the dihybrid cross studying the inheritance of two characters simultaneously © 2007 paul billiet...
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THE DIHYBRID CROSS
Studying the inheritance of two characters simultaneously
© 2007 Paul Billiet ODWS
Mendel’s peas
Character Trait Allele
Seed shape Round
Wrinkled
Cotyledon colour Yellow
Green
© 2007 Paul Billiet ODWS
Combinations
Genotype Phenotype
Round Yellow
Round Yellow
Round Yellow
Round Yellow
Round Green
Round Green
Wrinkled Yellow
Wrinkled Yellow
Wrinkled Green© 2007 Paul Billiet ODWS
Is the inheritance of one character
affected by the inheritance of another?
P Phenotypes x (Pure Bred)
F1 Phenotypes All (Selfed)
F2 Phenotypes Seed colour
Yellow Green TOTAL RATIO
Seed Round 315 108 423
shape Wrinkled 101 32 133
TOTAL 416 140 556
RATIO
© 2007 Paul Billiet ODWS
A dihybrid cross can be treated as two
separate monohybrid crosses
The expected probability of each type of seed can be calculated:
Probability of an F2 seed being round =
Probability of an F2 seed being wrinkled =
Probability of an F2 seed being yellow =
Probability of an F2 seed being green =
© 2007 Paul Billiet ODWS
Therefore
Probability of an F2 seed being round and yellow = ¾ x ¾ = 9/16 = 56.25%
Probability of an F2 seed being round and green =
Probability of an F2 seed being wrinkled and yellow =
Probability of an F2 seed being wrinkled and green =
© 2007 Paul Billiet ODWS
Predicting how many seeds we could expect to
get in a sample We could expect
What Mendel observed
556 x 9/16 round yellow 313 315
556 x 3/16 round green 108
556 x 3/16 wrinkled yellow
101
556 x 1/16 wrinkled green 32
© 2007 Paul Billiet ODWS
THE LAW OF INDEPENDENT ASSORTMENT
It appears that the inheritance of seed shape has no influence over the inheritance of seed colour
The two characters are inherited ___________________
The pairs of alleles that control these two characters assort themselves independently
© 2007 Paul Billiet ODWS
Mendel & Meiosis
The pairs of chromosomes could orientate in different ways at Anaphase 1
© 2007 Paul Billiet ODWS
P Phenotypes
Round Yellow seed
X Wrinkled Green seed
(Pure bred)
Genotypes
RRYY rryy
meiosis meiosis
Gametes RY ry
fertilisation
F
1
Phenotypes
RrYy (Selfed)
Genotypes
Round Yellow
Proportions
100%
Dihybrid cross genetic diagram
© 2007 Paul Billiet ODWS
F
1
Phenotypes
RrYy (Selfed)
Genotypes
Round Yellow
Proportions
100%
meiosis
Gametes Y y
R
r
Dihybrid cross genetic diagram
© 2007 Paul Billiet ODWS
Gametes Y y
R RY Ry
r rY ry
fertilisation
F2 Genotypes RY Ry rY ry
RY
Ry
rY
ry
Dihybrid cross genetic diagram
© 2007 Paul Billiet ODWS
Dihybrid cross proportions
Phenotypes ProportionsRound Yellow 9/16 or 56.25%
Round Green
Wrinkled Yellow
Wrinkled Green
© 2007 Paul Billiet ODWS
Dihybrid test cross
In monohybrid crosses, to know if a dominant trait is homozygous (RR) or heterozygous (Rr) it is necessary to carry out a _____________
This is done with a ___________________ (rr) individual
The same is true for a dihybrid cross where the test cross is made with an individual which is homozygous recessive for ______ characters (rryy)
© 2007 Paul Billiet ODWS
Dihybrid test cross
Phenotypes X
Genotypes
Gametes
Genotypes RY Ry rY ry
ry
Phenotypes
Round Yellow
Round Green
Wrinkled Yellow
Wrinkled Green
Proportions
© 2007 Paul Billiet ODWS
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