Sex Linkage

Key definitions

A sex-linked characteristic is a feature in which the gene responsible is located on a sex chromosome and that this makes the characteristic more common in one sex than in the other.
Red-green colour blindness is an example of sex linkage.

Your chromosomes determine which sex you are.
In humans, there are 46 chromosomes and these occur in 23 pairs.
- In females, all 46 chromosomes can be matched together into pairs. This is done by taking images of the chromosomes and matching them for size and shape.
- In males, there are two chromosomes that are not alike. These are known as the X and Y chromosomes.
Females have two X chromosomes and males have an X chromosome and a Y chromosome.

gametes	X
X	XX (50% female)
Y	XY (50% male)

In this Punnett square, you can see how sex chromosomes in the gametes determine the sex of individuals.
- Remember that the number of chromosomes is halved in meiosis so each gamete can have only one of the two sex chromosomes.
All egg cells have an X chromosome. Half of the sperm contain an X chromosome and half of the sperm contain a Y chromosome.
At fertilization, the egg could fuse with either a sperm with an X chromosome or a sperm with a Y chromosome.
- Since there are equal numbers of sperm with the X chromosome and sperm with the Y chromosome, there is an equal chance of the zygote being XX or XY and the child being female or male.

In this image with an X chromosome (left) and a Y chromosome (right), the Y chromosome is much smaller than the X chromosome.
- Therefore, the Y chromosome has far fewer genes than the X chromosome or any other chromosome.
One gene on the Y chromosome stimulates the development of testes in the embryo.
- If it is not present (or if it has mutated) then the body that develops is female.

The genes that are located on the X chromosome are described as sex-linked even though they have no relation to determining gender or controlling sexual characteristics.
Among the genes on the X chromosome are genes involved with:
- controlling vision
- blood clotting.
Males only have one copy of the genes that are on the X chromosomes. This means that if any of them are recessive, the effect will be seen.
Since women have two X chromosomes, they are less affected by sex-linked recessive alleles and this is why sex-linked conditions are more common in boys than in girls.

One of the genes on the X chromosome controls the ability to see red and green colours.
The gene works in the receptors, known as cones, in the retina of the eye.
- There is an allele of this gene that does not produce a protein necessary for colour vision. It is a recessive allele, so any girl or woman who is heterozygous, Rr, has normal colour vision.
Males only have one X chromosome so if they have inherited the allele r, they will have red-green colour blindness.

gametes	$X^{r}$	Y
$X^{R}$	$X^{R} X^{r}$	$X^{R} Y$

This Punnett square shows what to expect if the mother is homozygous dominant and the father is colour blind.
None of the children can expect to be colour blind, but the girls will inherit the allele for colour blindness on the X chromosome from their father.
- They have this allele, but it does not affect their phenotype. Any females who are heterozygous are carriers of red-green colour blindness.

gametes	$X^{R}$	$Y$
$X^{r}$	$X^{R} X^{r}$	$X^{r} Y$
$X^{R}$	$X^{R} X^{R}$	$X^{R} Y$

This Punnett square shows what to expect if the mother is a carrier and the father has normal colour vision.
In this case, there is a 1 in 4 chance that one of the children will be colour blind.
- Note that there is a 1 in 2 chance that any boy will be colour blind.