Dihybrid cross


Dihybrid cross is a cross between two different lines/genes that differ in two observed traits. According to 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.
In the name "Dihybrid cross", the "di" indicates that there are two traits involved, the "hybrid" means that each trait has two different alleles, and "cross" means that there are two individuals who are combining or "crossing" their genetic information.
The Dihybrid cross is easy to visualize using a Punnett square of dimensions 16:
RYRyrYry
RYRRYYRRYyRrYYRrYy
RyRRYyRRyyRrYyRryy
rYRrYYRrYyrrYYrrYy
ryRrYyRryyrrYyrryy

The rules of meiosis, as they apply to the dihybrid, are codified in Mendel's first law and Mendel's 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 produces four identical offspring, the second filial generation, which occurs by crossing the members of the first filial generation, shows a phenotypic ratio of 9:3:3:1, where:
RRYY + 2 x RRYy + 2 x RrYY + 4 x RrYy
RRyy + 2 x Rryy
rrYY + 2 x rrYy
rryy
The Dihybrid ratio=9:3:3:1
The genotypic ratios are: RRYY 1: RRYy 2: RRyy 1: RrYY 2: RrYy 4: Rryy 2: rrYY 1: rrYy 2: rryy 1.