Transcript Dihybrid Inheritance

Dihybrid Inheritance
Dihybrid inheritance refers to
the simultaneous inheritance
of two characters
Mendel’s experiment
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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
Ratio
315
108
101
32
9
3
3
1
round and yellow seeds
round and green seeds
wrinkled and yellow seeds
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
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Parents
Phenotype
Genotype
Gametes
F1 offspring
round
x
wrinkled
yellow seeds
green seeds
RR YY
rryy
RY
RY
gametes
gametes RY
RY
ry
RrYy
RrYy
ry
RrYy
RrYy
ry
ry
F1 intercross
Parents
phenotype
genotype
Gametes
round, yellow seeds x
Rr Yy
RY
Ry
rY
ry
round, yellow seeds
Rr Yy
RY Ry
rY
ry
What are the genotypes and phenotypes of the F2
offspring and in what ratio?
F2 offspring
gametes
RY
Ry
rY
ry
RY
RRYY
RRYy
RrYY
RrYy
genotypes
RRYY (1) RrYy (4)
Ry
RRYy
RRyy
RrYy
Rryy
gametes
rY
RrYY
RrYy
rrYY
rrYy
phenotypes
ry
RrYy
Rryy
rrYy
rryy
expected observed
yellow, round
RRYy (2) RrYY (2) yellow, round
9
315
RRyy (1) Rryy (2)
3
108
wrinkled, yellow
3
101
wrinkled, green
1
32
rrYY (1)
rryy (1)
rrYy (2)
round, green
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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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