Question

Two parents, John and Laura, had dominant genes for smartness, but a recessive genes for dumbness. Their great grand children were all dumb. what were the genotypes John and Laura's children?

Answer 1

Two alleles of one gene: D = smart and d = dumb.

John and Laura are both heterozygous for the allele, Dd. Their children could be DD, Dd, or dd. The problem says that all the children's children are all dumb, so we know they are recessive homozygotes, dd.

This could only happen if John and Laura's children were homozygous recessive (dd) and started families with other people who were recessive homozygotes for intelligence.