4. A woman with defective tooth enamel (autosomal recessive) and normal color vision, (X-linked) who had a color blind father with normal tooth enamel and a mother with normal vision and defective tooth enamel, marries a color blind first cousin with normal tooth enamel who had a father with defective tooth enamel. What is the probability of their having a child with normal tooth enamel and color blindness

The probability of their having a child with normal tooth enamel and color blindness is 3/8 or 0.375

Explanation:

Let the alleles for normal and defective tooth enamel be T and t respectively.

Color blindness is on the X-Chromosome. Let the allele normal vision and colour blindness be Xn and Xc respectively.

The woman had a colour blind father (XcY) and a normal vision mother, so her allelic pair for vision will be XnXc.

Her father had normal tooth enamel while her mother had a defective tooth enamel, therefore, she will have the allelic pair, Tt.

Her genotype for vision and tooth enamel will be XnXcTt.

Her husband who is color blind with normal tooth enamel had a father with defective tooth enamel, tt. Therefore, her husband's genotype for vision and tooth enamel will be XcYTt.

From the dihybrid cross of XnXcTt and XcYTt, the following offspring are obtained:

XnXcTT, XnXcTt, XnYTT, XnYTt

XnXcTt, XnXctt, XnYTt, XnYtt

XcXcTT, XcXcTt, XcYTT, XcYTt

XcXcTt, XcXctt, XcYTt, XcYtt

From the results of the cross, a child with normal tooth enamel and color blindness will have the following genotypes: XcXcTT, XcXcTt, XcYTT, XcYTt, XcXcTt, XcYTt.

Probability of their having a child with normal tooth enamel and color blindness = 6/16 = 3/8 or 0.375

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