2.1.9 - Punnet Squares

Punnett square

A grid used to predict the genotypes and phenotypes of offspring from a cross and to calculate the probabilities of each outcome for a single-gene trait.

Monohybrid cross

A cross between two individuals that differ in one trait, involving a single gene with two alleles.

Allele

A variant form of a gene; each parent contributes one allele to the offspring in a cross.

Genotype

The genetic makeup of an individual, shown as allelic combinations such as DD, Dd, or dd.

Phenotype

The observable trait expressed by an organism, such as dimples or no dimples, resulting from the genotype.

Dominant allele

An allele that is expressed in the phenotype when present in one or two copies; typically represented by a capital letter.

Recessive allele

An allele whose effect is masked by a dominant allele in heterozygotes; represented by a lowercase letter.

Heterozygous

Having two different alleles for a gene (e.g., Dd).

Homozygous

Having two identical alleles for a gene (e.g., DD or dd).

True-breeding

Individuals that, when crossed, produce offspring with the same genotype and phenotype for the trait; usually homozygous.

Hybrid breeding

Crossing true-breeding individuals with different traits to produce hybrids that are phenotypically different from the parents, with the dominant trait expressed.

Test cross

A cross between an individual with the dominant phenotype of unknown genotype and a recessive homozygote to reveal the unknown genotype.

Genotype ratio

The proportion of different genotypes among offspring from a cross (e.g., 2 Dd : 2 dd or 1:2:1 depending on the cross).

Phenotype ratio

The proportion of phenotypes among offspring (e.g., 2 dimples : 2 no dimples).

Dominant trait

A trait expressed in the phenotype when at least one dominant allele is present.

Recessive trait

A trait expressed only when both alleles are recessive (homozygous recessive).