal. We are all genetically &QUOT;pre-programmed&QUOT; to become female but at around week six of human development SRY becomes activated. SRY begins a developmental &QUOT;cascade&QUOT; in which other genes, many of them on autosomes, are also turned on and this starts a development pathway producing male features in the embryo. Testis and a penis are the obvious ones. Of course, only embryos with a Y chromosome, carrying a healthy SRY, will follow this alternative pathway. Embryos without a Y chromosome continue in their default pathway to produce female features in the embryo. Ovaries and a vagina are the obvious ones. Let's do a Punnett square for sex determination using the sex chromosomes. Normally we use Punnett squares to arrange alleles into gametes and then make zygotes but it is acceptable to use the square with entire chromosomes too. You might think that this Punnett square is obvious but I want to go through it with you so you will appreciate what is going on. First, notice that the female is producing only one type of egg with respect to sex chromosomes.

She is XX so, with respect to sex chromosomes, she is &QUOT;homozygous&QUOT;. This is true of all (normal) women. Second, notice that the male is producing two types of sperm, one carrying the X and the other the Y. He is a &QUOT;heterozygote&QUOT;, with respect to his sex chromosomes, as are all (normal) men. Any embryo that gets the father's Y chromosome will develop into a boy but any chromosome that gets his X chromosome will become a girl (because she doesn't get a Y chromosome so she doesn't have the gene SRY). Because men are &QUOT;heterozygotes&QUOT;, it is their sperm that determines whether the woman gives birth to a son or daughter. Men determine the sex of their children. Also notice that this mating produces sons and daughters in a 1 : 1 ratio because the father produces equal numbers of sperm carrying an X or Y. Indeed, this ratio is the ratio you expect for a test cross!

The female is &QUOT;recessive&QUOT; because neither of her gametes carries the &QUOT;dominant&QUOT; trait of &QUOT;maleness&QUOT; caused by SRY. However, unlike a test cross, the male's genotype - actually his karyotype (the number, form and types of chromosomes in a cell) with respect to his sex chromosomes - is known. Because we are dioecious, all matings involve a female that is &QUOT;homozygous&QUOT; (XX) and &QUOT;recessive&QUOT; (because no one can &QUOT;hide&QUOT; the phenotype produced by SRY). Also, because we are dioecious, all matings involve a male who is &QUOT;heterozygous&QUOT; (XY) &QUOT;dominant&QUOT; (only one copy of SRY is needed to be male). Strictly speaking, each mating is not really a test cross but I hope you will agree that there are a lot of parallels.

At this point, most students start to ask very good questions about &QUOT;mutant&QUOT; sex chromosomes. You may recall that Down syndrome (trisomy 21) is the only viable (&QUOT;liveable&QUOT;) autosomal aneuploidy. [You should be able to figure out that an &QUOT;autosomal aneuploidy&QUOT; means there is an unusual number of autosomes.] Aneuploidy among the sex chromosomes is more common and I expect you to understand these - like you understand Down syndrome. As with Down syndrome, these sex chromosome aneuploidies usually come about due to errors in meiosis during the gamete formation in one of the parents so the zygote starts out with the wrong chromosomal count. [Some aneuploidies can develop after fertilization has produced a normal zygote, but this has to do with errors occurring in the mitosis that follow. These exceptions are rare.] There is an interesting medical condition called Klinefelter syndrome where a person has two Xs and one Y. Remember, all you need to be male is a Y.

(&QUOT;If it's got a Y, it's a guy!&QUOT;) So you would be correct to guess that people with Klinefelter syndrome are males. He is XXY and has a total of 47 chromosomes. This is an example of a sex chromosome aneuploidy. There are rarer variations of Klinefelter syndrome in which the individual has even more extra Xs - like XXXY and XXXXY. Most men with Klinefelter syndrome are tall and thin, long legged, with small gonads and poorly developed secondary sexual characteristics - they don't develop beards, for example - and they are usually infertile. [Don't bother memorizing these medical details. I'm only providing them because people want to know. All you need to memorize is the name of these syndromes and their chromosome complement.] The &QUOT;opposite&QUOT; sex chromosome aneuploidy would be to have more than one Y chromosome. These XYY people are male, of course, and this condition has the easy to remember name of XYY syndrome.

These men do not have an abnormal phenotype (or anything &QUOT;excessive&QUOT;, in case you were wondering ). They are slightly taller and have normal fertility. Years ago there were reports that these XYY men, or &QUOT;supermen&QUOT; as the media called them, were more likely to end up in jail for violent crimes. Further research showed this was a statistical error although there is some evidence that XYYs have slight behavioral problems. So that's the two male aneuploidies - Klinefelter's and XYY syndromes. What about a zygote that, somehow, has two Ys and no Xs? Or for that matter, what about an embryo that only has a Y but no X? Well, YY and YO (as it is represented) are fatal. The embryo dies within a week of fertilization because there are lots of very important genes on the X chromosome that we simply cannot live without. We all must have at least one X chromosome to survive. There are two female aneuploidies. Trisomy X is a condition where the woman has three Xs, so she has a total of 47 chromosomes.

