Dihybrid Cross in Genetics

Monohybrid and Dihybrid Crosses

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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 or they share one 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 alternative version of gene expression inherited (one from each parent) during sexual reproduction.

In a dihybrid cross, 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 being studied. This means that all of the F1 individuals possess a hybrid genotype and express the dominant phenotypes for each trait.

Dihybrid Cross Example

Look at the above illustration. The drawing on the left shows a monohybrid cross and the drawing on the right shows a dihybrid cross. The two different phenotypes being tested in this 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)—this genotype can be expressed as (YYRR)—and the other plant displays homozygous recessive traits of green seed color and wrinkled seed shape (yyrr).

F1 Generation

When a true-breeding plant (organism with identical alleles) that is yellow and round (YYRR) is cross-pollinated with a true-breeding plant with green and wrinkled seeds (yyrr), as in the example above, the resulting F1 generation will all be heterozygous for yellow seed color and round seed shape (YyRr). The single round, yellow seed in the illustration represents this F1 generation.

F2 Generation

Self-pollination of these F1 generation plants results in offspring, an F2 generation, that exhibit a 9:3:3:1 phenotypic ratio in variations of seed color and seed shape. See this represented in the diagram. This ratio can be predicted using a Punnett square to reveal possible outcomes of a genetic cross.

In the resulting F2 generation: About 9/16 of F2 plants will have round, yellow seeds; 3/16 will have round, green seeds; 3/16 will have wrinkled, yellow seeds; and 1/16 will have wrinkled, green seeds. The F2 progeny exhibit four different phenotypes and nine different genotypes.

Genotypes and Phenotypes

Inherited genotypes determine the phenotype of an individual. Therefore, a plant exhibits a specific phenotype based on whether its alleles are dominant or recessive.

One dominant allele leads to a dominant phenotype being expressed, but two recessive genes lead to a recessive phenotype being expressed. The only way for a recessive phenotype to appear is for a genotype to possess two recessive alleles or be homozygous recessive. Both homozygous dominant and heterozygous dominant genotypes (one dominant and one recessive allele) are expressed as dominant.

In this example, yellow (Y) and round (R) are dominant alleles and green (y) and wrinkled (r) are recessive. The possible phenotypes of this example and all possible genotypes that may produce them are:

Yellow and round: YYRR, YYRr, YyRR, and YyRr

Yellow and wrinkled: YYrr and Yyrr

Green and round: yyRR and yyRr

Green and wrinkled: yyrr

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

A dihybrid cross deals with differences in two traits, while a monohybrid cross is centered around a difference in one trait. Parent organisms involved in a monohybrid cross have homozygous genotypes for the trait being studied but have different alleles for those traits that result in different phenotypes. In other words, one parent is homozygous dominant and the other is homozygous recessive.

As in a dihybrid cross, the F1 generation plants produced from a monohybrid cross are heterozygous and only the dominant phenotype is observed. The phenotypic ratio of the resulting F2 generation is 3:1. About 3/4 exhibit the dominant phenotype and 1/4 exhibit the recessive phenotype.