Also located on the X chromosome of a cat is a gene that codes for deafness. This gene is recessive. A black female cat that is heterozygous for deafness (Dd) is crossed with a orange male cat that is not deaf. Show the cross. What are the phenotypes of the offspring and in what proportion? Hint: place two letters on the X chromosome in your cross. (You will need to use two superscript letters on your X chromosome)

Explanation:

This gene is an X linked recessive trait, thus for it to be expressed in the females, it has to be in the homozygous recessive form (both X chromosomes will carry this trait dd). For males, only one copy which is in the recessive condition is needed for it to be expressed. Thus, a cross between

female           male

XDXd     x      XDY

                     XD             Y

XD              XDXD        XDY

Xd              XDXd         XdY

the offspring phenotype: normal females: 1/2

                                         normal male: 1/4

                                         deaf male: 1/4