In genetics, a dihybrid cross is a cross between two individuals identically heterozygous at two loci, for example, AaBb x AaBb. A dihybrid cross is often used to test for dominant and recessive alleles in two separate characteristics. Such a cross has a variety of uses in Mendelian genetics; it can help determine if genes are physically linked and whether alleles of different genes interact genetically (epistasis).
Meiosis is the cellular process of gamete creation and is how sperm and eggs get the unique set of genetic information that will be used in the development and growth of the offspring. The rules of meiosis as they apply to the dihybrid are codified in Mendel's First Law and Mendel's Second Law also called the Law of Segregation and the Law of Independent Assortment. For genes on separate chromosomes each allele pair shows independent segregation.
F1 and F2 generation
To set up a dihydrid cross it is common to cross two inbred parents that each carry one unique trait. A cross between two such parents (AAbb x aaBB) will give the first filial generation (F1 generation), all being heterozygous for the two genes (AaBb). A cross between the F1 siblings gives the second filial generation (F2 generation) and in this case the progeny often shows a phenotypic (appearance) ratio of 9:3:3:1.
The variation in the F2 progeny phenotypes is due to the segregation of the alleles into the different gametes (A can segregate from a and B can segregate from b) as well as the independent assortment of the two genes (The A allele of the first gene can pair with either the B or b allele of the second gene). Consequently, in the F1 generation, four different genotypes of gamete, either a sperm or an egg, can be produced by the process of meiosis (AB, Ab, aB or ab).
When the F1 generation are crossed together the four different gamete can combine in 16 different combinations when considering the source of both sperm and egg. These combinations are the seed that go on to produce the F2 generation. This generation can have every combination of alleles (9 different genotypes) and it is this variation that leads to the four different phenotypes in the Mendelian ration of 9:3:3:1.
Punnett square for a Dihybrid Cross
In the pea plant, two characteristics for the peas, shape and color, will be used to demonstrate an example of a dihybrid cross in a punnett square. W is the dominant gene for roundness for shape, with lower-case w to stand for the recessive wrinkled shape. G stands for the dominant yellow pea, and lower-case g stands for the recessive green color. By using a punnett square (the gametes are WG, Wg, wG, and wg):
The result in this cross is a 9:3:3:1 phenotypic ratio, as shown by the colors, where yellow represents a round yellow (both dominant genes) phenotype, green representing a round green phenotype, red representing a wrinkled yellow phenotype, and blue representing a wrinkled green phenotype (both recessive genes).
The resulting 9:3:3:1 phenotypic ratio is seen if the two traits are independent and show a simple dominant recessive relationship. Exceptions to the 9:3:3:1 ratio of the four phenotypes in the F2 generation are seen if the two genes interact genetically. These epistatic interactions between genetic alleles are common if the two genes are involved in the same biological process.