Freckles are small, spots of a skin pigment called melanin. Most fair-skinned, red-haired people have them. Freckles are control
led primarily by the MC1R gene. Freckles are a dominant trait: parents who have freckles tend to have children with freckles. Bill and Barb both have freckles. Yet of their four children, one child does not have freckles! What must be the genotypes of Bill and Barb?
A) BB and BB
B) BB and Bb
C) Bb and Bb
D) Bb and bb
A) BB and BB
B) BB and Bb
C) Bb and Bb
D) Bb and bb
2 answers:
Answer:
The correct answer would be C) Bb and Bb.
The phenotype of both parents is a dominant trait. Thus, both of them must contain at least one copy of the dominant allele.
Therefore, the genotype of both parents would be B_.
Case I: When both parents are homozygous dominant, that is, BB. All the offspring would be BB, that is, all have freckles.
Case II: When one parent is homozygous dominant and other is heterozygous, that is. BB and Bb. All the offspring would be either BB or Bb. Thus, all the offspring would have freckles.
Case III: When both the parents are heterozygous dominant, that is, both are Bb. The offspring can have BB, Bb, or bb. One out of four children would have no freckles.
Thus, the correct answer would be C) Bb and Bb.
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