In cats, there is also a gene for deafness located on the X chromosome. The gene is recessive. A homozygous black female cat, heterozygous at the deafness locus, is crossed with an orange, hearing, male cat.

What are the offspring phenotypes and in what proportion?

Question

In cats, there is also a gene for deafness located on the X chromosome. The gene is recessive. A homozygous black female cat, heterozygous at the deafness locus, is crossed with an orange, hearing, male cat.

What are the offspring phenotypes and in what proportion?

BlackTom AI · Accepted Answer

To tackle this, first restate the genetic setup and what each phenotype implies: the deafness gene is on the X chromosome and is recessive. The mother is a homozygous black female (so both of her X chromosomes carry the black-coated allele) and is heterozygous at the deafness locus (one X carries the deaf allele, the other does not). The father is orange and hearing, so his X carries the non-deaf allele and the orange coat color locus is on the X as well. The cross will produce daughters who inherit one X from the mother and one X from the father, and sons who inherit one X from the mother and a Y from the father. Coat color (black vs orange) is determined by the X-linked coat color locus and due to X-inactivation in females, a female with one black X and one orange X will appear calico.

Option 1: '25% hearing, black female; 25% deaf, black female; 25% hearing, black male; 25% deaf, black male'
- This option incorrectly assigns females to be black and deaf or black and deaf in equal 25% portions. Since the mother is homozygous black for coat color, daughters will be calico (one X from mother is black, one from father is orange) and all calico females will be hearing because the deaf allele is recessive and the father’s X provides a non-deaf allele. Additionally, there are no calico deaf females produced in this cross. This option misrepresents both coat color in daughters and the inheritance of the deaf allele.

Option 2: '25% hearing, calico female; 25% deaf, calico female; 25% hearing, black male; 25% deaf, black male'
- This option posits calico females that can be deaf or hearing. In this cross, daughters receive one X from the mother (which can be either X^B_d or X^B_D) and one X^O from the father. Because the father’s X is non-deaf, a daughter will be hearing regardless of whether she inherits the deaf allele on the mother’s X, since she would be D from father and d or D from mother, resulting in at least one normal allele. So there cannot be deaf calico daughters. The separation of deafness here is incorrect for females, and the coat color logic (calico vs black) is misrepresented for the female offspring.

Option 3: '50% deaf, calico female; 25% hearing, black male; 25% deaf, black male'
- This option mislabels all daughters as deaf calico females. As explained, daughters are calico but not deaf, because the paternal X provides the normal deaf allele and the recessive deafness shows only in females who are homozygous for the deaf allele, which cannot occur here since the father’s X is non-deaf. It also lists only three categories and misallocates proportions, ignoring that 50% of offspring are daughters (calico, hearing) and 50% are sons (black male, with either hearing or deaf depending on the maternal X).

Option 4: '50% hearing, calico female

25% hearing, black male

25% deaf, black male'
- This option matches the genetic reasoning: half of the offspring are daughters who inherit one black X from the mother and one orange X from the father, resulting in calico females. Because the deaf allele is recessive and the father contributes a non-deaf X, all daughters will be hearing. The other half are sons, who inherit the mother’s X and a Y from the father. If a son inherits the maternal X carrying the deaf allele, he will be deaf and black; if he inherits the maternal X carrying the non-deaf allele, he will be hearing and black. This yields 25% hearing black male and 25% deaf black male, with the remaining 50% being calico hearing females. The reasoning aligns with X-linked recessive inheritance and the provided cross.

In summary, the correct interpretation is that the offspring consist of 50% hearing calico females, 25% hearing black males, and 25% deaf black males. The other options either incorrectly assign deafness to females, misstate coat-color outcomes, or miscalculate proportions.

In cats, there is also a gene for deafness located on the X chromosome. The gene is recessive. A homozygous black female cat, heterozygous at the deafness locus, is crossed with an orange, hearing, male cat. What are the offspring phenotypes and in what proportion?单项选择题

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