Dihybrid Crosses

• Dihybrid crosses are those where we consider the inheritance of two characteristics at the same time.

Mendel’s Peas (again!)Mendel had observed that the seeds of his pea plants varied in several ways – among them, whether the peas were round or wrinkled and whether they were green or yellow.

He did monohybrid crosses on both characteristics, but then decided to look at how they were inherited together….

• There are four possible combinations of the two characteristics. Peas can be:

• 1) round and yellow

• 2) round and green

• 3) wrinkled and yellow

• 4) wrinkled and green

• From his monohybrid trials, Mendel had discovered that the allele for yellow color was dominant to that for green.

• He had also found that the allele for round peas was dominant to the allele for wrinkled peas.

• Starting with individuals that were homozygous, either for both dominant characteristics or both recessive characteristics, he set off breeding again…

Phenotype Round, yellow Wrinkled, green

Genotype RRYY rryy

Gametes all RY all ry

F1 all RrYy

All of the F1 generation would be heterozygous for both characteristics, round and yellow. As with monohybrid crosses, he then crossed two of the F1 generation together…

X

Phenotype Round, yellow

Genotype RrYy

Gametes RY, Ry, rY, ry

F2We really need a punnet square for this…

X Round, yellow

RrYy

RY, Ry, rY, 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

F2

9 3 3 1: : :Round

Yellow

Round

Green

Wrinkled

Yellow

Wrinkled

Green

This is the phenotypic ratio expected in a dihybrid cross.

• There are four possible combinations of the two characteristics. Peas can be:

• 1) round and yellow

• 2) round and green

• 3) wrinkled and yellow

• 4) wrinkled and green