Dihybrid Cross - A Genetics Definition

Monohybrid & Dihybrid Crosses
Mendel's Laws of Inheritance, pattern of the transmission of characteristics from one pea generation to the next based on genetic heritage. The drawing on the left demonstrates a monohybrid cross and the drawing on the right demonstrates a dihybrid cross. Credit: De Agostini Picture Library/Getty Images

Dihybrid Cross

Definition: A dihybrid cross is a breeding experiment between P generation (parental generation) organisms that differ in two traits. The individuals in this type of cross are homozygous for a specific trait. Traits are characteristics that are determined by segments of DNA called genes. Diploid organisms 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.

In a dihybrid cross, the parent organisms have different pairs of alleles for each trait being studied. One parent possesses homozygous dominant alleles and the other possesses homozygous recessive alleles. The offspring, or F1 generation, produced from the genetic cross of such individuals are all heterozygous for the specific traits. This means that all of the F1 individuals possess a hybrid genotype and express the dominant phenotypes for each trait.

Example: In the image above, the drawing on the left demonstrates a monohybrid cross and the drawing on the right demonstrates a dihybrid cross. The two different phenotypes in the dihybrid cross are seed color and seed shape. One plant is homozygous for the dominant traits of yellow seed color (YY) and round seed shape (RR). The genotype can be expressed as (YYRR). The other plant displays the homozygous recessive traits of green seed color and wrinkled seed shape (yyrr).

When a true-breeding plant with yellow seed color and round seed shape (YYRR) is cross-pollinated with a true-breeding plant with green seed color and wrinkled seed shape (yyrr), the resulting offspring (F1 generation) are all heterozygous for yellow seed color and round seed shape (YyRr).

Self pollination in the F1 generation plants results in offspring (F2 generation) that exhibit a 9:3:3:1 phenotypic ratio in variations of seed color and seed shape.

This ratio can be predicted by using a Punnett square to reveal the possible outcomes of a genetic cross based on probability. In the F2 generation, about 9/16 of the plants have yellow seeds with round shapes, 3/16 (green seed color and round shape), 3/16 (yellow seed color and wrinkled shape ) and 1/16 (green seed color and wrinkled shape). The F2 progeny exhibit four different phenotypes and nine different genotypes. It is the inherited genotype that determines the phenotype of the individual. For example, plants with genotypes (YYRR, YYRr, YyRR, or YyRr) have yellow seeds with round shapes. Plants with genotypes (YYrr or Yyrr) have yellow seeds and wrinkled shapes. Plants with genotypes (yyRR or yyRr) have green seeds and round shapes, while plants with the genotype (yyrr) have green seeds and wrinkled shapes.

Independent Assortment

Dihybrid cross pollination experiments led Gregor Mendel to develop his law of independent assortment. This law states that alleles are transmitted to offspring independently of one another. Alleles separate during meiosis, leaving each gamete with one allele for a single trait. These alleles are randomly united upon fertilization.

Dihybrid Cross vs Monohybrid Cross

As a dyhibrid cross deals with differences in two traits, a monohybrid cross is centered around a difference in one trait.

The parent organisms are both homozygous for the trait being studied, but have different alleles for those traits. One parent is homozygous dominant and the other is homozygous recessive. Like in the dyhbrid cross, the F1 generation produced in a monohybrid cross are all heterozygous and only the dominant phenotype is observed. However, the phenotypic ratio observed in the F2 generation is 3:1. About 3/4 exhibit the dominant phenotype and 1/4 exhibit the recessive phenotype.