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: How many chromosomes are there in a human body cell? | eNotes How many chromosomes are there in a human body cell?

lynn30k | High School Teacher | (Level 1) Educator Posted on May 30, 2012 at 3:06 PM I have to correct the first answerer--the "normal" number of 46 chromosomes is the DIPLOID number, not the haploid. In egg and sperm cells, half the number is present, and this is known as the haploid number. In some individuals, the diploid number is off; this is known as either a monosomy (only one of the usual pair is present), or a trisomy (3 copies, instead of two). The most well-known trisomy is Trisomy 21--3 copies of the 21st pair, otherwise known as Down Syndrome. This is not always obvious simply from counting chromosomes in a karyotype, as the extra material can be attached to another chromosome. Extra/missing X or Y chromosomes are other common genetic mistakes.

like 18 dislike 1 sanjeetmanna | College Teacher | (Level 3) Assistant Educator Posted on There are 46 chromosomes that is 23 pairs of chromosomes that is 22 pair of autosomes and 1 pair of sex chromosomes. like 9 dislike 0 aimeevd | (Level 1) Adjunct Educator Posted on January 12, 2013 at 10:36 PM 23 pairs of chromosomes, which results in 46 chromosomes. 22 of the pairs are automsomes, while 1 pair in the sex chromosomes (which XX results in a female and XY results in a male). I would like to clarify that sometimes people can have more or less than 46 chromosomes is not due to mutation, but rather nondisjunction. Basically it is caused by improper division of chromosomes during meiosis. The only time someone can survive from nondisjunction (result in a vilable baby) is when the person has an extra chromosomes 21, or an extra sex chromosomes. Also one may survive with only 1 X chromosome or missing part of chromosome 5.

Extra chromosome 21: Down Syndrome Missing a partial Chromosome 5: Cri du chat Syndrome (aka Cat's Cry) XXX: Tripe X Syndrome June 14, 2012 at 10:53 AM There are 22 pairs of autosomes( chromosomes which are responsible for any feature in the body other than sex determination) and 1 pair of allosomes or sex chromosomes( wwhich determines the sex of an individual). Therefore there is a total of 23 pairs of chromosome in a human body cell. However the human gametes formed by meiosis have haploid number of chromosomes i.e. they have only 23 chromosomes instead of 46; so that after fertilization the zygote has 23 pairs of chromosome(23 from the sperm and 23 from the ovum). like 10 dislike 0 DNA, genes and chromosomes — University of Leicester The DNA double helix showing base pairs The sides are sugar and phosphate molecules. The rungs are pairs of chemicals called 'nitrogenous bases', or 'bases' for short.

There are four types of base: adenine (A), thymine (T), guanine (G) and cytosine (C). These bases link in a very specific way: A always pairs with T, and C always pairs with G. The DNA molecule has two important properties. It can make copies of itself. If you pull the two strands apart, each can be used to make the other one (and a new DNA molecule). It can carry information. The order of the bases along a strand is a code - a code for making proteins. Genes A gene is a length of DNA that codes for a specific protein. So, for example, one gene will code for the protein insulin, which is important role in helping your body to control the amount of sugar in your blood. Genes are the basic unit of genetics. Human beings have 20,000 to 25,000 genes. These genes account for only about 3 per cent of our DNA. The function of the remaining 97 per cent is still not clear, although scientists think it may have something to do with controlling the genes.

Chromosomes If you took the DNA from all the cells in your body and lined it up, end to end, it would form a strand 6000 million miles long (but very, very thin)! To store this important material, DNA molecules are tightly packed around proteins called histones to make structures called chromosomes. The packaging of DNA into chromosomes Human beings have 23 pairs of chromosomes in every cell, which makes 46 chromosomes in total. A photograph of a person's chromosomes, arranged according to size, is called a karyotype. The sex chromosomes determine whether you are a boy (XY) or a girl (XX). The other chromosomes are called autosomes. The karyotype of a male human being The largest chromosome, chromosome 1, contains about 8000 genes. The smallest chromosome, chromosome 21, contains about 300 genes. (Chromosome 22 should be the smallest, but the scientists made a mistake when they first numbered them!). The DNA that contains your genes is stored in your cells in a structure called the nucleus.

A diagram of animal cell showing the nucleus 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. How many chromosomes does a zygote contain? | Reference.com How many chromosomes does a zygote contain? A: Quick Answer Normal human zygotes all have 46 chromosomes organized into 23 pairs, which is the same number that most adult humans have. Together, these 46 chromosomes contain an estimated 25,000 genes, according to the National Human Genome Research Institute. Sometimes, human embryos develop with an extra chromosome that influences the development of Down syndrome. Full Answer NHGRI points to a distinction between the first 22 pairs of chromosomes, or autosomes, and the last pair called "sex chromosomes." Every human has a full set of autosomes from conception and therefore carries them throughout the zygote phase of gesticulation. Some people are conceived with one fewer sex chromosome. This condition, known as Turner syndrome, always results in a female zygote, as the missing chromosome is always the male, or "Y," chromosome.

