Diabetes is a disorder of the pancreas in which insulin cannot be produced or is not effective. As a result, blood sugar cannot be properly controlled.

Diabetics have a homozygous recessive genotype (dd).

A father carries the recessive gene for diabetes and a mother does not (generation 1). They have two children (generation 2), a daughter and a son. The daughter does not suffer from diabetes and marries a man who does not carry the gene for diabetes. They have one child, a girl who is a carrier (generation 3). The son marries a diabetic woman and they have two boys, both who suffer from diabetes (generation 3).