In some breeds of domestic cats, the gene controlling fur colour lies on the X chromosome. This gene has two alleles. One allele codes for black fur and the other codes for ginger fur. The two alleles are codominant, so a heterozygous cat has fur with patches of black and ginger. A cat with fur of two colours is called tortoiseshell. Using suitable symbols, construct a genetic diagram to show the outcome of a cross between a female tortoiseshell cat and a male ginger cat. Include symbols, parent phenotypes, parent genotypes, gametes, offspring genotypes and offspring phenotypes.
In humans, the $TYR$ gene takes part in the production of the dark pigment melanin in some cells. Describe how expression of the $TYR$ gene leads to melanin being produced.