If both husband and wife are carriers, then the genotypes PP and Pp would be phenotypically normal (3 out of 4) and the genotype pp would express the phenotype for the disease Phenyllketouria (1 out of 4).
Hence probability of a normal child=3/4 and probability of a child who is affected=1/4.
a. P(All will have a normal, unaffected phenotype)=(3/4)3=0.4219 (since whether the child is affected or not does not depend on other child)
b. P(4 children, 3 will be unaffected and 1 will be affected)
c. P(All three will be affected)=(1/4)3=0.0156
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