Dihybrid cross

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Dihybrid cross is a cross between two individuals who differ in two observed traits that are controlled by two distinct genes If the two parents are homozygous for both genes, then the generation of offspring will be uniformly heterozygous for both genes and will display the dominant phenotype for both traits. Self-pollination or crossing of these individuals will result in predictable ratios of both genotype and phenotype in the generation. Deviations from these expected ratios may indicate that the two traits are linked or that one or both traits has a non-Mendelian mode of inheritance.

Expected genotype and phenotype ratios[]

The phenotypic ratio of the F2 offspring of the cross is 9:3:3:1, where 9/16 of the F2 individuals possess the dominant phenotype for both traits, and 1/16 are recessive for both traits. (Valid only for Angiosperms / similar sexually reproducing organisms)

According to Gregor Johann Mendel's statement, between the alleles of both these loci, there is a relationship of completely dominant - recessive traits. In the example pictured to the right, RRYY/rryy parents result in F1 offspring that are heterozygous for both R and Y (RrYy).[1]

This is not a dihybrid cross. This is a cross between two purebreds, which will produce a F1 generation consisting entirely of dihybrids. Only when you allow the F1 generation offspring to self-pollinate will you perform a dihybrid cross. The resulting F2 generation will have a phenotypic ratio of 9:3:3:1.

In the name "Dihybrid cross", the "di" indicates that there are two traits involved (e.g. R and Y), the "hybrid" means that each trait has two different alleles (e.g. R and r, or Y and y), and "cross" means that there are two individuals (usually a mother and father) who are combining or "crossing" their genetic information.

This figure illustrates the process of dihybrid crosses between pea plants with multiple traits, their different generations, and their phenotypic ratio patterns
  • The Dihybrid cross is easy to visualize using a Punnett square of dimensions 16:
  • The dominant traits are uppercase, and the recessive traits of the same characteristic is lowercase.
  • In the following case the example of Pea plant seed is chosen. The two characteristics being compared are
  1. Shape: round or wrinkled
  2. colour: yellow or green
  • R (roundness) is dominant and Y (Yellow colour of seed) is Dominant. This implies that Rr will be a round seed and Yy will be a yellow seed. Only rr will be wrinkled seed and yy will be green seed.
RY Ry rY ry
RY RRYY RRYy RrYY RrYy
Ry RRYy RRyy RrYy Rryy
rY RrYY RrYy rrYY rrYy
ry RrYy Rryy rrYy rryy

The rules of meiosis, as they apply to the dihybrid, are codified in Mendel's first and second law, which are also called the Law of Segregation and the Law of Independent Assortment, respectively.

For genes on separate chromosomes, each allele pair showed independent segregation. If the first filial generation (F1 generation) produces four identical offspring, the second filial generation, which occurs by crossing the members of the first filial generation, shows a phenotypic (appearance) ratio of 9:3:3:1, where:

  • the 9 represents the proportion of individuals displaying both dominant traits:
    1 x RRYY + 2 x RRYy + 2 x RrYY + 4 x RrYy
  • the first 3 represents the individuals displaying the first dominant trait and the second recessive trait:
    1 x RRyy + 2 x Rryy
  • the second 3 represents those displaying the first recessive trait and second dominant trait:
    1 x rrYY + 2 x rrYy
  • the 1 represents the homozygous, displaying both recessive traits:
    1 x rryy
  • The Dihybrid phenotypic ratio= 9:3:3:1
  • The genotypic ratio are: RRYY 1: RRYy 2: RRyy 1: RrYY 2: RrYy 4: Rryy 2: rrYY 1: rrYy 2: rryy 1.

See also[]

References[]

  1. ^ "The Dihybrid Cross" - Open Door Archived February 7, 2010, at the Wayback Machine

External links[]

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