Sex-linked Characteristics Flashcards

All 6 cards in this deck

What is a sex-linked characteristic?

A characteristic controlled by a gene carried on a sex chromosome, usually the X chromosome.

Why are males more likely than females to be affected by an X-linked recessive disorder?

Males have only one X chromosome, so a single recessive allele is expressed; females would need the recessive allele on both X chromosomes.

Name two examples of sex-linked recessive genetic disorders in humans.

Haemophilia and Duchenne muscular dystrophy (also red–green colour blindness).

How are the genotypes of a female carrier and an affected male for haemophilia written?

Carrier female $X^HX^h$; affected male $X^hY$.

What are the steps to find the chance of an affected child from a carrier mother and an unaffected father for an X-linked recessive disorder?

e.g. $X^HX^h \times X^HY$

1. Write the gametes: mother $X^H$ and $X^h$; father $X^H$ and Y.
2. Fill the Punnett square: $X^HX^H$, $X^HX^h$, $X^HY$, $X^hY$.
3. Count affected offspring: 1 in 4 (25%) — an affected son.

True or false? A male can be an unaffected carrier of an X-linked recessive disorder.

False. He has only one X chromosome, so if he has the recessive allele he is affected.