Mendelian Genetics

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…And Now the Test Cross

Mendel then crossed a pure & a hybrid from his F2 generation

This is known as an F2 or test cross

There are two possible testcrosses: Homozygous dominant x Hybrid Homozygous recessive x Hybrid

Slide 35

Trait: Seed Shape

Alleles: R – Round r – Wrinkled

Cross: Round seeds x Round seeds

RR x Rr

F2 Monohybrid Cross (1st)

R

R

r

R

RR

Rr

RR

Rr

Genotype: RR, Rr

Phenotype: Round

Genotypic Ratio: 1:1

Phenotypic Ratio: All alike

Slide 36

Trait: Seed Shape

Alleles: R – Round r – Wrinkled

Cross: Wrinkled seeds x Round seeds

rr x Rr

F2 Monohybrid Cross (2nd)

r

r

r

R

Rr

rr

Rr

rr

Genotype: Rr, rr

Phenotype: Round & Wrinkled

G. Ratio: 1:1

P.Ratio: 1:1

Slide 37

F2 Monohybrid Cross Review

Homozygous x heterozygous(hybrid)

Offspring: 50% Homozygous RR or rr 50% Heterozygous Rr

Phenotypic Ratio is 1:1

Called Test Cross because the offspring have SAME genotype as parents

Slide 38

Practice Your Crosses

Work the P1, F1, and both F2 Crosses for each of the other Seven Pea Plant Traits

Slide 39

Mendel’s Laws

Slide 40

Results of Monohybrid Crosses

Inheritable factors or genes are responsible for all heritable characteristics

Phenotype is based on Genotype

Each trait is based on two genes, one from the mother and the other from the father

True-breeding individuals are homozygous ( both alleles) are the same

Slide 41

Law of Dominance

In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation.

All the offspring will be heterozygous and express only the dominant trait.

RR x rr yields all Rr (round seeds)

Slide 42

Law of Dominance

Slide 43

Law of Segregation

During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other.

Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.

Slide 44