These &QUOT;triple X&QUOT; women look normal (although they are sometimes on the tall side) but they often have lower IQs. There are rarer variations on this aneuploidy in which a woman has four or five extra X chromosomes! It appears that with each extra X the mental retardation becomes more severe. [Don't bother memorizing these medical details. I'm only providing them because people want to know. All you need to memorize is the name of these syndromes and their chromosome complement.] Women with Turner syndrome have only one X chromosome. This is the only (viable) human aneuploidy involving less than 46 chromosomes. Specifically, women with Turner syndrome have only 45 chromosomes. (Right?) This condition is the rarest of the sex chromosome aneuploidies and these girls/women show a variety of subtle but diagnostic clinical features and a variety of medical problems. [Don't bother memorizing these medical details. I'm only providing them because people want to know. All you need to memorize is the name of these syndromes and their chromosome complement.] Clearly the X and Y chromosomes are different.

We can live without a Y chromosome (half of us do) but we must have at least one X chromosome. The Y chromosome is the smallest chromosome and it is made mostly of useless &QUOT;junk DNA&QUOT;. [You'll learn all about &QUOT;junk DNA&QUOT; in a later course.] The only gene on the Y (of any importance) is SRY - and, of course &QUOT;importance&QUOT; is a matter of opinion . The X chromosome, on the other hand, is one of the largest chromosomes and it is packed with very important genes. For example, on the X chromosome (at locus Xq28.2) is the gene for an enzyme called glucose-6-phosphate dehydrogenase. [This is just an example. Don't memorize it!] This enzyme is required to complete a very important biochemical pathway that nearly every cell uses to metabolize glucose. Without it we are dead before we are born. There are other genes on the X chromosome involved in other important events such as DNA recycling, muscle development, immunity and blood clotting. Genes on the X chromosome are said to be X-linked and they present us with a very special puzzle.

Think about this. Normal men have only one X chromosome but normal women have two copies. Don't you think that should cause a problem? After all, an extra #21 chromosome causes Down syndrome and the X chromosome has many more genes than the tiny #21. If a condition like Down syndrome can be caused by extra copies of a few genes (from the #21) then you should expect &QUOT;normal&QUOT; women to be burdened by extra copies too. Women should be &QUOT;over dosed&QUOT; with X-linked gene products. No, wait - perhaps it's the other way around. &QUOT;Normal&QUOT; men are missing an entire chromosome! Men have only one copy of every gene on the X chromosome so it is the men who are receiving an &QUOT;under dose&QUOT;. Surprisingly, when you measure the amounts of gene products (proteins made by the gene) from X-linked genes you find that men and women produce (roughly) equal amounts. No one is getting &QUOT;over dosed&QUOT; or &QUOT;under dosed&QUOT;.

It turns out that there is enough gene product from one X chromosome to keep men alive. Women avoid being &QUOT;over dosed&QUOT; by turning off one of their X chromosomes in a process called X inactivation. Which X chromosome they turn off is determined at random during embryo development. This is a fascinating and important effect and we will spend the rest of this lesson understanding it. The dosage compensation mentioned above was a clue to X inactivation but two more clues came from very different sources. Mice, cats and several other mammals (but not humans) have a gene on their X chromosomes involved in coat-color. Female mice heterozygote for two coat-colors do not show a simple pattern of dominance. Instead, their fur is mottled, made up of patches of the two colors arranged randomly. Male mice never have this arrangement. Males always have coats of uniform color. [The genetics of X-linked genes in males is another fascinating story and we will come back to it.] Another observation had to do with structures seen in the interphase nucleus of all female mammals. A dark pack of sex chromatin, called a Barr body, is seen in all mammal cells with more than one X chromosome.

Normal males never have a Barr body and normal women always have one Barr Body. Furthermore, women with Turner syndrome (the condition with only one X) did not show a Barr body - just like a man! On the other hand all the other conditions involving extra X chromosomes always showed one Barr body for each extra X chromosome. (Read that again and think about it. Hmmm.) In 1961 Mary Lyon proposed an explanation that we now called the Lyon hypothesis, although it should more properly be called a rule because it has been proven correct and there are no known exceptions. Lyon proposed that in the cells of female mammals only one X chromosome is active. The other X becomes inactivated and remains inactivated. This inactivated X chromosome is the condensed pack of chromatin we see as a Barr body in normal women. Normal men would not survive if their only X chromosome were inactivated but normal women inactivate one X chromosome and all is well. So men have no Barr body but women have one. This also explains why women with Turner syndrome (2n = 45, with only a single X) don't show a Barr body. They cannot inactivate their only X chromosome because they are in the same predicament as men.

