Exploring Mendelian Genetics

Law of Independent Assortment

• Does the segregation of one pair of alleles affect the segregation of another pair of alleles?– Mendel performed an experiment to follow two

different genes from generation to generation in a Two-Factor Cross = Dihybrid Cross

Dihybrid Cross: F1 Generation

• Mendel crossed true-breeding plants that produced only round yellow peas (RRYY) with plants that produced wrinkled green peas (rryy). All F1 offspring from this cross had round yellow peas (RrYy)

• This cross produced hybrid plants for the next cross…

Dihybrid CrossCross a Round, Yellow peas plant (RRYY) with

plants that produce Wrinkled, Green peas (rryy).

RRYY x rryy

R = round seed

r = wrinkled seed

Y = yellow seed

y = green seed

RY RY RY RY

ry

ry

ry

ry

RrYy RrYy RrYy RrYy

RrYy RrYy RrYy RrYy

RrYy RrYy RrYy RrYy

RrYy RrYy RrYy RrYy

All offspring produced have Round, Yellow peas.

Round, Yellow

Round, Yellow

Round, Yellow

Round, Yellow

Dihybrid Cross: F2 Generation

– Mendel crossed two F1 plants (RrYy) heterozygous for both seed shape and color

– Using a Punnett square, we can see that alleles for seed shape segregate independently of those for seed color = Law of Independent Assortment – the inheritance of one character has no effect on the inheritance of another.

Dihybrid CrossCross two plants that are heterozygous for round,

yellow seeds (RrYy).

RrYy x RrYy

R = round seed

r = wrinkled seed

Y = yellow seed

y = green seed

RY Ry rY ry

RY

Ry

rY

ry

RRYY RRYy RrYY RrYy

RRYy RRyy RrYy Rryy

RrYY RrYy rrYY rrYy

RrYy Rryy rrYy rryy

Phenotype Ratio -Round, Yellow

Round, Yellow

Round, Yellow

Round, Yellow

Round, Green

Round, Yellow

Round, Yellow

Round, Yellow

Round, Yellow

Round, Yellow

Round, Green

Round, Green

Wrinkled,Yellow

Wrinkled,Yellow

Wrinkled,Yellow

Wrinkled, Green

9 Round, Yellow

3 Round, Green

3 Wrinkled, Yellow

1 Wrinkled, Green

9 : 3 : 3 : 1 Ratio

Dihybrid Cross

Genotype Ratio –

1 RRYY : 2 RRYy :

2 RrYY : ….Ahhh!

Punnett square shows a 9:3:3:1 phenotypic ratio –

9 yellow round : 3 yellow wrinkled : 3 green round: 1 green wrinkled

Blind dog

Phenotypes

(a) Possible phenotypes of Labrador retrievers

B_N_

Blind dog

Genotypes

(b) A Labrador dihybrid cross

Black coat,normal vision

B_nn

Black coat,blind (PRA)

bbnn

Chocolate coat,blind (PRA)

bbN_

Chocolate coat,normal vision

Mating of double heterozygotes

(black coat, normal vision)BbNn BbNn

Phenotypicratio ofoffspring

9 black coat, normal vision

3 black coat, blind (PRA)

3 chocolate coat, normal vision

1 chocolate coat, blind (PRA)

Blind Blind

Summary of Mendel’s Principles

– Inheritance of biological information is determined by genes and there are different versions (alleles)

– Each adult organism has two copies of each gene – one from each parent (could then be heterozygous or homozygous)

– Some forms of a gene may be dominant or recessive– Alleles for different genes usually segregate

independently of one another (law of segregation/law of independent assortment)