Explanation:
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.
<h3>a) What type of crosses would be expected to produce female tortoiseshell cats? </h3>
The female tortoiseshell cats have patches of cells that express the dominant <em>O</em> allele and patches of cells that express the recessive <em>o</em> 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
<h3>b. Suggest a hypothesis to explain the origin of male tortoiseshell cats</h3>
Male cats normally have one Y chromosome and <u>one</u> 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.
<h3>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.</h3>
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 <em>SS </em>produces large patches of white fur, the heterozygous <em>Ss</em> genotype causes smaller patches of white, and the homozygous <em>ss </em>genotype determines the lack of white color.
Female Calico cats would have the genotypes XᴼXᵒ SS or XᴼXᵒ Ss.