Also men with Klinefelter syndrome (2n = 47, with the sex chromosomes of XXY) show a Barr body because they can, and do, inactivate the extra copy just like normal women do. Think about that before reading further. Now, what about those female mice with the patchy coat-colors? It turns out that during the first few days of embryonic life the X chromosome inactivation occurs. At that point in development there are hundreds or thousands of cells. In any one female cell the inactivated X chromosome may be either the paternal (Xp) or maternal (Xm). Which one is inactivated is a matter of chance but once that X has been inactivated in that cell, all that nucleus' descendants (clones through mitosis) will have the same inactive X chromosome. X inactivation is randomly determined but once it happens it's permanent. Each cell's descendants contribute to the overall body of the organism. Because some cells have inactivated paternal Xs and others have inactivated maternal Xs, the embryo and the organism it becomes is a mixture of two types of cells - expressing different alleles.

We call such an organism, made of two different cell &QUOT;types&QUOT;, a mosaic (because it is like each cell contributes a &QUOT;tile&QUOT; to make the final &QUOT;image&QUOT;). Perhaps this diagram will help you to understand the Lyon hypothesis and how it causes female mammals, including women, to be mosaic. Early in development the X chromosomes are randomly inactivated in each cell. In any particular cell it could be the paternal (Xp) or the maternal (Xm) that remains active. The inactivated X chromosome becomes a tiny blob of dense, useless chromatin called the Barr body - shown here as a dark oval. These cells pass their inactivation patterns to all their descendants. These descendants contribute to the body plan of the organism, often in a complicated way that produces a mottled pattern. The complete organism is a mosaic of cells expressing alleles from either the Xp or the Xm chromosome, depending on which one has escaped inactivation.

X inactivation allows for a dosage compensation and explains the mosaicism seen in heterozygous female mice - and cats, where it is called a &QUOT;tortoise shell&QUOT; or &QUOT;calico&QUOT;. The astute student will wonder, &QUOT;If women with Turner syndrome (45, with X0) don't make a Barr body, they don't inactivate their single X chromosome, so why do they have problems? Why aren't they as healthy as any male? On the other hand, why is Klinefelter syndrome (47, XXY) a problem since these men inactivate the extra X chromosome anyway (producing a Barr body like a normal female)?&QUOT; Good questions! The fact is that a few of the thousands of genes on the X chromosome escape inactivation. Also, there are some delicate interactions between the X chromosome genes and other genes on the autosomes (and perhaps the Y chromosome too)! All together, these poorly understood effects contribute to the unusual medical problems caused by an &QUOT;over dose&QUOT; or &QUOT;under dose&QUOT; effect.

Evolution has figured out a way for normal males and normal females to compensate for these dosage differences (Barr bodies) but the sex chromosome aneuploidies are too rare for evolution to work on them. Besides, most of them are infertile anyway, so evolution doesn't have much to work with! It is important to understand X inactivation and the mosaicism it produces because it causes important differences between male and female mammals and even differences between cells within the same female! Females that are heterozygous for X-linked genes are a mosaic for those genes and, therefore, are not &QUOT;truly&QUOT; heterozygotes on a cell-to-cell basis. Males, on the other hand cannot be heterozygote at all for any X-linked genes because they have only one location (one locus) to place only one allele. This lesson has some complicated materials. Both sex determination and X inactivation are very important events that occur in the development of the early embryo but affect each individual for the rest of his or her life. Please reread the above materials to be sure you understand it.

In our next lesson we move on to important points about sex chromosomes - the way the X-linked genes are inherited and the complications that arise. End of Lesson Twelve Chromosomes: Definition & Structure Chromosomes: Definition & Structure By Jessie Szalay, LiveScience Contributor | February 19, 2013 05:51pm ET MORE Humans have 22 chromosome pairs and two sex chromosomes. Females have two X chromosomes; males have an X chromosome and a Y chromosome. Credit: U.S. National Library of Medicine Chromosomes are thread-like molecules that carry hereditary information for everything from height to eye color. They are made of protein and one molecule of DNA, which contains an organism’s genetic instructions, passed down from parents. In humans, animals, and plants, most chromosomes are arranged in pairs within the nucleus of a cell. Humans have 22 of these chromosome pairs, called autosomes. How sex is determined Humans have an additional pair of sex chromosomes for a total of 46 chromosomes. The sex chromosomes are referred to as X and Y, and their combination determines a person’s sex. Human females have two XX chromosomes while males possess an XY pairing.

