THE DIHYBRID CROSS Studying the inheritance of two traits simultaneously © 2007 Paul Billiet...

THE DIHYBRID CROSS

Studying the inheritance of two traits simultaneously

© 2007 Paul Billiet ODWS

Student Moving Demo…

4 students2 male2 female

Each Pair (male and female) represent a possible gene we can inherit from either dad (male) or mom (female)Traits considered = eye color & heightWhat are the possible combinations?

What are the possible combinations of these traits?T or t B or b Phenotype

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© 2007 Paul Billiet ODWS

How is this possible?

MENDELS LAW OF INDEPENDENT ASSORTMENT It appears that the inheritance of

seed shape has no influence over the inheritance of seed color

The two characters are inherited INDEPENDENTLY

The pairs of alleles that control these two characters assort themselves independently


Mendel & Meiosis

The pairs of chromosomes can be lined up 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

fertilization

F

1

Phenotypes

RrYy (Selfed)

Genotypes

Round Yellow

Proportions

100%

Dihybrid cross genetic diagram


AND CHECK OUT THE GENERATIONAL NOTATION…

F

1

Phenotypes

RrYy (Selfed)

Genotypes

Round Yellow

Proportions

100%

meiosis

Gametes Y y

R RY Ry

r rY ry



Gametes Y y

R RY Ry

r rY ry

fertilization

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



HINT: Do a Punnett square for each parent’s 2 traits first!!!

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%


THE DIHYBRID CROSS Studying the inheritance of two traits simultaneously © 2007 Paul Billiet...

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