a. The pedigree shows autosomal recessive disease. The two key points of autosomal recessive inheritance is that,
• The affected individuals inherited the recessive allele from the unaffected parents
• Both males and females are affected by the disease.
Thus, in this pedigree, the mother, and father are unaffected. But one son and one daughter is having the disease.
b. Parent 1 to be heterozygous, for the disease, the probability is 1/2. Because, his paternal grandmother was homozygous, his father must be heterozygous.
Parent 2 is heterozygous, for this disease because, her father is homozygous, for that allele and the probability is 1. So, both parent 1 and parent 2 must be heterozygous, for their first child to be homozygous, for this alles. And the probability is 1/4.
So, altogether the probability of the parent 1 and 2 having a first child with disease = Probability of mother being heterozygous x probability of father being heterozygous x the child being homozygous for that allele and having the disease
= 1 x1/2 x 1/4
= 1/8.