This XY sex-determination system is found in most mammals as well as some reptiles and plants. Whether a person has XX or XY chromosomes is determined when a sperm fertilizes an egg. Unlike the body’s other cells, the cells in the egg and sperm — called gametes or sex cells — possess only one chromosome. Gametes are produced by meiosis cell division, which results in the divided cells having half the number of chromosomes as the parent, or progenitor, cells. In the case of humans, this means that parent cells have two chromosomes and gametes have one. During fertilization, gametes from the sperm combine with gametes from the egg to form a zygote. The zygote contains two sets of 23 chromosomes, for the required 46. All of the gametes in the mother’s eggs possess X chromosomes. The father’s sperm contains about half X and half Y chromosomes. The sperm are the variable factor in determining the sex of the baby. If the sperm carries an X chromosome, it will combine with the egg’s X chromosome to form a female zygote. If the sperm carries a Y chromosome , it will result in a male.

The structure of X and Y chromosomes While the chromosomes for other parts of the body are the same size and shape — forming an identical pairing — the X and Y chromosomes have different structures. The X chromosome is significantly longer than the Y chromosome and contains hundreds more genes. Because the additional genes in the X chromosome have no counterpart in the Y chromosome, the X genes are dominant. This means that almost any gene on the X, even if it is recessive in the female, will be expressed in males. These are referred to as X-linked genes. Genes found only on the Y chromosome are referred to as Y-linked genes, and expressed only in males. Genes on either sex chromosome can be called sex-linked genes. There are approximately 1,098 X-linked genes, though most of them are not for female anatomical characteristics. In fact, many are linked to disorders such as hemophilia, Duchenne muscular dystrophy, fragile-X syndrome, and more. They are responsible for red-green color blindness, considered the most common genetic disorder and found most often in males. The non-sex feature X-linked genes are also responsible for male pattern baldness . In contrast to the large X chromosome, the Y chromosome contains only 26 genes.

Sixteen of these genes are responsible for cell maintenance. Nine are involved in sperm production, and if some are missing or defective it can cause low sperm counts or infertility. One gene, called the SRY gene, is responsible for male sexual traits. The SRY gene triggers the activation and regulation of another gene, found on a non-sex chromosome, called the Sox9. The Sox9 triggers the development of non-sexed gonads into testes instead of ovaries. Sex chromosome abnormalities Abnormalities in the sex chromosome combination can result in a variety of gender-specific conditions that are rarely lethal. Female abnormalities result in Turner syndrome or Trisomy X. Turner syndrome occurs when females have only one X chromosome instead of two. Symptoms include failure of the sex organs to normally mature leading to infertility, small breasts, and no menstruation; short stature; a wide, shield-shaped chest; and a wide, webbed neck. Trisomy X syndrome is caused by three X chromosomes instead of two. Symptoms include tall stature, speech delays, premature ovarian failure or ovarian abnormalities, and weak muscle tone—although many girls and women exhibit no symptoms. Males can be affected by Klinefelter syndrome .

Symptoms include breast development, abnormal proportions such as large hips, tall height, infertility, and small testicles. Further reading: WHO | Gender and Genetics Gender and Genetics Genetic Components of Sex and Gender Humans are born with 46 chromosomes in 23 pairs. The X and Y chromosomes determine a person’s sex. Most women are 46XX and most men are 46XY. Research suggests, however, that in a few births per thousand some individuals will be born with a single sex chromosome (45X or 45Y) (sex monosomies) and some with three or more sex chromosomes (47XXX, 47XYY or 47XXY, etc.) (sex polysomies). In addition, some males are born 46XX due to the translocation of a tiny section of the sex determining region of the Y chromosome. Similarly some females are also born 46XY due to mutations in the Y chromosome. Clearly, there are not only females who are XX and males who are XY, but rather, there is a range of chromosome complements, hormone balances, and phenotypic variations that determine sex.

The biological differences between men and women result from two processes: sex determination and differentiation.(3) The biological process of sex determination controls whether the male or female sexual differentiation pathway will be followed. The process of biological sex differentiation (development of a given sex) involves many genetically regulated, hierarchical developmental steps. More than 95% of the Y chromosome is male-specific (4) and a single copy of the Y chromosome is able to induce testicular differentiation of the embryonic gonad. The Y chromosome acts as a dominant inducer of male phenotype and individuals having four X chromosomes and one Y chromosome (49XXXXY) are phenotypically male. (5) When a Y chromosome is present, early embryonic testes develop around the 10th week of pregnancy. In the absence of both a Y chromosome and the influence of a testis-determining factor (TDF), ovaries develop. Gender, typically described in terms of masculinity and femininity, is a social construction that varies across different cultures and over time. (6) There are a number of cultures, for example, in which greater gender diversity exists and sex and gender are not always neatly divided along binary lines such as male and female or homosexual and heterosexual.

