the dihybrid cross

THE DIHYBRID CROSS

Studying the inheritance of two characters simultaneously

SBI3U

Independent Independent AssortmentAssortment

Are Different Characters Like Color and Shape Inherited Together or Inherited Independently?

Mendel performed dihybrid crosses to find out.

Mendel’s conclusion: Different characters are inherited independently.

Why Did Mendel Conclude That The Inheritance of one Trait is Independent of Another?

The alternative and incorrect hypothesis: dependent inheritance.

Because it’s the only way to explain the pattern of inheritance.

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The Punnett Square

Note that we’re simultaneously applying the Principles of Segregations and Independent Assortment.

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The Independent Alignment of Different Pairs of Homologous Chromosomes At Meiosis Accounts for the Principle of Independent Assortment

The alignment of one pair of homologs is independent of any other.

Principle of Independent Assortment: The assortment of one pair of genes into gametes is independent of the assortment of another pair of genes.

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Peas & Humans

Consider a cross between parents heterozygous for both deafness and albinism.

This is the same 9:3:3:1 ratio seen for Mendel’s cross involving pea color and shape.

Mendel’s peas

Character Trait Allele

Seed shape Round R

Wrinkled r

Cotyledon colour Yellow Y

Green y

Combinations

Genotype Phenotype

RRYY Round Yellow

RRYy Round Yellow

RrYY Round Yellow

RrYy Round Yellow

RRyy Round Green

Rryy Round Green

rrYY Wrinkled Yellow

rrYy Wrinkled Yellow

rryy Wrinkled Green

Is the inheritance of one character

affected by the inheritance of another?

P Phenotypes Round Yellow

x Wrinkled Green

(Pure Bred)

F1 Phenotypes All Round Yellow

(Selfed)

F2 Phenotypes Seed colour

Yellow Green TOTAL RATIO

Seed Round 315 108 423

shape Wrinkled 101 32 133

TOTAL 416 140 556

RATIO

Is the inheritance of one character

affected by the inheritance of another?

P Phenotypes Round Yellow

x Wrinkled Green

(Pure Bred)

F1 Phenotypes All Round Yellow

(Selfed)

F2 Phenotypes Seed colour

Yellow Green TOTAL RATIO

Seed Round 315 108 423 3.18

shape Wrinkled 101 32 133 1

TOTAL 416 140 556

RATIO 2.97 1

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 = 75% or ¾

Probability of an F2 seed being wrinkled =

Probability of an F2 seed being yellow =

Probability of an F2 seed being green =

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 = 75% or ¾

Probability of an F2 seed being wrinkled = 25% or ¼

Probability of an F2 seed being yellow = 75% or ¾

Probability of an F2 seed being green = 25% or ¼

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 =

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 = ¾ x ¼ = 3/16 = 18.75%

Probability of an F2 seed being wrinkled and yellow = ¼ x ¾ = 3/16 = 18.75%

Probability of an F2 seed being wrinkled and green = ¼ x ¼ = 1/16 = 6.25%

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

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 104 108

556 x 3/16 wrinkled yellow

104 101

556 x 1/16 wrinkled green 35 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 INDEPENDENTLY

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 RY Ry

r rY ry

Dihybrid cross genetic diagram

Gametes Y y

R RY Ry

r rY ry

fertilisation

F2 Genotypes RY Ry rY ry

RY RRYY RRYy RrYY RrYy

Ry RRYy RRyy RrYy Rryy

rY RrYY RrYy rrYY rrYy

ry RrYy Rryy rrYy rryy

Dihybrid cross genetic diagram

Dihybrid cross proportions

Phenotypes ProportionsRound Yellow 9/16 or 56.25%

Round Green

Wrinkled Yellow

Wrinkled Green

Dihybrid cross proportions

Phenotypes ProportionsRound Yellow 9/16 or 56.25%

Round Green 3/16 or 18.75%

Wrinkled Yellow

3/16 or 18.75%

Wrinkled Green

1/16 or 6.25%

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 test cross

This is done with a homozygous recessive (rr) individual

The same is true for a dihybrid cross where the test cross is made with an individual which is homozygous recessive for both characters (rryy)

Dihybrid test cross

Phenotypes

Round Yellow X Wrinkled Green

Genotypes RrYy rryy

Gametes RY, Ry, rY, ry ry

Genotypes RY Ry rY ry

ry RrYy Rryy rrYy rryy

Phenotypes

Round Yellow

Round Green

Wrinkled Yellow

Wrinkled Green

Proportions

1/4 or 25%

1/4 or 25%

1/4 or 25%

1/4 or 25%