Numerical abnormalities such as these are not present in the majority of developing zygotes, however, and the majority of humans carry the normal complement of 46. Interestingly, this number is two fewer than the complement carried by most apes. The Department of Energy's Office of Science describes the way the human chromosome number two seems to be arranged as if it resulted from the fusion of an ancestral pair of chromosomes, which it identifies as being the most parsimonious explanation for the difference in chromosome count between humans and apes. Genes and Chromosomes - Fundamentals - Merck Manuals Consumer Version A gene is a segment of DNA containing the code used to synthesize a protein. A chromosome contains hundreds to thousands of genes. Every human cell contains 23 pairs of chromosomes, for a total of 46 chromosomes. A trait is any gene-determined characteristic and is often determined by more than one gene. Some traits are caused by abnormal genes that are inherited or that are the result of a new mutation. Proteins are probably the most important class of material in the body. Proteins are not just building blocks for muscles, connective tissues, skin, and other structures. They also are needed to make enzymes.

Enzymes are complex proteins that control and carry out nearly all chemical processes and reactions within the body. The body produces thousands of different enzymes. Thus, the entire structure and function of the body is governed by the types and amounts of proteins the body synthesizes. Protein synthesis is controlled by genes, which are contained on chromosomes. The genotype is a person’s unique combination of genes or genetic makeup. Thus, the genotype is a complete set of instructions on how that person’s body synthesizes proteins and thus how that body is supposed to be built and function. The phenotype is the actual structure and function of a person’s body. The phenotype typically differs somewhat from the genotype because not all the instructions in the genotype may be carried out (or expressed). Whether and how a gene is expressed is determined not only by the genotype but also by the environment (including illnesses and diet) and other factors, some of which are unknown. The karyotype is the full set of chromosomes in a person’s cells. Genes DNA Genes consist of deoxyribonucleic acid (DNA). DNA contains the code, or blueprint, used to synthesize a protein. Genes vary in size, depending on the sizes of the proteins for which they code.

Each DNA molecule is a long double helix that resembles a spiral staircase containing millions of steps. The steps of the staircase consist of pairs of four types of molecules called bases (nucleotides). In each step, the base adenine (A) is paired with the base thymine (T), or the base guanine (G) is paired with the base cytosine (C). Structure of DNA DNA (deoxyribonucleic acid) is the cell’s genetic material, contained in chromosomes within the cell nucleus and mitochondria. Except for certain cells (for example, sperm and egg cells and red blood cells), the cell nucleus contains 23 pairs of chromosomes. A chromosome contains many genes. A gene is a segment of DNA that provides the code to construct a protein. The DNA molecule is a long, coiled double helix that resembles a spiral staircase. In it, two strands, composed of sugar (deoxyribose) and phosphate molecules, are connected by pairs of four molecules called bases, which form the steps of the staircase. In the steps, adenine is paired with thymine and guanine is paired with cytosine. Each pair of bases is held together by a hydrogen bond.

A gene consists of a sequence of bases. Sequences of three bases code for an amino acid (amino acids are the building blocks of proteins) or other information. Synthesizing proteins Proteins are composed of a long chain of amino acids linked together one after another. There are 20 different amino acids that can be used in protein synthesis—some must come from the diet (essential amino acids), and some are made by enzymes in the body. As a chain of amino acids is put together, it folds upon itself to create a complex three-dimensional structure. It is the shape of the folded structure that determines its function in the body. Because the folding is determined by the precise sequence of amino acids, each different sequence results in a different protein. Some proteins (such as hemoglobin) contain several different folded chains. Instructions for synthesizing proteins are coded within the DNA. Coding Information is coded within DNA by the sequence in which the bases (A, T, G, and C) are arranged. The code is written in triplets. That is, the bases are arranged in groups of three. Particular sequences of three bases in DNA code for specific instructions, such as the addition of one amino acid to a chain.

For example, GCT (guanine, cytosine, thymine) codes for the addition of the amino acid alanine, and GTT (guanine, thymine, thymine) codes for the addition of the amino acid valine. Thus, the sequence of amino acids in a protein is determined by the order of triplet base pairs in the gene for that protein on the DNA molecule. The process of turning coded genetic information into a protein involves transcription and translation. Transcription and translation Transcription is the process in which information coded in DNA is transferred (transcribed) to ribonucleic acid (RNA). RNA is a long chain of bases just like a strand of DNA, except that the base uracil (U) replaces the base thymine (T). Thus, RNA contains triplet-coded information just like DNA. When transcription is initiated, part of the DNA double helix splits open and unwinds. One of the unwound strands of DNA acts as a template against which a complementary strand of RNA forms. The complementary strand of RNA is called messenger RNA (mRNA).

