Mendel also investigated the inheritance of two pairs of contrasting characters in the pea plant. This is referred to as the dihybrid cross.

Pure breedingXpure breeding round seedswrinkled seeds yellow cotyledonsgreen cotyledons F1 progenyall round yellow peas

The F1 progeny were selfed:- F2 progeny Round roundwrinkled wrinkled Yellowgreenyellowgreen

The ratio between the numbers of different seeds reduces to: 9.8 : 3.38 : 3.15 : 1.0 Or about 9 : 3 : 3 : 1

Mendel also noticed that two new combinations not represented in the parents appeared in the progeny Round or wrinkled seeds can turn up with either green or yellow cotyledons. The two pairs of factors were inherited independently.

Either one of the contrasting characters could be passed on to the offspring Therefore the heterozygote plant (F1 – round and yellow) must produce four types of gamete in equal numbers

F1 phenotype RrYy Produces 4 different gametes RYRyrYry

Use this information to draw the initial cross and the F1 cross on your worksheet. How many different phenotypes are there and what will they be?

Answers CROSS 1 ParentsRound yellow Xwrinkled green genotypeRRYYrryy gametes RYry

RY ryRrYy ryRrYy

F1 phenotype – all round and yellow F1 genotype – all RrYy (heterozygous for both characters)

F1 generation crossed F1 phenotype – round yellow Xround yellow Genotype - RrYyXRrYy Gametes RYRyrYry

RYRyrYry RYRRYYRRYyRrYYRrYy RyRRYyRRyyRrYyRryy rYRrYYRrYyrrYYrrYy ryRrYyRryyrrYyrryy

F2 generation Phenotypes – Round and yellow – 9 Round and green – 3 Wrinkled and yellow - 3 Wrinkled and green - 1

The Dihybrid Test Cross As with monohybrids the appearance of the heterozygous dominant offspring will be identical to that of the homozygous dominant organism. How could they be distinguished?

If the organism in question is heterozygous dominant and is crossed with a homozygous recessive individual then the resulting progeny would show all possible combinations of characters in the ratio 1 : 1 : 1 : 1

This is described as a test cross in which the genotype of the F1 offspring can be determined. Perform a genetic cross to show the possible genotypes resulting from a test cross for a homozygous dominant individual (RRYY) and a heterozygote (RrYy)