Question

A pedigree is shown below for a disease that is autosomal dominant. What would be the genetic makeup of the first generation?

(a) AA, Aa
(b) Aa, aa
(c) Aa, AA
(d) Aa, Aa

Answer 1

The term “autosomal” means that the gene for that particular disease in question is on the non-sex chromosomes. The other term “dominant” means that only a single copy of the disease-causing allele is enough to cause that disease in the individuals.

Complete answer:
In the given pedigree, the genetic makeup of the first generation should be ‘Aa’ for the female and ‘aa’ for the male because as the disease is autosomal dominant, the male parent who is normal must have the genetic makeup of a homozygous recessive genotype i.e. aa and there is only one option in which homozygous recessive condition has been given.
-Since the disease is autosomal dominant, it will affect the genotypes in which the dominant allele will be present i.e. AA (homozygous dominant) and Aa (heterozygous).
-If we analyze the progeny in the third generation, in the left group only one is affected out of the four. This means the affected female in the second generation must be heterozygous i.e. Aa. Because if she will have genotype as homozygous dominant i.e. AA, all of the progenies in the third generation will be affected.

-In the same way, the affected male in the second generation will also have the same genotype as heterozygous i.e. Aa because only $50\%$ of the progenies are affected in the second group.

So, the correct answer is ‘Aa, aa’.

Note:
For an autosomal dominant disease, two types of conditions will be for affected individuals i.e they will either be homozygous dominant or heterozygous. The homozygous recessives will be unaffected or normal.