The diagram shows a pedigree of a sex-linked recessive trait. In generation II,

Question

The diagram shows a pedigree of a sex-linked recessive trait. In generation II, female number 5 marries a man and has three children. One daughter and one son do not have the recessive trait; the other son does have the recessive trait. Based on these results, one can conclude that

a. the mother (generation I) is heterozygous for the trait.

b. the mother (generation I) is homozygous for the trait.

c. a mutation has occurred.

d. the father (generation I) is heterozygous for the trait.

2-A new student of genetics suspects that a particular recessive trait in fruit flies (dumpy wings, which are somewhat smaller and more bell-shaped than the wild type) is sex-linked. A single mating between a fly with dumpy wings (dp; female) and a fly with wild-type wings (Dp; male) produces three dumpy-winged females and two wild-type males in the F1 generation. On the basis of these data, is the trait sex-linked or autosomal? What were the genotypes of the parents? Explain how these conclusions can be reached on the basis of so few data.

Explanation / Answer

1. The answer is a, that the mother was heterozygous for the trait.
b is incorrect, because if the mother had been homozygous for a recessive trait, she would have had the trait herself and therefore her circle would have been shaded in on the chart (shaded-in shapes have the trait)

c could be possible, but the chances of that particular mutation occurring are rare, and a is totally possible, as I'll explain below

d is impossible because a male cannot be heterozygous for a sex-linked trait. Since males only have one X chromosome, they are what is called "hemizygous" because they can only have one allele for a trait

a is the correct answer because if a female is heterozygous, since not having the trait is dominant, she would still appear as a white circle (which she did). However, she would carry the recessive allele and could therefore pass it on to her children.

her genotype would be XAXa, whereas her husband would be XAY. All the girls would receive at least one copy of the dominant allele (from the father), but the boys would only receive alleles from their mother, and could thsu receive either one, which explains why one boy could have the trait while the other would not, as seen in the pedigree.

2. If this trait were sex-linked recessive as the student suggests, a female with dumpy wings would have a genotype of XbXb and her husband would just have the one wild-type dominant X chromosome. If this were the case, all the girls would receive one X from their mother and AT LEAST ONE DOMINANT X from their dad, meaning all the girls would have the dominant trait and NOT have dumpy wings. However, this was not the case reported in the data set. Thus, even though there is not very much data, it is clear that the student's hypothesis was incorrect and it must just be an autosomal trait (meaning that if the father were heterozygous, both sexes could potentially inherit the trait).

I hope this helped!

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