Dihybrid cross

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A dihybrid cross is a cross in which two hybrids are mated to test for dominant genes and recessive genes in two separate characteristics, and such a cross has a variety of uses in Mendelian genetics and genetic linkage experiments. The dihybrid genotype is usually created when two different parental individuals that are true breeding (homozygous) for different alleles of two genes are sexually crossed or mated together. Their resulting progeny possesses the "dihybrid" genotype and is heterozygous for alleles of two genes. The dihybrid is also often referred to as the double heterozygote. When two dihybrids with the same genotype are mated together, the mating is referred to as a dihybrid cross.

Alleles in genetic crosses are usually indicated by capital and lower case letters with dominant alleles designated by capitals and recessive alleles designated by lower case letters. Thus, a dihybrid cross is usually shown as following the form Aa Bb X Aa Bb. Here one parent (usually female by convention) precedes the X and another parent (usually male) follows the X.

Meiosis is the cellular process of gamete creation, it is quite literally where sperm and eggs get the unique set of genetic information that will be used in the development and growth of the offspring of the mating. 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. If the first filial generation (F1 generation) produces four 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.

[edit] 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. R is the dominant gene for roundness for shape, with lower-case r to stand for the recessive wrinkled shape. Y stands for the dominant yellow pea, and lower-case y stands for the recessive green color. By using a punnett square (the gametes are RY, Ry, rY, and ry):

	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 result 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, orange representing a wrinkled yellow phenotype, and blue representing a wrinkled green phenotype (both recessive genes).