Single gene inheritance

• Some characteristics are controlled by a single gene. Examples are fur colour in mice and red–green colour blindness in humans.
• Each gene may have different forms called alleles.
• A dominant allele is always expressed, even if only one copy is present. A recessive allele is only expressed if two copies are present (therefore no dominant allele present).
• If the two alleles present are the same the organism is homozygous for that trait, but if the alleles are different they are heterozygous.