Monohybrid inheritance is the inheritance of a single gene that has two alleles. In genetic diagrams the dominant allele is shown with a capital letter, such as T, and the recessive allele with the matching small letter, t. An organism with two copies of the same allele (TT or tt) is homozygous. One with two different alleles (Tt) is heterozygous.
To work out a cross, first write the genotype of each parent. Each gamete contains only one allele for the gene. For a cross between two heterozygous parents, Tt and Tt, half the gametes carry T and half carry t. A Punnett square shows every possible combination of gametes from the two parents. It gives the possible genotypes of the offspring: TT, Tt, Tt and tt. That is 1 TT : 2 Tt : 1 tt. Three of the four offspring show the dominant phenotype and one shows the recessive phenotype.
A family pedigree shows how a characteristic is passed through generations. Squares stand for males and circles stand for females, and the people who show the characteristic are shaded. If two unaffected parents have an affected child, the characteristic must be caused by a recessive allele, and both parents must be heterozygous carriers.
