4. Sex-Linked Inheritance
What determines if a baby is a boy or a girl? Recall that you have 23 pairs of chromosomes, one pair of which are the sex chromosomes. Everyone has two sex chromosomes, X or Y, that determine our sex. Females have two X chromosomes, while males have one Y chromosome and one X chromosome. So if a baby inherits an X from the father and an X from the mother, it will be a girl. If the father’s sperm carries the Y chromosome, it will be a boy. Notice that a mother can only pass on an X chromosome, so the sex of the baby is determined by the father. The father has a 50 percent chance of passing on the Y or X chromosome, hence it is a 50 percent chance whether a child will be a boy or a girl.
One special pattern of inheritance that doesn’t fit Mendel’s rules is sex-linked inheritance, referring to the inheritance of traits which are due to genes located on the sex chromosomes. The X chromosome and Y chromosome carry many genes and some of them code for traits that have nothing to do with determining sex. Since males and females do not have the same sex chromosomes, there will be differences between the sexes in how these sex-linked traits are expressed.
One example of a sex-linked trait is red-green colorblindness. People with this type of colorblindness cannot distinguish between red and green and often see these colors as shades of brown (Figure below). Boys are much more likely to be colorblind than girls. That’s because colorblindness is a sex-linked recessive trait. Boys only have one X chromosome, so if that chromosome carries the gene for colorblindness, they will be colorblind. As girls have two X chromosomes, a girl can have one X chromosome with the colorblind gene and one X chromosome with a normal gene for color vision. Since colorblindness is recessive, the dominant normal gene will mask the recessive colorblind gene. For a girl to be colorblind, she would have to inherit two genes for colorblindness, which is very unlikely. Many sex-linked traits are inherited in a recessive manner.
A person with red-green colorblindness would not be able to see the number.
A woman can be a carrier of colorblindness, however. A carrier appears normal but is capable of passing on a genetic disorder to her child. Carriers for colorblindness have a heterozygous genotype of one colorblind allele and one normal allele. We can use a Punnett square to predict the probability of a carrier passing on the trait to her children. For example, if a woman who is a carrier for colorblindness has children, her boys would have a 50% chance of being colorblind and her girls have a 50% chance of being carriers.