Mendelian Genetics

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Applying the Law of Segregation

Slide 45

Law of Independent Assortment

Alleles for different traits are distributed to sex cells (& offspring) independently of one another.

This law can be illustrated using dihybrid crosses.

Slide 46

Dihybrid Cross

A breeding experiment that tracks the inheritance of two traits.

Mendel’s “Law of Independent Assortment”

a. Each pair of alleles segregates independently during gamete formation

b. Formula: 2n (n = # of heterozygotes)

Slide 47

Question: How many gametes will be produced for the following allele arrangements?

Remember: 2n (n = # of heterozygotes)

1. RrYy

2. AaBbCCDd

3. MmNnOoPPQQRrssTtQq

Slide 48

Answer:

1. RrYy: 2n = 22 = 4 gametes

RY Ry rY ry

2. AaBbCCDd: 2n = 23 = 8 gametes

ABCD ABCd AbCD AbCd

aBCD aBCd abCD abCD

3. MmNnOoPPQQRrssTtQq: 2n = 26 = 64 gametes

Slide 49

Dihybrid Cross

Traits: Seed shape & Seed color

Alleles: R round r wrinkled Y yellow y green

RrYy x RrYy

RY Ry rY ry

RY Ry rY ry

All possible gamete combinations

Slide 50

Dihybrid Cross

Slide 51

Dihybrid Cross

Slide 52

Dihybrid Cross

Round/Yellow: 9 Round/green: 3 wrinkled/Yellow: 3 wrinkled/green: 1

9:3:3:1

Slide 53

Test Cross

A mating between an individual of unknown genotype and a homozygous recessive individual.

Example: bbC x bbcc

BB = brown eyes

Bb = brown eyes

bb = blue eyes

CC = curly hair

Cc = curly hair

cc = straight hair

Slide 54

Test Cross

Possible results:

Slide 55

Summary of Mendel’s laws

Slide 56

Incomplete Dominance and Codominance

Slide 57

Incomplete Dominance

F1 hybrids have an appearance somewhat in between the phenotypes of the two parental varieties.