In a cross between a male and female, both heterozygous for sickle cell anaemia gene, what percentage of the progeny will be diseased?

Since both the parents are heterozygous for sickle-cell anaemia, i.e., shows sickle-cell trait, they are represented by the symbol ${\mathrm{Hb}}^{\mathrm{A}}{\mathrm{Hb}}_{}^{\mathrm{S}}$. Each parent thus will produce two types of gametes; one having a composition ${\mathrm{Hb}}^{\mathrm{A}}$ and the other having a composition for sickle cell anaemia as ${\mathrm{Hb}}^{\mathrm{S}}$. The progeny cross will be as follows:

Progeny:

Phenotypic ratio: Normal: Sickle-cell trait: Diseased: $1: 2: 1$
Thus, $25\%$ of the progeny will be affected.