Since both the parents are heterozygous for sickle-cell anaemia, i.e., shows sickle-cell trait, they are represented by the symbol HbAHbS. Each parent thus will produce two types of gametes; one having a composition HbA and the other having a composition for sickle cell anaemia as HbS. The progeny cross will be as follows:
Progeny:
| HbA | HbS | |
| HbA | HbAHbA (Normal) | HbAHbS (Sickle-cell trait) |
| HbS | HbAHbS (Sickle-cell trait) | HbSHbS (Diseased) |
Thus, 25 of the progeny will be affected.