Sex linkage

Sex linkage is the phenotypic expression of an allele related to the chromosomal sex of the individual. This mode of inheritance is in contrast to the inheritance of traits on autosomal chromosomes, where both sexes have the same probability of inheritance. Since humans have many more genes on the X than the Y, there are many more X-linked traits than Y-linked traits.

In mammals, the female is the homozygous sex, with two X chromosomes (XX), while the male is heterozygous, with one X and one Y chromosome (XY). Genes on the X or Y chromosome are called sex linked genes.

In birds, the opposite is true: the male is the homozygous sex, having two Z chromosomes (ZZ), and the female (hen) is heterozygous, having one Z and one W chromosome (ZW).

Linkage overview
X-linked recessive traits are expressed in all heterogametics, but are only expressed in those homogametics that are homozygous for the recessive allele. For example, an X-linked recessive allele in humans causes hemophilia. Hemophilia is much more common in males than females because males are hemizygous - they only have one copy of the gene in question - and therefore express the trait when they inherit one mutant allele. In contrast, a female must inherit two mutant alleles, a less frequent event since the mutant allele is rare in the population. Tsarevich Alexei of Russia was the most famous sufferer of X-linked hemophilia, and his disease may have played an important role in the overthrow of the imperial regime, which changed the course of history for millions of people.

The incidence of recessive X-linked phenotypes in females is the square of that in males (squaring a proportion less than one gives an outcome closer to 0 than the original). If 1 in 20 males in a human population are green colour blind, then 1 in 400 females in the population are expected to be colour blind (1/20)*(1/20).

X-linked traits are maternally inherited from carrier mothers or from an affected father. Each son born to a carrier mother has a 50% probability of inheriting the X-chromosome carrying the mutant allele. There are a few Y-linked traits; these are inherited from the father.

In classical genetics, a reciprocal cross is performed to test if a trait is sex-linked.

Types of links
Diseases well known for their X-linked recessive inheritance are Haemophilia (types A and B), and color blindness.

X-linked recessive
X-linked inherited diseases occur far more frequently in males because they only have one X chromosome. Females must receive a copy of the gene from both parents to have such a recessive disease. However, they will still be carriers if they receive one copy of the gene. Recessive genes on the X chromosome that cause serious diseases are usually passed from female carriers to their ill sons and carrier daughters. This is because males, who always have the disease and are not just carriers, would have to father a daughter to pass on the gene. This is unlikely because severe genetic diseases often cause death in childhood or early adulthood. However, if the disease shows up late in life, or is not severe, he will pass the gene to all of his daughters. He can not pass it to his sons because a male receives his X chromosome from his mother, and the father passes on a Y chromosome. A mother with one copy of the gene has a 50% chance of passing it to her children of both sexes, but her daughters will just be carriers of the gene unless their father has it, too.

X-linked dominant
There are few examples of X-linked dominant diseases; the best known in this category is vitamin D resistant rickets.


 * Incontinentia pigmenti
 * CHILD syndrome

Y-linked

 * Various failures in the SRY genes

Sex-linked traits in other animals

 * Calico or tortoiseshell cats, which have orange-and-black fur
 * White eyes in Drosophila melanogaster flies&mdash;the first sex-linked gene ever discovered