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