Question

A pedigree chart of a family with an autosomal recessive disease Z is given. Assume that inheritance is Mendelian, and that all individuals with homozygous recessive genotype have the disease. Circles in the pedigree represent females and squares represent males. Filled shapes indicate affected individuals, while unfilled shapes indicate unaffected individuals. If J does not have the recessive allele, the probability that I is a 'carrier of disease Z' is?

• A. $0.67$
• B. $0.5$
• C. $0.25$
• D. $0$

Answer 1

Answer: (A)

$0.67$

From the given pedigree, it is seen that generation II - 3 is having the disease. It indicates that both the parents are heterozygous (Zz) for the given disease. In the second generation, 'I' does not have a disease. It indicates that she is either having homozygous dominant alleles (ZZ) or having heterozygous alleles (Zz). For genotype ZZ, she would have received Z allele from each parents. For genotype Zz, she would have received Z allele from either of the parent and z allele from the other parent. Hence, there are three possibilities of receiving alleles from the parents. Hence, there is 0.67 probability that 'I' is a carrier of disease Z and there is 0.33 probability that 'I' is having normal alleles.

Thus, the correct answer is option A.