The mRNA separates from the DNA, leaves the nucleus, and travels into the cell cytoplasm (the part of the cell outside the nucleus—see see Figure: Inside a Cell ). There, the mRNA attaches to a ribosome, which is a tiny structure in the cell where protein synthesis occurs. With translation, the mRNA code (from the DNA) tells the ribosome the order and type of amino acids to link together. The amino acids are brought to the ribosome by a much smaller type of RNA called transfer RNA (tRNA). Each molecule of tRNA brings one amino acid to be incorporated into the growing chain of protein, which is folded into a complex three-dimensional structure under the influence of nearby molecules called chaperone molecules. Control of gene expression There are many types of cells in a person’s body, such as heart cells, liver cells, and muscle cells. These cells look and act differently and produce very different chemical substances. However, every cell is the descendant of a single fertilized egg cell and as such contains essentially the same DNA. Cells acquire their very different appearances and functions because different genes are expressed in different cells (and at different times in the same cell). The information about when a gene should be expressed is also coded in the DNA.

Gene expression depends on the type of tissue, the age of the person, the presence of specific chemical signals, and numerous other factors and mechanisms. Knowledge of these other factors and mechanisms that control gene expression is growing rapidly, but many of these factors and mechanisms are still poorly understood. The mechanisms by which genes control each other are very complicated. Genes have markers to indicate where transcription should begin and end. Various chemical substances (such as histones) in and around the DNA block or permit transcription. Also, a strand of RNA called antisense RNA can pair with a complementary strand of mRNA and block translation. Replication Cells reproduce by splitting in two. Because each new cell requires a complete set of DNA molecules, the DNA molecules in the original cell must reproduce (replicate) themselves during cell division. Replication happens in a manner similar to transcription, except that the entire double-strand DNA molecule unwinds and splits in two. After splitting, bases on each strand bind to complementary bases (A with T, and G with C) floating nearby. When this process is complete, two identical double-strand DNA molecules exist. Mutation To prevent mistakes during replication, cells have a “proofreading” function to help ensure that bases are paired properly.

There are also chemical mechanisms to repair DNA that was not copied properly. However, because of the billions of base pairs involved in and the complexity of the protein synthesis process, mistakes can happen. Such mistakes can occur for numerous reasons (including exposure to radiation, drugs, or viruses) or for no apparent reason. Minor variations in DNA are very common and occur in most people. Most variations do not affect subsequent copies of the gene. Mistakes that are duplicated in subsequent copies are called mutations. Mutations that affect the reproductive cells may be passed on to offspring. Mutations that do not affect reproductive cells affect the descendants of the mutated cell (for example, becoming a cancer) but are not passed on to offspring. Mutations may be unique to an individual or family, and most mutations are rare. Mutations that become so common that they affect more than 1% of a population are called polymorphisms (for example, the human blood types A, B, AB, and O). Most polymorphisms have no effect on the phenotype (see Genes and Chromosomes ). Mutations may involve small or large segments of DNA.

Depending on its size and location, the mutation may have no apparent effect or it may alter the amino acid sequence in a protein or decrease the amount of protein produced. If the protein has a different amino acid sequence, it may function differently or not at all. An absent or nonfunctioning protein is often harmful or fatal. For example, in phenylketonuria, a mutation results in the deficiency or absence of the enzyme phenylalanine hydroxylase. This deficiency allows the amino acid phenylalanine (absorbed from the diet) to accumulate in the body, ultimately causing severe intellectual disability. In rare cases, a mutation introduces a change that is advantageous. For example, the sickle cell gene causes sickle cell anemia but also provides protection against malaria. Although the protection against malaria can help a person survive, sickle cell anemia causes symptoms and complications that may shorten life span, so that survival is not always prolonged. Natural selection refers to the concept that mutations that impair survival in a given environment are less likely to be passed on to offspring (and thus become less common in the population), whereas mutations that improve survival progressively become more common. Thus, beneficial mutations, although initially rare, eventually become common.

The slow changes that occur over time caused by mutations and natural selection in an interbreeding population collectively are called evolution. Did You Know... Not all gene abnormalities are purely harmful—the gene that causes sickle cell disease also provides protection against malaria. People carry an average of 100 to 400 abnormal genes. Chromosomes A chromosome is made of a very long strand of DNA and contains many genes (hundreds to thousands). The genes on each chromosome are arranged in a particular sequence, and each gene has a particular location on the chromosome (called its locus). In addition to DNA, chromosomes contain other chemical components that influence gene function. Pairing Except for certain cells (for example, sperm and egg cells or red blood cells), the nucleus of every human cell contains 23 pairs of chromosomes, for a total of 46 chromosomes. Normally, each pair consists of one chromosome from the mother and one from the father. There are 22 pairs of nonsex (autosomal) chromosomes and one pair of sex chromosomes. Paired nonsex chromosomes are, for practical purposes, identical in size, shape, and position and number of genes.

