# X-linked genetic disease

An X-linked genetic disease is a disease inherited through a genetic defect on the X chromosome. In human cells, there is a pair of non-matching sex chromosomes, labelled X and Y. Females carry two X chromosomes, whereas males have one X and one Y chromosome. A disease or trait determined by a gene on the X chromosome demonstrates X-linked inheritance, which can be divided into dominant and recessive patterns.
The first X-linked genetic disorder described on paper was by John Dalton in 1794, then later in 1910, following Thomas Hunt Morgan's experiment, more about the sex-linked inheritance was understood. In 1961, Mary Lyon proposed the hypothesis of random X-chromosome inactivation providing the fundamental for understanding the mechanism of X-linked inheritance.
There is currently an estimation of 867 X-linked genes identified, with over 533 diseases related to X-linked genes. Common X-linked genetic diseases include Red-green colour blindness, which affects an individual's ability to see red or green images; X-linked agammaglobulinemia, resulting in a deficiency of immunity; Duchenne Muscular Dystrophy, causing muscle weakness and immobility; Hemophilia A, leading to blood clotting deficiency. X-linked recessive diseases are more frequently encountered than dominant ones and predominantly affect males, with Red-green colour blindness having the highest prevalence among all.
Genetic screening including carrier screening, prenatal screening and newborn screening could be done on individuals for early detection of genetic defects. As there are many X-linked genetic diseases, the pathology and mechanism of each varies significantly, there is no clear-cut diagnosis and treatment for all diseases. Methods of diagnosis range from blood tests to genetic tests, while treatments range from specific medications to blood infusion.

## History

Red-green colour blindness was the first X-linked genetic disorder described on paper, in 1794 by John Dalton, who is affected by the disorder himself. However, it was not until later that the inheritance pattern and genetics were worked out. The X-chromosome was discovered in 1890 by Hermann Henking, then in 1910, Thomas Hunt Morgan discovered an X-linked mutation on a Drosophila, who then conducted experiments and observations to understand the X-linked inheritance.
In 1961, Mary Lyon proposed that one of the two X chromosomes in female mammalian cells would experience random inactivation (see X-chromosome inactivation) in the early embryonic stage. According to her hypothesis, both males and females should have one single X chromosome that is active. This provided an enhancement for understanding the fundamental mechanisms of X-linked inheritance.

## Mode of inheritance

Every human cell consists of 23 chromosome pairs, with one of each pair inherited from each parent. 22 of these are homologous chromosomes, meaning they have similar structure and composition. The remaining pair is non-matching sex chromosomes labeled X and Y, which determine the sex of an individual. In humans, females have two X chromosomes while males have one X and one Y.
In each chromosome, there is unique genetic information for different traits encoded by sets of genes found on specific loci. Genes have different versions called alleles, and when an allele is dominant, it can override the effect of the other (recessive). For a dominant trait to be displayed, an individual only requires one dominant allele, whereas expressing a recessive trait requires the possession of two recessive alleles at the same time.
X-linked genetic disorders can arise when there is a spontaneous and permanent change in the DNA sequence of an X-linked gene, known as mutation. Traits or diseases caused by X chromosome genes follow X-linked inheritance, the difference between recessive and dominant inheritance affects the probability of an offspring acquiring it from the parents.

### X-linked recessive inheritance

X-linked recessive inheritance is coded by the recessive version of a gene. The mutation of a gene on the X chromosome causes the phenotype to be always present in the male because they have only one X chromosome. The phenotype only occurs in a female if she is homozygous for the mutation. A female with one copy of the mutated gene is considered a carrier.
A carrier female with only one copy of the mutated gene does not often express the diseased phenotype, although X-chromosome inactivation (or skewed X-inactivation), which is common in the female population, may lead to different levels of expression. There are characteristic patterns for X-linked recessive inheritance. As each parent contributes one sex chromosome to their offspring, sons cannot receive the X-linked trait from affected fathers, who provide only a Y chromosome. Consequently, affected males must inherit the mutated X chromosome from their mothers. X-linked recessive traits are more common in males as they only have one X chromosome, they need only one mutated X chromosome to be affected. In contrast, females have two X chromosomes and must inherit two mutated recessive X alleles, one from each parent, to be affected. X-linked recessive phenotypes tend to skip generations. A grandfather will not affect the son but could affect the grandson by passing the mutated X chromosome to his daughter who is, therefore, the carrier.
Common X-linked recessive disorders include Red green colour blindness, Hemophilia A, Duchenne muscular dystrophy.