The Berdache in North America, the fa’afafine (Samoan for “the way of a woman”) in the Pacific, and the kathoey in Thailand are all examples of different gender categories that differ from the traditional Western division of people into males and females. Further, among certain North American native communities, gender is seen more in terms of a continuum than categories, with special acknowledgement of “two-spirited” people who encompass both masculine and feminine qualities and characteristics. It is apparent, then, that different cultures have taken different approaches to creating gender distinctions, with more or less recognition of fluidity and complexity of gender. XYY Males Case Example 1: Genetics as an important determinant of biological sex In 1959, chromosomal analysis of two human disorders, Turner syndrome and Klinefelter syndrome, demonstrated for the first time that genetic factors on the Y chromosomes of mammals are important determinants in male sex. (7) The sex determining region on the Y chromosome, referred to as the SRY, has a fundamental role in sex determination and is believed to be the switch that initiates testis development. It is, therefore, responsible for the initiation of male sex determination during embryo development.

(8) SRY-box-related (SOX) genes have been identified on autosomes (9) and mutations in the SRY or SOX gene have been implicated in sex reversal. (10) In other words, individuals with SRY mutations should develop as females. (11) However, it is likely that there are other sex determining genes involved in phenotypic sex. (12) The DNA sequence of the human X chromosome and male-specific region of the Y chromosome have recently been published, enabling further research into sex-specific genes. (13) Genetically Determined Disorders of Sexual Differentiation Typical sexual development is the result of numerous genes, and mutation in any of these genes can result in partial or complete failure of sex differentiation. These include mutations or structural anomalies of the SRY region on the Y chromosome resulting in XY gonadal dysgenesis, XX males, or XY females; defects of androgen biosynthesis or androgen receptors, and others. Androgen Insensitivity Syndrome ELSI of Genetic Components of Sex and Gender The issues of gender assignment, gender verification testing, and legal definitions of gender are especially pertinent to a discussion on the ELSI of gender and genetics.

These practices, however, are misnomers as they actually refer to biological sex and not gender. Such a discrepancy is highlighted by the existence of intersex individuals whose psychosexual development and gender sometimes do not match the biological sex assigned to them as infants. In this report the term ‘sex’ will be used where the practice refers to biological sex and not the more social construct of ‘gender’. Sex Chromosome Abnormalities Chromosomes are the structures that carry genes which in turn transmit hereditary characteristics from parents to offspring. Humans have 23 pairs of chromosomes, one half of each pair inherited from each parent. The Y chromosome is small, carries few genes, and has abundant repetitive sequence, while the X chromosome is more autosome-like in form and content. (14)Despite being relatively gene-poor overall due to reduced recombination, the X and Y sex chromosomes are enriched for genes that relate to sexual development. (15) Aneuploidy is the condition of having less than (monosomy) or more than (polysomy) the normal diploid number of chromosomes. Aneuploidy occurs in at least 5% of all pregnancies and is the most commonly recognized chromosome abnormality in humans.

(16) Divergence from the normal number of X and Y chromosomes, called sex chromosome aneuploidy (SCA), accounts for approximately half of all chromosomal anomalies in humans with a total frequency of 1:400. (17) Sex chromosome physical abnormalities are able to be diagnosed prenatally via amniocentesis and chorionic villi sampling (CVS). Prenatal diagnosis of SCA is increasing because of the widespread use of these technologies. (18) The high frequency of individuals with SCA is due to the fact that their effects are generally not as severe as autosomal abnormalities and are rarely lethal. Indeed, most cases of SCA are compatible with normal life expectancy and often go undiagnosed. (19) Still, it is estimated that1 in 3 miscarriages is due to aneuploidy affecting the foetus. (20) The induced abortion rate for SCA has steadily decreased from 100% in the 1970s to 69% in the 1980s and 49% in the 1990s.

(21) It has been suggested that this decrease is directly the result of improved knowledge of the conditions associated with SCA combined with the influence of genetic counseling. (22) Turner syndrome This disorder, also referred to as monosomy X (45X) occurs in individuals that have one X chromosome, no Y chromosome, and are phenotypically female. Although 45X is a frequent chromosomal anomaly, Turner syndrome is rare with a live-birth frequency of 1:3000, (23) as only 1 in 40 affected zygotes develops to term. (24) Affected individuals experience abnormal growth patterns, are short in stature, generally lack prominent female secondary sexual characteristics and are sterile. In some instances of Turner syndrome, there is slight mental retardation. XXX Females Women with three X chromosomes (47XXX) experience normal development of sexual traits and are fertile. Affected individuals are usually taller than average and have slender builds. The frequency of women obtaining an extra X chromosome is approximately 1:1000. There is no severe phenotype associated with three X chromosomes in women. (25) These women may have slight learning difficulties.

