Punnett Squares: Dominant & Recessive Traits

Gregor Mendel is often called the “Father of Genetics”.

He studied the heredity of pea plants & was the first to identify dominant & recessive traits.

Our focus will be on the following 3 traits: Height, Color, and Pea Texture.

Our focus will be on the following 3 traits: Height Color Pea Texture

A CAPITAL letter is used to represent a dominant allele while a lower case letter represents a recessive allele.

TraitDominant trait Recessive trait Height Color Pea texture Tall (T) Short (t) Green (G) Yellow (g) Smooth (R) Wrinkled(r)

The physical appearance or trait that is visible, like freckles or dimples, is known as the phenotype.

The actual gene combination or alleles that determine a trait is known as the genotype.

Offspring inherit one allele of a genotype from each parent.

If an organism carries two dominant alleles (TT) or two recessive alleles (tt) for a trait they are said to be purebred or homozygous.

“homo” = same

An organism that carries both a dominant allele & a recessive allele (Tt) for a trait is called a hybrid or heterozygous.

“hetero” = different

In a heterozygous mix, the dominant trait will mask the recessive trait.

Some traits are determined by several genes & are therefore hard to predict in offspring, for example, height, hair color & eye color in humans.

AllelesGenotypePhenotype GG Gg gg Homozygous dominant green Heterozygousgreen Homozygous recessive yellow

A table called a Punnett square can be used to predict the genotypes & phenotypes offspring will have based on the genotypes or traits of the parents.

**Unless noted otherwise, use the following rules:

Mom’s genes will be first in a cross (ladies first) & should go on the left of a Punnett square. Dad’s genes will be last in a cross & should go on the top of a Punnett square. Capital letter is written BEFORE the lowercase letter in a heterozygous genotype.

G = green g = yellow GG X Gg MomDad

Mom Dad G G G g GGG g G G G g

GG X gg Mom Dad G = green g = yellow

Mom Dad G G g g GgG g G g G g

Genotypes (%) GG: 0% Gg: 100% gg: 0% Phenotype ratio: 4:0 G = green g = yellow

Gg X Gg Mom Dad G = green g = yellow

Mom Dad G g G g GGG g G g g g

Genotypes (%) GG: 25% Gg: 50% gg: 25% Phenotype ratio: 3:1 G = green g = yellow

Mom Dad R R r r RrR r R r R r

Genotypes (%) Homozygous dominant: 0% Homozygous recessive: 0% Heterozygous: 100% Phenotype ratio: 4:0 R = Smooth r = Wrinkled

Mom Dad R r R r RRR r R r r r

Genotypes (%) Homozygous dominant: 25% Homozygous recessive: 25% Heterozygous: 50% Phenotype ratio: 3:1 R = Smooth r = Wrinkled

Mom Dad R r r r RrR r r r r r

Genotypes (%) Homozygous dominant: 0% Homozygous recessive: 50% Heterozygous: 50% Phenotype ratio: 2:2 R = Smooth r = Wrinkled

Mom Dad R R R R RRR R R R R R

Genotypes (%) Homozygous dominant:100% Homozygous recessive: 0% Heterozygous: 0% Phenotype ratio: 4:0 R = Smooth r = Wrinkled