Monohybrid Monohybrid crosscross

Complete DominanceComplete Dominance

Complete dominanceComplete dominance A kind of A kind of dominancedominance wherein the wherein the dominant genedominant gene

completely masks the effect of the completely masks the effect of the recessive generecessive gene in in heterozygousheterozygous condition. condition.

Dominant genes are usually represented by CAPITAL Dominant genes are usually represented by CAPITAL lettersletters

Recessive genes are usually represented by Recessive genes are usually represented by lowercase letterlowercase letter

Source: http://www.biology-online.org/dictionary/Complete_dominanceSource: http://www.biology-online.org/dictionary/Complete_dominance

Example: Tail length in Example: Tail length in catscats

Long tails in cats are caused by a dominant gene (T)Long tails in cats are caused by a dominant gene (T) Short tails are caused by having two recessive genes (tt)Short tails are caused by having two recessive genes (tt)

Cross two individuals that are heterozygous for the trait Cross two individuals that are heterozygous for the trait

Tt x TtTt x Tt

AllelesAlleles

TT ttTTtt

Tt X Tt

AllelesAlleles TT t t

TT TTTT TtTt

t t TtTt tttt

Tt X Tt

What are we looking for?What are we looking for?

Genotypic ratio – Genotypic ratio – the ratio of the the ratio of the offspring’s offspring’s genotypegenotype

Genotype – the set Genotype – the set of alleles that of alleles that determines the determines the expression of a expression of a traittrait

Phenotypic ratio – Phenotypic ratio – the ratio of the the ratio of the offspring’s offspring’s phenotypephenotype

Phenotype – the Phenotype – the physical physical appearance or appearance or expression of a expression of a traittrait

Genotypic outcomes ofGenotypic outcomes ofTt X TtTt X Tt

3 possible combinations3 possible combinations 1 TT homozygous dominant1 TT homozygous dominant 2 Tt heterozygous 2 Tt heterozygous 1 tt homozygous recessive1 tt homozygous recessive Genotypic ratio = 1:2:1Genotypic ratio = 1:2:1

Phenotype outcomes of Phenotype outcomes of Tt X TtTt X Tt

1 TT long tail1 TT long tail 2 Tt long tail2 Tt long tail 1 tt short tail1 tt short tail Phenotypic ratio = 3:1Phenotypic ratio = 3:1