Dihybrid Inheritance

1
Dihybrid Inheritance

• Dihybrid inheritance refers to the simultaneous
  inheritance of two characters

2
Mendels 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)

3
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 Ratio 315 9 round and yellow
seeds 108 3 round and green seeds 101 3
wrinkled and yellow seeds 32 1 wrinkled and
green seeds However, the ratio of dominant to
recessive features is still 31
4
Explanation of Mendels 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
5
F1 intercross

• Parents
• phenotype round, yellow seeds x round, yellow
  seeds
• genotype Rr Yy Rr Yy
• Gametes

RY
Ry
rY
ry
ry
rY
Ry
RY
What are the genotypes and phenotypes of the F2
offspring and in what ratio?
6
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
7

• Allowing for statistical error, Mendels results
  were a reasonable approximation to the expected
  9331 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?

8
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