In cats, the dominant O allele of the X-Iinked orange gene is required to produce orange fur, the recessive o allele of this gene yields black fur. a. Tortoiseshell cats have coats with patches of orange fur alternating with patterns of black fur. Approximately 90% of all tortoiseshell cats are females. What type of crosses would be expected to produce female tortoiseshell cats? b. Suggest a hypothesis to explain the origin of male tortoiseshell cats. c. Calico cats (most of which are females) have patches of white, orange, and black fur. Suggest a hypothesis for the origin of calico cats.

Female cats have two X chromosomes. In the early stages of mammal development each cell in the embryo will randomly inactivate one of the X chromosomes, and all of its daughter cells will keep that same X chromosome inactive (its genes won't be expressed).

For that reason, the adult individual will have patches of cells that express the genes of only one X chromosome, and patches of cells that express the genes of the other one.

a) What type of crosses would be expected to produce female tortoiseshell cats?

The female tortoiseshell cats have patches of cells that express the dominant O allele and patches of cells that express the recessive o allele. That means that they have the genotype XᴼXᵒ.

These females can come from the crosses of:

XᴼXᴼ x XᵒY: Orange female x black male XᵒXᵒ x XᴼY: black female x orange male XᴼXᵒ x XᵒY: tortoiseshell female x black male XᴼXᵒ x XᴼY: tortoiseshell female x orange male

b. Suggest a hypothesis to explain the origin of male tortoiseshell cats

Male cats normally have one Y chromosome and one X chromosome, and their fur will have the color determined by the allele of the O/o gene in the X chromosome they have. However, in rare cases males can have two X chromosomes and a Y chromosome. If the genotype is XᴼXᵒY, the males will also inactivate one of the X chromosomes randomly in each cell, giving the tortoiseshell phenotype.

c. Calico cats (most of which are females) have patches of white, orange, and black fur. Suggest a hypothesis for the origin of calico cats.

The patches of white are determined by another gene (Piebald Spotting, with the alleles S/s) in an autosomal chromosome, that basically causes unpigmented fur. A homozygous genotype SS produces large patches of white fur, the heterozygous Ss genotype causes smaller patches of white, and the homozygous ss genotype determines the lack of white color.

Female Calico cats would have the genotypes XᴼXᵒ SS or XᴼXᵒ Ss.