Because each member of a pair of nonsex chromosomes contains one of each corresponding gene, there is in a sense a backup for the genes on those chromosomes. The 23rd pair is the sex chromosomes (X and Y). Sex chromosomes The pair of sex chromosomes determines whether a fetus becomes male or female. Males have one X and one Y chromosome. A male’s X comes from his mother and the Y comes from his father. Females have two X chromosomes, one from the mother and one from the father. In certain ways, sex chromosomes function differently than nonsex chromosomes. The smaller Y chromosome carries the genes that determine male sex as well as a few other genes. The X chromosome contains many more genes than the Y chromosome, many of which have functions besides determining sex and have no counterpart on the Y chromosome. In males, because there is no second X chromosome, these extra genes on the X chromosome are not paired and virtually all of them are expressed. Genes on the X chromosome are referred to as sex-linked, or X-linked, genes. Normally, in the nonsex chromosomes, the genes on both of the pairs of chromosomes are capable of being fully expressed.

However, in females, most of the genes on one of the two X chromosomes are turned off through a process called X inactivation (except in the eggs in the ovaries). X inactivation occurs early in the life of the fetus. In some cells, the X from the father becomes inactive, and in other cells, the X from the mother becomes inactive. Thus, one cell may have a gene from the person’s mother and another cell has the gene from the person’s father. Because of X inactivation, the absence of one X chromosome usually results in relatively minor abnormalities (such as Turner syndrome—see see Turner Syndrome ). Thus, missing an X chromosome is far less harmful than missing a nonsex chromosome. If a female has a disorder in which she has more than two X chromosomes, the extra chromosomes tend to be inactive. Thus, having one or more extra X chromosomes causes far fewer developmental abnormalities than having one or more extra nonsex chromosomes. For example, women with three X chromosomes (triple X syndrome) are often physically and mentally normal (see Triple X Syndrome ). Most males who have more than one Y chromosome (see XYY Syndrome ) are not physically and mentally normal.

Chromosomal abnormalities There are several types of chromosomal abnormalities (see Overview of Chromosomal Disorders ). A person may have an abnormal number of chromosomes or have abnormal areas on one or more chromosomes. Many such abnormalities can be diagnosed before birth (see Testing for chromosomal and genetic abnormalities ). Abnormal numbers of nonsex chromosomes usually result in severe abnormalities. For example, receiving an extra nonsex chromosome can be fatal to a fetus or can lead to abnormalities such as Down syndrome, which commonly results from a person having three copies of chromosome 21. Absence of a nonsex chromosome is typically fatal to the fetus. Large areas on a chromosome may be abnormal, usually because a whole section was left out (called a deletion) or mistakenly placed in another chromosome (called translocation). For example, chronic myelogenous leukemia is sometimes caused by translocation of part of chromosome 9 onto chromosome 22. This abnormality can be inherited or be the result of a new mutation (see Mutation ). Mitochondrial chromosomes Mitochondria are tiny structures inside cells that synthesize molecules used for energy. Unlike other structures inside cells, each mitochondrion contains its own circular chromosome.

This chromosome contains DNA (mitochondrial DNA) that codes for some, but not all, of the proteins that make up that mitochondrion. Mitochondrial DNA usually comes only from the person’s mother because, in general, when an egg is fertilized, only mitochondria from the egg become part of the developing embryo. Mitochondria from the sperm usually do not become part of the developing embryo. Traits A trait is any gene-determined characteristic. Many traits are determined by the function of more than one gene. For example, a person's height is likely to be determined by genes affecting growth, appetite, muscle mass, and activity level. However, some traits are determined by the function of a single gene. Variation in some traits, such as eye color or blood type, is considered normal. Other variations, such as albinism, Marfan syndrome, and Huntington disease, harm body structure or function and are considered disorders. However, not all such gene abnormalities are uniformly harmful. For example, the sickle cell gene causes disease (sickle cell anemia) but also provides protection against malaria. Genetic Disorders A genetic disorder is a detrimental trait caused by an abnormal gene.

The abnormal gene may be inherited or may arise spontaneously as a result of a mutation. Abnormalities of one or more genes are fairly common. Humans carry an average of 100 to 400 abnormal genes. However, most of the time the corresponding gene on the other chromosome in the pair is normal and prevents any harmful effects. In the general population, the chance of a person having two copies of the same abnormal gene (and hence a disorder) is very small. However, in children who are offspring of close blood relatives, the chances are higher. Chances are also high among children of parents who have married within an isolated population, such as the Amish or Mennonites. Resources In This Article 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.

Question: A human-being has how many pairs of chromosomes?
Answer: Twenty-three