And (s) represents the gene for the deformed Haemoglobin.

There are three possible genotypes for Human individuals:

- SS : Normal persons. They are homozygous, and have normal Haemoglobin.

- Ss : Carrier persons. They are heterozygous, and the symptoms of the disease

Appear on them after physical exertion, or if present in places where

Oxygen is scarce.

- ss : Sick persons that die before maturity. They are homozygous recessive.

If a carrier man marries a carrier woman, the following crossing represents F1 resulted:

A carrier man X A carrier woman

P1 Ss Ss

G1

SS Ss Ss ss

F1

Normal Carrier Sick and die

1 : 2 : 1

N.B.: This disease represents a case of incomplete dominance. Since, the heterozygous individuals (the carriers) have a special phenotype when Oxygen is scarce. This is different from Lack of dominance.