Complete dominance is a type of genetic dominance where the dominant gene fully masks the expression of the recessive gene in heterozygous individuals. In a monohybrid cross exhibiting complete dominance, such as tail length in cats, the dominant gene for long tails (T) will be represented by capital letters while the recessive gene for short tails (t) uses lowercase. Crossing two heterozygous individuals (Tt x Tt) will result in a genotypic ratio of 1 TT : 2 Tt : 1 tt and a phenotypic ratio of 3 long tails : 1 short tail.

1. Monohybrid cross Complete Dominance

2. Complete dominance A kind of dominance wherein the dominant gene completely masks the effect of the recessive gene in heterozygous condition. Dominant genes are usually represented by CAPITAL letters Recessive genes are usually represented by lowercase letter Source: http://www.biology-online.org/dictionary/Complete_dominance

3. Example: Tail length in cats Long tails in cats are caused by a dominant gene (T) Short tails are caused by having two recessive genes (tt) Cross two individuals that are heterozygous for the trait Tt x Tt

4. Tt X Tt t T t T Alleles

5. Tt X Tt tt Tt t Tt TT T t T Alleles

6. What are we looking for? Genotypic ratio – the ratio of the offspring’s genotype Genotype – the set of alleles that determines the expression of a trait Phenotypic ratio – the ratio of the offspring’s phenotype Phenotype – the physical appearance or expression of a trait

7. Genotypic outcomes of Tt X Tt 3 possible combinations 1 TT homozygous dominant 2 Tt heterozygous 1 tt homozygous recessive Genotypic ratio = 1:2:1

8. Phenotype outcomes of Tt X Tt 1 TT long tail 2 Tt long tail 1 tt short tail Phenotypic ratio = 3:1