Mendelian Genetics

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•Mendel stated that physical traits are inherited as “particles”

•Mendel did not know that the “particles” were actually genes located on chromosomes

Particulate Inheritance

Inheritance We inherit genes from

our parents

Different versions of a gene are called alleles

Alleles are represented by letters

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Alleles in an Organism

How many alleles does an organism have for a single trait?

Genes come in pairs because chromosomes come in pairs (homologues)

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Where does each allele come from?

• Half from mother

• Half from father

• What process divides the genome in half?

• What happens during the formation of gametes to give each sex cell receives one allele for each trait?

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Homologous Pairs Separate

Replicated Chromosomes Separate

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Anaphase I Anaphase II

Genes Come in Pair

•An individual can only have two alleles of a given gene

•A population can have more than two alleles of a given gene

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Two Types of Alleles

Dominant - stronger of two alleles that masks the trait of other allele; represented by a capital letter (R)

Recessive – allele is hidden by the dominant; represented by a lowercase letter (r)

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Genetic Make-Up

Genotype - allele combination for a trait (e.g. RR, Rr, rr)

Phenotype - the physical feature resulting from a genotype ( seven traits of pea plants)

Genotype dictates phenotype!

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Two Types of Genotypes

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•Heterozygous – genotype of two

different alleles (Rr)

•Homozygous – genotype of the same

two alleles (rr or RR)

Which allele’s trait will we see in a

heterozygote?

Classifying Homozygotes

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RR

rr

Homozygous

Dominant

Homozygous

Recessive

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Odds of Inheritance

•What are the odds of inheriting each

allele from a parent? Why?

50%

Crosses (Breeding)

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Practice Cross • Trait: Seed Shape

• Alleles: R – Round r – Wrinkled

• Cross: Round seeds x Round seeds

• Rr x Rr

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R

r

r R

RR

rr Rr

Rr

Genotypic Ratio

1RR:2Rr:1rr

Phenotypic Ratio

3 Round:1 Wrinkled

Practice Cross • Trait: Seed Shape

• Alleles: R – Round r – Wrinkled

• Cross: Round seeds x Wrinkled seeds

• RR x rr

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R

R

r r

Rr

Rr Rr

Rr

Genotype: Rr

Phenotype: Round

Genotypic Ratio: All alike

Phenotypic Ratio: All alike

Practice Cross • Trait: Seed Shape

• Alleles: R – Round r – Wrinkled

• Cross: Round seeds x Round seeds

• RR x Rr

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R

R

r R

RR

Rr RR

Rr

Genotype: RR, Rr

Phenotype: Round

Genotypic Ratio: 1:1

Phenotypic Ratio: All alike

Practice Cross • Trait: Seed Shape

• Alleles: R – Round r – Wrinkled

• Cross: Wrinkled seeds x Round seeds

• rr x Rr

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r

r

r R

Rr

rr Rr

rr

Genotype: Rr, rr

Phenotype: Round & Wrinkled

Genottypic Ratio: 1:1

Phenotypic Ratio: 1:1

Dihybrid Cross

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RRYY

RRYy

RrYY

RrYy

RRYy

RRyy

RrYy

Rryy

RrYY

RrYy

rrYY

rrYy

RrYy

Rryy

rrYy

rryy

Round/Yellow: 9

Round/green: 3

wrinkled/Yellow: 3

wrinkled/green: 1

9:3:3:1 phenotypic ratio

RY Ry rY ry

RY

Ry

rY

ry