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Sex linkage

Sex determination in humans is controlled by the inheritance of X and Y chromosomes from the parents.

The human genome contains 23 pairs of chromosomes. 22 of these are homologous pairs (autosomal chromosomes). The 23rd pair are called the sex chromosomes.

There are two types of sex chromosome: the X chromosome and the Y chromosomes .

The X and the Y chromosomes are non-homologous; they do not share all the same genes

The sex chromosomes an offspring inherits will determine its sex:

  • Males have an X and a Y .
  • Females have two Xs .

Not all animals use the XY system to determine sex. Alligators determine the sex of the offspring by the temperature at which the egg is incubated!

Sex determination is 50:50 as a child has an equal chance of inheriting the X or Y chromosome from its father.
Sex determination is 50:50 as a child has an equal chance of inheriting the X or Y chromosome from its father.

The Y chromosome is much smaller than the X chromosome. This means that a large section of the X chromosome has no homologous section on the Y chromosome.

As a result, there will only be a single copy of many sex-linked genes in males. This has a significant effect on the expression of the traits that those genes code.

If a recessive gene is present on the X chromosome in females, two copies of the gene must be present for that trait to be expressed.

However, in males, only one copy of a recessive gene needs to be present on the X chromosome for that trait to be expressed. This is because there is no corresponding gene on the Y chromosome that can suppress it.

Sex-linked traits are traits that are coded for by sex-linked genes.

Sex-linked genes are present on only one sex chromosome (on X but not Y or only on Y but not X). As a result, they have an unusual pattern of expression.

A recessive sex-linked gene will need two copies to be expressed in females, but just one copy in males. This is a form of non-Mendelian inheritance.

There are several human traits that show this pattern of inheritance, such as haemophilia, Duchenne muscular dystrophy and red-green colour-blindness.

Haemophilia is a condition that prevents the blood from clotting properly. A person suffering from haemophilia is more likely to suffer from internal and external bleeding and bruising due to a lack of blood clotting factors.

Haemophilia is an example of a recessive sex-linked trait. The gene is present on the X chromosome but not on the Y chromosome. Males only need to carry a single allele (on the single X chromosome they have) to have the symptoms of haemophilia, whereas females need two.

As a result, haemophilia and other sex-linked traits are far more common in the male population.

Haemophilia appeared in the British royal family through a spontaneous mutation in either Queen Victoria or her mother. The disease has been passed on to the Spanish, German and Russian royal families.

If a female is a carrier of haemophilia, half her male children can be expected to have the disease, and half her female children will be carriers.
If a female is a carrier of haemophilia, half her male children can be expected to have the disease, and half her female children will be carriers.