Question

Part A - Identifying X-linked inheritance in pedigrees

Answer 1

Pedigree from other source attached:

Answer:

Condition A inheritance mode: Autosomal recessive

Condition B inheritance mode: X-linked recessive

Explanation:

1. Condition A appears in generation 2 when neither parent in generation I is affected. This means at least one of them must carry the allele for the trait. However, because neither parent displays the trait, it must be a recessive allele. Thus, individual 6 must have inherited two copies of the allele, meaning both parents are carriers.

Because we know both parents are carriers but do not display the trait, we can conclude that the trait is not X-linked, because even if it was recessive, the dad (generation I, individual 3) would display the trait (because he only has only 1 X chromosome).

2. Condition B is more common in males, suggesting it could be X-linked. This is shown to be the case if you look at generation 3. Individual 1 of generation II (father) is unaffected, as is individual 2 of generation II (mother), although her father is. If the trait is X-linked, then she must carry the allele. Therefore, all her sons would carry it and display the trait, this is true (generation II), meaning it is likely X-linked,

To determine whether it is dominant or recessive, we need to look at the females (who have 2 copies of the X chromosome and hence would not display a recessive trait). The only affected female is Generation IV, individual 2. She is also the only individual who might have 2 copies (parents III.2 and III.3). Other females that must have a copy (II.2) and (II.4) are unaffected, meaning it is X-linked recessive.