Dihybrid Crosses Learning Objective (DOT point) Describe outcomes of dihybrid crosses involving simple dominance using Mendel’s explanations

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

Dihybrid Cross monohybrid = 1 trait with 2 alleles = 1 allele gamete each for 2x2 box dihybrid = 2 traits with 2 alleles EACH = 4 alleles = 2 allele gametes each for 4x4 box [one allele for each trait in each gamete]

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…

FOIL method First, Outer, Inner, Last (ab) (cd) First (ac) Outer (ad) Inner (bc) Last (bd)

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

Dihybrid cross proportions PhenotypesProportions Round Yellow9/16 or56.25% Round Green3/16 or18.75% Wrinkled Yellow3/16 or18.75% Wrinkled Green1/16 or6.25%

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