Klinefelter Syndrome Klinefelter syndrome (47XXY or XY/XXY mosaic) with male phenotype is the most pervasive sex chromosomal anomaly (26) affecting approximately 1:600 males. (27) Males with Klinefelter syndrome carry two or more X chromosomes which results in abnormal development of the testis, leading to hypogonadism and infertility. (28) Affected individuals are often tall and produce relatively small amounts of testosterone. As a result of this hormone imbalance, affected males have incompletely developed secondary male sex characteristics. XYY Males Men inheriting an additional Y chromosome are usually taller than average and are prone to acne because they produce higher than average levels of testosterone. Affected males are typically fertile and many are unaware that they have a chromosomal abnormality. The frequency of males born with an additional Y chromosome is approximately 1:1000. Hermaphroditism ‘True’ hermaphroditism is a genetic condition in which affected individuals have both mature ovarian and testicular tissue. (29) There are no published population-wide estimates of the frequency of true hermaphrodites.

(30) The autosomal inheritance of this condition suggests that genes controlling sexual development and differentiation are not limited to the sex chromosomes.(31) Blackless et al. suggest that such familial inheritance “opens the possibility that, as with other inherited forms of sexual ambiguity, there may be pockets, perhaps even large geographical regions, with relatively high frequencies of true hermaphroditism.” (32) Congenital Adrenal Hyperplasia Congenital adrenal hyperplasia (CAH) is an inherited autosomal recessive condition that can affect both boys and girls. It is the most common cause of intersexuality in females with 46XX, where untreated girls develop an outwardly male appearance. This disorder, also called adrenogenital syndrome (AGS), results from a genetically caused deficiency of cortisol, a steroid hormone produced by the adrenal cortex. The disorder occurs with a frequency of 1:5000 and results in incomplete female sex differentiation and increased androgenic effects due to a compensatory increase in adrenocortical hormone (ACTH).

(33) Androgen Insensitivity Syndrome Androgen Insensitivity Syndrome (AIS) is an X-linked recessive disorder in which affected individuals have external female genitalia and breast development despite being genetically male (46XY). Tissues of affected individuals are unresponsive to male hormones (androgens) yet respond to estrogens. Wisniewski et al. assessed the physical and psychosexual status of 14 women with complete androgen insensitivity syndrome (CAIS) by questionnaire as well as medical examination in order to determine the participants' knowledge of their disorder as well as their opinion of medical and surgical treatment. (34) Most of the women questioned expressed satisfaction with their psychosexual development and sexual function. All of the women who participated in the study were satisfied at having been raised as females, and none of the participants desired gender reassignment. (35) Gender Assignment of Intersex Infants and Children Intersex is defined as a congenital anomaly of the reproductive and sexual system. An estimate about the birth prevalence of intersex is difficult to make because there are no concrete parameters to the definition of intersex.

The Intersex Initiative, a North-American based organization, estimates that one in 2,000 children, or five children per day in the United States, are born visibly intersex. (36) This estimate sits within range; from genital anomalies, such as hypospadias, with a birth prevalence of around 1:300 to complex genital anomalies in which sex assignment is difficult, with a birth prevalence of about 1:4500. (37) Many intersex children have undergone medical intervention for health reasons as well as for sociological and ideological reasons. An important consideration with respect to sex assignment is the ethics of surgically altering the genitalia of intersex children to “normalize” them. Clitoral surgery for intersex conditions was promoted by Hugh Hampton Young in the United States in the late 1930s. Subsequently, a standardized intersex management strategy was developed by psychologists at Johns Hopkins University (USA) based on the idea that infants are gender neutral at birth. (38) Minto et al.

note that “the theory of psychosexual neutrality at birth has now been replaced by a model of complex interaction between prenatal and postnatal factors that lead to the development of gender and, later, sexual identity”. (39) However, currently in the United States and many Western European countries, the most likely clinical recommendation to the parents of intersex infants is to raise them as females, often involving surgery to feminize the appearance of the genitalia. (40) Minto et al. conducted a study aiming to assess the effects of feminizing intersex surgery on adult sexual function in individuals with ambiguous genitalia. As part of this study, they noted a number of ethical issues in relation to this surgery, including that: there is no evidence that feminizing genital surgery leads to improved psychosocial outcomes; feminizing genital surgery cannot guarantee that adult gender identity will develop as female; and that adult sexual function might be altered by removal of clitoral or phallic tissue. (41) Developmental biology suggests that a strict belief in absolute sexual dimorphism is incorrect. (42) Instead, Blackless et al. suggest two overlapping bell-shaped curves to conceptualize sexual variations across populations.

