Dihybrid Crosses A2 Biology Meiosis, genetics & gene control A Graham Carmel RC Technology College Darlington

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 colour 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…

PhenotypeRound, yellowWrinkled, green Genotype RRYY rryy Gametes all RY all ry F1 all RrYy All of the F1 generation would be heterozygous for both characteristics, meaning that they would all be round and yellow. As with monohybrid crosses, he then crossed two of the F1 generation together… X

PhenotypeRound, 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 ¼ Ry ¼ rY ¼ ry ¼ RY ¼ RRYYRRYyRrYYRrYy Ry ¼ RRYyRRyyRrYyRryy rY ¼ RrYYRrYyrrYYrrYy ry ¼ RrYyRryyrrYyrryy

F ::: Round Yellow Round Green Wrinkled Yellow Wrinkled Green This is the typical ratio expected in a dihybrid cross.