The following question is a sex-linked trait. Review the patterns of inheritance

Question

The following question is a sex-linked trait. Review the patterns of inheritance of sex-linked traits before answering the following questions. Make sure to indicate chromosomes and alleles in your answers when necessary.

Hemophilia is a caused by a recessive mutation found on the X chromosome in humans. The recessive allele (Xh) is associated with abnormal blood clotting (hemophilia), while the dominant allele (XN) causes normal blood clotting.

A man with normal blood clotting and woman who is heterozygous are going to have a child. What is the probability of an offspring who is a carrier of hemophilia, and if so, the sex(es) of the offspring?

A. Chances are 25% of a son who is a carrier

B. There is no chance of an offspring who is a carrier

C. Chances are 50% of either a son or a daughter who is a carrier

D. All offspring, male or female, will be carriers

E. Chances are 1 in 4 of a daughter who is a carrier

2. Two fruit flies, one with red eyes and one with white eyes mate and have an offspring with white eyes. Having red eyes is a dominant trait (R) while white eyes (w) is recessive. Based on this information, what must be the genotypes of the parental white-eyed fruit fly and the parental red-eyed fruit fly?

A. Red-Eyed fruit fly: Rw

White-eyed fruit fly: ww

B. Red-Eyed fruit fly: RR

White-eyed fruit fly: wR

C. Red-Eyed fruit fly: R

White-eyed fruit fly: w

D. Red-Eyed fruit fly: rr

White-eyed fruit fly: wr

E. Red-Eyed fruit fly: RR

White-eyed fruit fly: ww

Explanation / Answer

Answer 1-

Genotype of mother is XN Xh, and that of the father is XN Y.
Now after the cross, the possible genotypes of the offsprings would be-

XN XN
XN Y
XN Xh
Xh Y

From here we can see that, there is only one child out of four, who is the carrier, i.e. XN Xh. And the gender of this child is "Girl".
Boy can't be a carrier, since this disease is X linked trait, so males can either be affected or normal as they have just one copy of X chromosome.

Answer : Option E.

Answer 2-

Now here the genotypes of the parental flies would be -

Red eyed: RR or Rw
White eyed: ww

Since the question says that one of the offspring has white eyes, we can say that red eyed parental fly is Rw.

So the correct answer is Option A.

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