Question

Study the following pedigree chart and answer the questions that
follow:
(a) Is the trait sex linked or autosomal?
(b) Write the genotypes of the parents and the third child in no 1
generation.
(c) Why has the trait appeared in the son of a normal couple
(the second daughter in first generation and her husband)?

Answer 1

Answer:

a) Trait is sex linked (X-linked) recessive (mother heterozygous carrier, Father affected)

b) X⁺X⁻ = Mother,  X⁻Y = Father,  X⁺X⁻ = Third child (unaffected daughter)

c) The trait has appeared because the Mother of affected son is carrier even if the The father of affected son is normal, still due to mother being carrier the case here shows that X⁻ from mother went into the son thus making his genotype X⁻Y which is genotype of male being affected by recessive sex linked disorder.

Explanation:

In (b), the Sex chromosome from mother carries dominant allele and father's recessive allele is also there hence due to dominance the daughter is not affected.

In (a), if it was autosomal then all should have been affected or none should have been affected by becoming carriers.

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