Dihybrid Cross

Mendel brought about a cross between a pea plant producing rounded and yellow-coloured seeds and a pea plant with wrinkled and green-coloured seeds. In this cross, two pairs of contrasting characteristics were considered: colour of seeds and shape of seeds. Hence, it is called a dihybrid cross.

Parental Generation:

Mendel selected the pea plants producing rounded yellow seeds and wrinkled green seeds as parent plants. In seed shape, round (R) is dominant over wrinkled (r); in cotyledon colour, yellow (Y) is dominant over green (y). Hence, the pure breeding round yellow parent is represented by the genotype RRYY and the pure breeding green wrinkled parent is represented by the genotype rryy.

• In RRYY plants, only RY gametes are formed, not combinations like RR or YY. Similarly, in rryy plants, only ry gametes are formed.
• This happens because each pair of genes contributes only one allele to the gamete, following Mendel's principle of segregation.

F₁ Generation:

Crossing RRYY with rryy produces RrYy plants, which are dihybrids. These plants have the dominant traits of round seeds (R) and yellow colour (Y), as round is dominant over wrinkled and yellow is dominant over green.

F₂ Generation:

In the F₂ generation, the dihybrid plants (RrYy) produce seeds in a phenotypic ratio of 9:3:3:1, showing Mendel's principle of independent assortment. Among these, the majority of plants produce rounded yellow seeds, like the parent plants, due to the dominance of the round and yellow traits.

Plants of the F1 generation of the dihybrid cross produce four types of gametes: RY, Ry, rY, and ry. Of these gametes, RY and ry are similar to those of the P1 generation. The F2 generation is formed through the selfing of F1 plants. The 16 different possible combinations through the union of 4 types of male gametes and 4 types of female gametes are shown in a chessboard-like table (Punnett Square / Checkerboard). Male gametes are shown at the top of the table, and female gametes are displayed in the left column.

Dihybrid Cross in Garden peas

Phenotypic ratio: 9:3:3:1

Round yellow- 9

Round green- 3

Wrinkled yellow- 3

Wrinkled green- 1

Genotypic ratio: 1:2:2:4:1:2:1:2:1

RRYY- 1

RRYy-2

RRYY- 1

RRYy-2

RRyy- 1

Rryy-2

rrYY- 1

rrYy- 2

rryy- 1 

Mendel's experiments were carefully planned and involved large samples. He carefully recorded the number of plants of each type and expressed his results in terms of ratios. In the pea plant, contrasting characters can be easily recognised. The seven different characters in the pea plant were controlled by a single factor. He introduced the concepts of dominance and recessiveness. 

How these traits get expressed: 

• Cellular DNA is the information source for making proteins in the cell. 
• A section of DNA that provides information for one protein is called the gene for that protein. 
• Plant height can thus depend on the amount of a particular plant hormone. 
• The amount of the plant hormone made will depend on the efficiency of the process for making it. 
• If this enzyme works efficiently, a lot of hormones will be made, and the plant will be tall. 
• If the gene for that enzyme has an alteration that makes the enzyme less efficient, the amount of hormone will be less, and the plant will be short. Thus, genes control characteristics or traits. 

In one experiment, the tall pea plants were pollinated with the pollen from true-breeding dwarf plants, and the result was all tall plants.

• When the parental types were reversed, the pollen from a tall plant was used to pollinate a dwarf pea plant, which gave only tall plants.
• The result was the same: all tall plants. Tall (♀) × Dwarf (♂) and Tall (♂) × Dwarf (♀) matings are done in both ways, which are called reciprocal crosses.
• The results of the reciprocal crosses are the same. So it was concluded that the trait is not sex dependent.
• The results of Mendel’s monohybrid crosses were not sex dependent. The gene for plant height has two alleles: tall (T) × dwarf (t).
• The phenotypic and genotypic analysis of the crosses has been shown by the chequerboard method or by the forkline method.