Dihybrid Inheritance

Dihybrid inheritance refers to the simultaneous inheritance of two characters

Mendel’s experiment

Mendel investigated the inheritance of seed shape (round v wrinkled) and seed colour (green v yellow) at the same time.

From his monohybrid crosses he knew that round seeds were dominant to wrinkled seeds and yellow seeds were dominant to green seeds.

He chose to cross plants that were pure breeding for both dominant features (round and yellow seeds) with plants that were pure breeding for both recessive features (wrinkled and green seeds)

Results Parents plants with round x plants with wrinkled

and yellow seeds and green seeds

F1 offspring 100% plants with round and yellow seeds

F1 intercross plants with round x plants with round and yellow seeds and yellow seeds

F2 offspring Total 556 seeds collected from F2 plants

Ratio315 9 round and yellow seeds108 3 round and green seeds101 3 wrinkled and yellow seeds321 wrinkled and green seeds

However, the ratio of dominant to recessive features is still 3:1

Explanation of Mendel’s results Let the allele for round seeds be R and the

allele for wrinkled seeds be r Let the allele for yellow seeds be Y and the

allele for green seeds be y

Parents Phenotype round x wrinkled yellow seeds green

seeds Genotype RR YY rryy

Gametes

F1 offspring

RY RY ry ry

gametes RY RYry RrYy RrYyry RrYy RrYy

gametes

F1 intercross

Parents

phenotype round, yellow seeds x round, yellow seeds

genotype Rr Yy Rr Yy

GametesRY Ry rY ry ryrYRyRY

What are the genotypes and phenotypes of the F2 offspring and in what ratio?

gametes RY Ry rY ryRY RRYY RRYy RrYY RrYyRy RRYy RRyy RrYy RryyrY RrYY RrYy rrYY rrYyry RrYy Rryy rrYy rryy

gametes

F2 offspring

genotypes phenotypes expected observed

RRYY (1) RrYy (4) yellow, round

RRYy (2) RrYY (2) yellow, round 9 315

RRyy (1) Rryy (2) round, green 3 108

rrYY (1) rrYy (2) wrinkled, yellow 3 101

rryy (1) wrinkled, green 1 32

Allowing for statistical error, Mendel’s results were a reasonable approximation to the expected 9:3:3:1 ratio

Each of a member of an allelic pair may combine randomly with either of another pair

A test cross can be performed to check the possible genotypes of a plant producing round, yellow seeds (RRYY, RrYY, RrYy, RRYy) by crossing the plants with a homozygous recessive plant with wrinkled green seeds genotype rryy

What phenotypes and genotypes would the offspring possess from the 4 crosses, and in what ratios?

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