Qualitative variation in chromosome complement, genital morphology and hormonal activity falls under the area of overlap. (43) Such an opinion challenges the need for medical intervention in cases of intersexuality. Legal Definitions of Gender An example of the implications of legal definitions of sexual affiliation In addition to social implications, sex and gender categorization has important political and legal implications as well. The identification of an individual as either biologically male or female can have legal ramifications for marriage licenses, spousal support and eligibility for parenthood. The issue of sexual classification, however, is complicated by factors such as chromosomal complement, external genitalia, gender identification and surgical alteration. In the United States, as in most countries, there is an absence of controlling legislation that defines exactly how a person’s gender is determined. (44) The following example demonstrates some of the complexities this may raise. In 1999, the Texas Court of Appeals considered the validity of a marriage between a man and a person born genetically as a man, but surgically altered to have the physical characteristics of a woman.

(45) As in most American states, Texas family law prohibits same-sex marriages, and the United States Congress has passed the Defense of Marriage Act that, for federal purposes, defines marriage as a “legal union between one man and one woman”. (46) Despite the fact that medical experts gave testimony to the effect that the individual in question was psychologically and psychiatrically female before and after the sex reassignment surgery, the court held that chromosome complement, and not sex reassignment surgery, outward gender characteristics, nor psychosocial self-identification, determine a person’s gender. The court invalidated the marriage on the grounds that the transsexual woman was legally a man. (47) This case is one example of how the legal system in one country has dealt with the complexities of defining sex and gender and the implications of defining gender and sex. Clearly, there are many other potential legal implications of such definitions and the complicated issues they raise are many. X chromosome - Genetics Home Reference X chromosome Description The X chromosome is one of the two sex chromosomes in humans (the other is the Y chromosome). The sex chromosomes form one of the 23 pairs of human chromosomes in each cell.

The X chromosome spans about 155 million DNA building blocks (base pairs) and represents approximately 5 percent of the total DNA in cells. Each person normally has one pair of sex chromosomes in each cell. Females have two X chromosomes, while males have one X and one Y chromosome. Early in embryonic development in females, one of the two X chromosomes is randomly and permanently inactivated in cells other than egg cells. This phenomenon is called X-inactivation or Lyonization. X-inactivation ensures that females, like males, have one functional copy of the X chromosome in each body cell. Because X-inactivation is random, in normal females the X chromosome inherited from the mother is active in some cells, and the X chromosome inherited from the father is active in other cells. Some genes on the X chromosome escape X-inactivation. Many of these genes are located at the ends of each arm of the X chromosome in areas known as the pseudoautosomal regions. Although many genes are unique to the X chromosome, genes in the pseudoautosomal regions are present on both sex chromosomes. As a result, men and women each have two functional copies of these genes. Many genes in the pseudoautosomal regions are essential for normal development.

Identifying genes on each chromosome is an active area of genetic research. Because researchers use different approaches to predict the number of genes on each chromosome, the estimated number of genes varies. The X chromosome likely contains 800 to 900 genes that provide instructions for making proteins. These proteins perform a variety of different roles in the body. Related Information 46,XX testicular disorder of sex development In most individuals with 46,XX testicular disorder of sex development, the condition results from an abnormal exchange of genetic material between chromosomes (translocation). This exchange occurs as a random event during the formation of sperm cells in the affected person's father. The translocation affects the gene responsible for development of a fetus into a male (the SRY gene). The SRY gene, which is normally found on the Y chromosome, is misplaced in this disorder, almost always onto an X chromosome. A fetus with an X chromosome that carries the SRY gene will develop as a male despite not having a Y chromosome. 48,XXYY syndrome 48,XXYY syndrome is caused by the presence of an extra X chromosome and an extra Y chromosome in a male's cells.

Extra genetic material from the X chromosome interferes with male sexual development, preventing the testes from functioning normally and reducing the levels of testosterone in adolescent and adult males. Extra copies of genes from the pseudoautosomal regions of the extra X and Y chromosome contribute to the signs and symptoms of 48,XXYY syndrome; however, the specific genes have not been identified. intestinal pseudo-obstruction Intestinal pseudo-obstruction, a condition characterized by impairment of the muscle contractions that move food through the digestive tract (peristalsis), can be caused by genetic changes within the X chromosome. Some individuals with intestinal pseudo-obstruction have mutations, duplications, or deletions of genetic material in the X chromosome that affect the FLNA gene. Researchers believe that these genetic changes may impair the function of the filamin A protein, causing abnormalities in the cytoskeleton of nerve cells (neurons) in the gastrointestinal tract. These abnormalities result in impaired peristalsis, which causes abdominal pain and the other gastrointestinal symptoms of intestinal pseudo-obstruction. Deletions or duplications of genetic material that affect the FLNA gene can also include adjacent genes on the X chromosome.

