In a cross between a male and female, both heterozygous for sickle...

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