Question

In mice, the autosomal locus coding for the β-globin chain of hemoglobin is 1 m.u. from the albino locus. Assume for the moment that the same is true in humans. The disease sickle-cell anemia is the result of homozygosity for a particular mutation in the β-globin gene.a. A son is born to an albino man and a woman with sickle-cell anemia. What kinds of gametes will the son form, and in what proportions? (2 points)b. A daughter is born to a normal man and a woman who has both albinism and sickle-cell anemia. What kinds of gametes will the daughter form, and in what proportions? (2 points)c. If the son in part a grows up and marries the daughter in part b, what is the probability that a child of theirs will be an albino with sickle-cell anemia? (1 point)

Answer 1

Answer:

Explanation:

The disease sickle cell anemia is caused by homozygosity for a mutation:

• SS or Ss genotypes: normal
• ss: anemia

Albinism is a recessive trait:

• AA or Aa: normal
• aa: albinism

The genes S/s and A/a are linked and separated by 1  map unit.

Remember that 1 mu means that 1% of the gametes produced by an individual will be recombinant.

a) Albino man (Sa/Sa) X woman with sickle-cell anemia (sA/sA)

Son: Sa/sA

The gametes he can produce are:

• Parentals: Sa and sA
• Recombinant: SA and sa

Since the frequency of recombination is 1% (or 0.01), each recombinant gamete has a frequency of 0.005 (since there are two possible recombinant gametes).

The parental gametes will appear with a frequency of 0.99, each of them with a frequenct of 0.495.

b) Normal man (SA/SA) X woman with anemia and albinism (sa/sa)

Daughter: SA/sa

The gametes she can produce are:

• Parentals: SA (0.495) and sa (0.495)
• Recombinant: Sa (0.005) and sA (0.005)

c) Son (Sa/sA) X daughter (SA/sa)

In order for them to have an albino child with anemia (sa/sa), the gamete sa from each parent had to be produced and fused into the zygote. Since the production of gametes by each parent is an independent event, the probability of having sa/sa offspring can be calculated as:

Prob. sa gamete son × Prob. sa gamete daughter = 0.005 × 0.495 = 0.0025

The probability that a child of theirs will be an albino with sickle-cell anemia is 0.25%