the dihybrid cross studying the inheritance of two characters simultaneously © 2007 paul billiet...

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


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


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 =


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 =


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


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


Mendel & Meiosis

The pairs of chromosomes could orientate in different ways at Anaphase 1


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


F

1

Phenotypes

RrYy (Selfed)

Genotypes

Round Yellow

Proportions

100%

meiosis

Gametes Y y

R

r



Gametes Y y

R RY Ry

r rY ry

fertilisation

F2 Genotypes RY Ry rY ry

RY

Ry

rY

ry



Dihybrid cross proportions

Phenotypes ProportionsRound Yellow 9/16 or 56.25%

Round Green

Wrinkled Yellow

Wrinkled Green


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)


Dihybrid test cross

Phenotypes X

Genotypes

Gametes

Genotypes RY Ry rY ry

ry

Phenotypes

Round Yellow

Round Green

Wrinkled Yellow

Wrinkled Green

Proportions


the dihybrid cross studying the inheritance of two characters simultaneously © 2007 paul billiet...

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