Genes controlling the same trait occur in a variety of forms alleles. One allele may be dominant over the others.
Offspring receive one allele of each pair from each parent.
Only one allele for a trait is present in each sex cell. The genes controlling traits do not blend during inheritance, but are transmitted as discrete units. Some variation in populations is accounted for by alternate expression of alleles. Chromosomes occur in pairs, with a copy of a given gene on each chromosome. Egg and sperm merge to form a fertilized egg with the full chromosome number.