### X-linked dominant inheritance

X-linked dominant inheritance occurs less frequently. Only one copy of the mutated alleles on the X chromosomes is sufficient to cause the disorder when inherited from an affected parent.
Unlike in X-linked recessive inheritance, X-linked dominant traits can affect females as much as males. Affected fathers alone will not lead to affected sons. However, if the mother is also affected, there will be a chance for the sons to be affected depending on which of the X chromosomes (recessive or dominant) is inherited. If a son displays the trait, the mother must also be affected. Some X-linked dominant traits, such as Aicardi syndrome, cause embryonic death in males, leading them to appear only in born females who continue to survive with these conditions.
Examples of X-linked dominant disorders include Rett syndrome, Fragile-X Syndrome, and the most cases in Alport syndrome.

## Common X-linked genetic diseases
### Red-green colour blindness

Red-green colour blindness is a type of colour vision deficiency (CVD) caused by a mutation in X-linked genes, affecting cone cells responsible for absorbing red or green light.
The perception of red and green light is attributed to the Long (L) wavelength cones and Medium (M) wavelength cones respectively. In Red-green colour blindness, mutations take place on the OPN1LW and OPN1MW genes coding for the photopigments in the cones. In milder cases, those affected exhibit reduced sensitivity to red or green light, as a result of hybridisation of the genes, shifting the response of one cone towards that of the other. In the more extreme conditions, there is a deletion or replacement of the respective coding genes, resulting in the absence of L or M cones photopigments and thus losing the ability to differentiate between red or green light completely.

### X-linked agammaglobulinemia

X-linked agammaglobulinemia (XLA) is a primary immunodeficiency disorder that impairs the body’s ability to produce antibodies, which are proteins protecting us from disease-causing antigens, resulting in severe bacterial infections.
XLA is associated with a mutation in the Bruton's tyrosine kinase (BTK) gene on the X chromosome, which is responsible for producing BTK, an enzyme regulating B cells development. B cells are a type of white blood cells essential in the production of antibodies, when at an early stage, called pre-B cells, they rely on expansion and survival signals involving BTK to mature.
In affected individuals, their BTK genes have an amino acid substitution mutation, altering the amino acid sequence and the structure of BTK making it faulty. Therefore, they have a normal pre-B cell counts but cannot develop mature B cells, resulting in antibody deficiency.

### Duchenne Muscular Dystrophy

Duchenne Muscular Dystrophy (DMD) is a severe neuromuscular disease causing progressive weakness and damage of muscle tissues, leading to mobility loss and difficulties in daily activities. In a later stage of DMD, as respiratory and cardiac muscles start to degenerate, affected individuals are likely to develop complications such as respiratory failure, cardiomyopathy and heart failure.
DMD arises from a mutation, likely to be the deletion of the exons, a nucleotide sequence in the DMD gene that codes for dystrophin. Dystrophin is a protein responsible for strengthening and stabilising muscle fibres. With the loss of the dystrophin complex, the muscle cells would no longer be protected and therefore result in progressive damage or degeneration.

### Haemophilia A

Haemophilia A is a blood clotting disease caused by a genetic defect in clotting factor VIII. It causes significant susceptibility to both internal and external bleeding. Individuals having more severe haemophilia can experience more frequent and intense bleeding.
Severe haemophilia A affects most patients. Patients with mild haemophilia often do not experience heavy bleeding except for surgeries and significant trauma.

## Screening
### Carrier screening

Carrier screening aims to screen for recessive diseases. Targets of carrier screening typically do not show any symptoms but rather might have a family history of the disease or are in a stage of family planning. Carrier screening is done by performing a blood test on the individual, to identify the specific allele.

### Prenatal screening

Prenatal screening is offered to females during pregnancy, it involves both maternal blood tests and ultrasound to check for possible defect genes in developing fetus. The screening result only confirms a possibility of genetic disease, so parents would be prepared psychologically, or could consider the option of pregnancy termination.

### Newborn screening

The heel prick test is commonly used. A few drops of blood would be collected with a cotton paper from the heel of a newborn that is less than a week old, samples would then be analysed for a variety of disorders.