2.2 Autosomal Recessive Inheritance

The symptoms of the character only appear when the individual is having homozygous genotype.  Both sexes are affected.  Heterozygous of a single gene is normal and is considered as a carrier.  Most individuals who have the disease arise from mating between two heterozygotes. In such cases there is a 25% chance of normal, 50% chance heterozygote and 25% chance of affected outcome (Fig-1-b). Most of the recessive conditions are quite rare. The frequency of heterozygotes is always much greater e.g. cystic fibrosis, phenylketonurea, sickle cell anemia, b-thalassaemia.