Quick questions on Sex-linked inheritance explained: HSC Biology Module 5

Carrier mother ($X^H X^h$) × unaffected father ($X^H Y$).

Affected father ($X^h Y$) × unaffected non-carrier mother ($X^H X^H$).

Always write the allele as a superscript on the X. Writing just "H" or "h" without the X is a 1-mark deduction because it hides the sex-linkage.

"50% of sons are affected" is different from "25% of all children are affected sons." Read the question.

Duchenne muscular dystrophy is X-linked recessive. A non-carrier woman marries an affected man. Predict the genotypes and phenotypes of all children, distinguishing daughters from sons.

In a population of 10 000 Australian-born males, approximately 800 are red-green colour blind. Calculate the expected frequency of affected females and explain why it is much lower. [3 marks]

A pedigree shows an unaffected man and a phenotypically unaffected woman with two affected sons and one unaffected daughter. (a) Deduce the most likely inheritance pattern. (b) State the mother's genotype using $X^A$/$X^a$ notation.