Question

In cats with tortoiseshell coat color, the fur is white with patchy black and orange spots. In cats, the tortoiseshell gene is on the X chromosome, and the tortoiseshell phenotype requires the presence of 2 different alleles at this locus: one for orange fur (o) and one for black fur (b). Which phenomenon best explains why female cats can inherit tortoiseshell coat color?
(A. In females, the alleles for coat color are pleiotropic, resulting in patches of orange and black coat color.

  
B In females, these coat color alleles show incomplete dominance, but in males, the coat color alleles do not.

       
C Females typically express both the orange and the black allele in each cell (alleles are co-dominant), making orange and black coat color possible.

       
D Females are mosaic with respect to their X chromosomes, because in each cell, one of the X chromosomes is randomly inactivated.)

If a black female cat mates with an orange male cat, what proportion of their female offspring are expected to be tortoiseshell cats?

Answer 1

Question: Which phenomenon best explains why female cats can inherit tortoiseshell coat color?

Answer- D. Females are mosaic with respect to their X chromosomes, because in each cell, one of the X chromosomes is randomly inactivated.

This is the correct option because female cat carry double X linked gene which is linked to coat color  resulting in expressing double dose of X-linked product which is toxic in comparision to male cate who has only one copy of X linked gene. In order to maintain  the balance , the female cat follow the dosage compensation by randomly inactivating anyone X chromosome.

In heterozygous condition, ( Suppose B code for orange color and b code for black color then heterozygous condition will be X^BX^b in famale cat)

In a patch of skin where the X chromosome carrying the B allele is inactivated , that will express only the b allele and will have black fur.In a patch of skin where the X chromosome carrying the b allele is inactivated, that will express only the B allele and will have orange fur. This is masaic expression of X-linked gene in female cat.

Question:If a black female cat mates with an orange male cat, what proportion of their female offspring are expected to be tortoiseshell cats?

Answer: Suppose B code for orange color and b code for black color,

genotype of black female cat: X^bX^b

genotype of orange male cat : X^BY

X^bX^b X X^BY

genotype of female offspring : X^bX^B

same proportion of their female offspring are expected to be tortoiseshell cats because all female offspring genotype will be heterozygous so it will express mosaic patterns of X linked gene for coat color.