Monohybrid Cross - A Genetics Definition

Monohybrid Cross
Monohybrid Cross Between True Breeding Green and Yellow Pod Pea Plants. Adapted From Work by Mariana Ruiz/Wikimedia Commons/Public Domain

Definition: A monohybrid cross is a breeding experiment between P generation (parental generation) organisms that differ in a single given trait. The P generation organisms are homozygous for the given trait, however each parent possesses different alleles for that particular trait. A Punnett square may be used to predict the possible genetic outcomes of a monohybrid cross based on probability. This type of genetic analysis can also be performed in a dihybrid cross, a genetic cross between parental generations that differ in two traits.

Traits are characteristics that are determined by discrete segments of DNA called genes. Individuals typically inherit two alleles for each gene. An allele is an alternate version of a gene that is inherited (one from each parent) during sexual reproduction. Male and female gametes, produced by meiosis, have a single allele for each trait. These alleles are randomly united at fertilization.

Example: In the image above, the single trait being observed is pod color. The organisms in this monohybrid cross are true-breeding for pod color. True-breeding organisms have homozygous alleles for specific traits. In this cross, the allele for green pod color (G) is completely dominant over the recessive allele for yellow pod color (g). The genotype for the green pod plant is (GG) and the genotype for the yellow pod plant is (gg). Cross-pollination between the true-breeding homozygous dominant green pod plant and the true-breeding homozygous recessive yellow pod plant results in offspring with phenotypes of green pod color.

All genotypes are (Gg). The offspring or F1 generation are all green because the dominant green pod color obscures the recessive yellow pod color in the heterozygous genotype.

Monohybrid Cross: F2 generation

Should the F1 generation be allowed to self-pollinate, the potential allele combinations will be different in the next generation (F2 generation).

The F2 generation would have genotypes of (GG, Gg, and gg) and a genotypic ratio of 1:2:1. One-fourth of the F2 generation would be homozygous dominant (GG), one-half would be heterozygous (Gg), and one-fourth would be homozygous recessive (gg). The phenotypic ratio would be 3:1, with three-fourths having green pod color (GG and Gg) and one-fourth having yellow pod color (gg).

F2 Generation

What Is A Test Cross?

How can the genotype of an individual expressing a dominant trait be determined to be either heterozygous or homozygous if it is unknown? The answer is by performing a test cross. In this type of cross, an individual of unknown genotype is crossed with an individual that is homozygous recessive for a specific trait. The unknown genotype can be identified by analyzing the resulting phenotypes in the offspring. The predicted ratios observed in the offspring can be determined by using a Punnett square. If the unknown genotype is heterozygous, performing a cross with a homozygous recessive individual would result in a 1:1 ratio of the phenotypes in the offspring.

Test Cross 1

Using pod color from the earlier example, a genetic cross between a plant with recessive yellow pod color (gg) and a plant heterozygous for green pod color (Gg) produces both green and yellow offspring.

Half are yellow (gg) and half are green (Gg). (Test Cross 1)

Test Cross 2

A genetic cross between a plant with recessive yellow pod color (gg) and a plant that is homozygous dominant for green pod color (GG) produces all green offspring with heterozygous genotype (Gg). (Test Cross 2)