Sex-Linked Inheritance

What sex-linked means

Most genes discussed in genetics are carried on autosomes (the 22 non-sex chromosome pairs) and are inherited without reference to sex. Sex-linked genes are instead carried on a sex chromosome, almost always the X chromosome, since the Y chromosome carries very few genes.

Because the sex chromosomes an individual has depends on their sex, so does their pattern of inheriting X-linked genes.

Notation for X-linked alleles

X-linked alleles are written as a superscript on the X chromosome itself, e.g. X^H for a normal allele and X^h for a recessive allele. A female genotype always has two X's (X^H X^H, X^H X^h, or X^h X^h).

A male genotype has one X and one Y (X^H Y or X^h Y), the Y carries no matching allele for this gene.

Why males are more often affected

A female needs two copies of a recessive X-linked allele (X^h X^h) to show the recessive condition, because a single X^H allele is enough to mask it. A male, however, has only one X chromosome, so a single X^h allele is enough to produce the recessive phenotype, there is no second X chromosome to mask it.

This is why X-linked recessive conditions such as colour blindness and haemophilia occur far more often in males than in females.

Carriers

A female with genotype X^H X^h is called a carrier. She does not show the recessive condition herself, because the dominant X^H allele masks it, but she can pass the X^h allele on to her children, who may then be affected depending on which X chromosome and which parent's gamete they inherit.

Worked problem 1, easy: carrier mother x normal father

Question. Colour blindness is X-linked recessive: X^B is the normal allele and X^b causes colour blindness. A carrier mother (X^B X^b) has children with a father of normal vision (X^B Y).

Draw the Punnett square and describe the sons and daughters.

Model answer. The mother forms X^B and X^b eggs; the father forms X^B and Y sperm. All daughters receive X^B from the father, so none is colour-blind, but half are carriers (X^B X^b).

Sons receive only the mother's X, so they are 1 normal (X^B Y) : 1 colour-blind (X^b Y).

Worked problem 2, medium: carrier mother x colour-blind father

Question. A carrier mother (X^B X^b) has children with a colour-blind father (X^b Y). Predict the genotypes and phenotypes of the daughters and the sons.

Model answer. The mother forms X^B and X^b eggs; the father forms X^b and Y sperm. Daughters are 1 carrier (X^B X^b) : 1 colour-blind (X^b X^b), here a daughter can be colour-blind because she can receive X^b from both parents.

Sons are 1 normal (X^B Y) : 1 colour-blind (X^b Y). Overall, half of all the children are colour-blind.

Worked problem 3, SPM-level: deducing a genotype

Question. A woman with normal colour vision, whose father was colour-blind, marries a colour-blind man. (a) Work out the woman's genotype.

(b) Draw the cross and state the proportion of their children, and of which sex, expected to be colour-blind.

Model answer. (a) The woman has normal vision, so she has at least one X^B, but her father was colour-blind (X^b Y) and passed his single X^b to her; therefore she is a carrier, X^B X^b. (b) The cross is X^B X^b (mother) x X^b Y (father).

As the grid shows, daughters are 1 carrier (X^B X^b) : 1 colour-blind (X^b X^b), and sons are 1 normal (X^B Y) : 1 colour-blind (X^b Y). So half of the children are colour-blind, made up of colour-blind daughters and colour-blind sons in equal numbers.

Practice problems

• 1. A man with normal colour vision (X^B Y) marries a carrier woman (X^B X^b). What fraction of their sons is expected to be colour-blind?
• 2. Can a colour-blind daughter be born to a father who has normal colour vision? Explain your answer.
• 3. A woman is a carrier for haemophilia (X^H X^h) and her husband does not have haemophilia (X^H Y). State the probability that a son has haemophilia.

Practice answers

How it is examined

SPM questions typically use superscript notation on X and Y, ask you to identify carriers, and require a Punnett square to predict the genotypes and phenotypes of children from parents with given X-linked genotypes.

Common mistakes

Source:SRC-DSKP-EN

Frequently asked questions