Retinitis pigmentosa, a form of blindness, can be caused by either a recessive a

Question

Retinitis pigmentosa, a form of blindness, can be caused by either a recessive allele of the A/a gene or a dominant allele of the B/b gene. These two genes assort independently. The following genotypes and phenotypes describe the inheritance of retinitis pigmentosa.

           genotype     phenotype

           A_ bb              normal vision

           aa __            retinitis pigmentosa

           __ B_            retinitis pigmentosa

A woman with retinitis pigmentosa and whose parents have normal vision marries a man of genotype AaBb.

Part A

What proportion of their children are expected to have retinitis pigmentosa?

Enter your answer as a reduced fraction.

Thanks in advance! Please explain

Explanation / Answer

The woman's parents had normal vision, means they did not have B. So, the genotype of the mother is aabb.

aa crossed with Aa; Chances of having aa will be 2/4 or 1/2.

bb crossed with Bb; Chances of having B_ will be 1/2 or 2/4. Then chances of having the disease are 2/4*2/4 that is equal to 4 out of 16, i.e one fourth of children may be effected by the disease.

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