Changes in adjacent genes may account for some of the other signs and symptoms, such as neurological abnormalities and unusual facial features, that occur in some affected individuals. Klinefelter syndrome Klinefelter syndrome is caused by the presence of one or more extra copies of the X chromosome in a male's cells. Extra genetic material from the X chromosome interferes with male sexual development, preventing the testes from functioning normally and reducing the levels of testosterone (a hormone that directs male sexual development). A shortage of testosterone can lead to delayed or incomplete puberty, genital abnormalities, breast enlargement (gynecomastia), reduced facial and body hair, and an inability to have biological children (infertility). Children with Klinefelter syndrome may also have learning disabilities, delayed speech and language development, and a shy and unassuming personality. Typically, people with Klinefelter syndrome have one extra copy of the X chromosome in each cell, for a total of two X chromosomes and one Y chromosome (47,XXY). Less commonly, affected individuals may have two or three extra X chromosomes (48,XXXY or 49,XXXXY).

As the number of extra sex chromosomes increases, so does the risk of learning problems, intellectual disability, birth defects, and other health issues. Some people with features of Klinefelter syndrome have the extra X chromosome in only some of their cells; in these individuals, the condition is described as mosaic Klinefelter syndrome (46,XY/47,XXY). Individuals with mosaic Klinefelter syndrome may have milder signs and symptoms, depending on how many cells have an additional X chromosome. microphthalmia with linear skin defects syndrome A deletion of genetic material in a region of the X chromosome called Xp22 causes microphthalmia with linear skin defects syndrome. This region includes a gene called HCCS , which carries instructions for producing an enzyme called holocytochrome c-type synthase. This enzyme helps produce a molecule called cytochrome c. Cytochrome c is involved in a process called oxidative phosphorylation, by which mitochondria generate adenosine triphosphate (ATP), the cell's main energy source. It also plays a role in the self-destruction of cells (apoptosis).

A deletion of genetic material that includes the HCCS gene prevents the production of the holocytochrome c-type synthase enzyme. In females (who have two X chromosomes), some cells produce a normal amount of the enzyme and other cells produce none. The resulting overall reduction in the amount of this enzyme leads to the signs and symptoms of microphthalmia with linear skin defects syndrome. In males (who have only one X chromosome), a deletion that includes the HCCS gene results in a total loss of the holocytochrome c-type synthase enzyme. A lack of this enzyme appears to be lethal very early in development, so almost no males are born with microphthalmia with linear skin defects syndrome. A few affected individuals with male appearance but who have two X chromosomes have been identified. A reduced amount of the holocytochrome c-type synthase enzyme can damage cells by impairing their ability to generate energy. In addition, without the holocytochrome c-type synthase enzyme, the damaged cells may not be able to undergo apoptosis. These cells may instead die in a process called necrosis that causes inflammation and damages neighboring cells.

During early development this spreading cell damage may lead to the eye and skin abnormalities characteristic of microphthalmia with linear skin defects syndrome. triple X syndrome Triple X syndrome (also called 47,XXX or trisomy X) results from an extra copy of the X chromosome in each of a female's cells. Females with triple X syndrome have three X chromosomes, for a total of 47 chromosomes per cell. An extra copy of the X chromosome is associated with tall stature, learning problems, and other features in some girls and women. Some females with triple X syndrome have an extra X chromosome in only some of their cells. This phenomenon is called 46,XX/47,XXX mosaicism. Females with more than one extra copy of the X chromosome (48,XXXX or 49,XXXXX) have been identified, but these chromosomal changes are rare. As the number of extra sex chromosomes increases, so does the risk of learning problems, intellectual disability, birth defects, and other health issues. Turner syndrome Turner syndrome results when one normal X chromosome is present in a female's cells and the other sex chromosome is missing or structurally altered.

The missing genetic material affects development before and after birth, leading to short stature, ovarian malfunction, and the other features of Turner syndrome. About half of individuals with Turner syndrome have monosomy X (45,X), which means each cell in an individual's body has only one copy of the X chromosome instead of the usual two sex chromosomes. Turner syndrome can also occur if one of the sex chromosomes is partially missing or rearranged rather than completely absent. Some women with Turner syndrome have a chromosomal change in only some of their cells, which is known as mosaicism. Some cells have the usual two sex chromosomes (either two X chromosomes or one X chromosome and one Y chromosome), and other cells have only one copy of the X chromosome. Women with Turner syndrome caused by X chromosome mosaicism (45,X/46,XX or 45,X/46,XY) are said to have mosaic Turner syndrome. Researchers have not determined which genes on the X chromosome are responsible for most of the features of Turner syndrome. They have, however, identified one gene called SHOX that is important for bone development and growth. The SHOX gene is located in the pseudoautosomal regions of the sex chromosomes.

Missing one copy of this gene likely causes short stature and skeletal abnormalities in women with Turner syndrome.

Question: Most people have 23 pairs of these. Women and men share 22. What are they?
Answer: Chromosomes