Mendelelian Genetics 6.3

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Mendelelian Genetics6.3
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Gregor Mendel(1822-1884)

• Austrian monk
• Studied the inheritance of traits in pea plants
• Developed the laws of inheritance

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Before Mendel, people believed traits blended.
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Mendel Performed crosses on different pea
plants

• Parents Yellow Seeds X Green Seeds
• F1 All offspring had yellow seeds

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• Parents
• X
• F1 X
• F2 3 1

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Generation

• Parental P1 Generation the parental generation
• F1 generation the first-generation
• F2 generation the second-generation

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Inheritance

• Mendel found plants were able to pass on traits
• He said these physical traits are inherited as
  factors
• Mendels factors are actually genes

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• What we know now
• Genes code for traits
• We have two copies of each gene because we have
  two copies of each chromosome

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Genetics Terms

• Gene codes for a trait (tallness)
• Alleles - forms of a gene
• Dominant always expressed(T)
• Recessive - allele that shows only if dominant is
  NOT there (t)
• Dominant MASKS the recessive

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• Homozygous SAME alleles (TT or tt) also called
  pure 
• Heterozygous DIFFERENT alleles (Tt) also called
  hybrid

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More Terminology

• Genotype what GENE is there (e.g. TT, Tt, tt)
• Phenotype - the PHysical feature (e.g. tall,
  short)

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Genotype Phenotype in Flowers
Genotype of allelesT tall plant t short
plant
Genotypes TT Tt tt Phenotypes tall tall
short
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Punnett Square

• male

• Used to predict outcomes of genetic crosses

• female

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Genetic Practice Problems
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Breed the P1 generation

• tall (TT) x dwarf (tt) pea plants

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Solution
tall (TT) vs. dwarf (tt) pea plants
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Breed the F1 generation

• tall (Tt) vs. tall (Tt) pea plants

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Solution
tall (Tt) x tall (Tt) pea plants
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Monohybrid Crosses
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P1 Monohybrid Cross

• Trait Seed Shape
• Alleles R Round r Wrinkled
• Cross Round seeds x Wrinkled seeds
• RR x rr

Genotype Rr Phenotype Round GenotypicRatio
All alike PhenotypicRatio All alike
r
r
Rr
Rr
R
R
Rr
Rr
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P1 Monohybrid Cross Review

• Homozygous dominant x Homozygous recessive
• Offspring all Heterozygous (hybrids)
• Offspring called F1 generation
• Genotypic Phenotypic ratio is ALL ALIKE

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F1 Monohybrid Cross

• Trait Seed Shape
• Alleles R Round r Wrinkled
• Cross Round seeds x Round seeds
• Rr x Rr

Genotype RR, Rr, rr Phenotype Round
wrinkled G.Ratio 121 P.Ratio 31
r
R
RR
Rr
R
r
rr
Rr
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F1 Monohybrid Cross Review

• Heterozygous x heterozygous
• Offspring25 Homozygous dominant RR50
  Heterozygous Rr25 Homozygous Recessive rr
• Offspring called F2 generation
• Genotypic ratio is 121
• Phenotypic Ratio is 31

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Mendels Laws
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Law of Dominance

• Some alleles are dominant and other alleles are
  recessive. (T or t)
• An organism with at least one dominant allele for
  a trait will always have that trait. (TT, Tt)
• Recessive traits are only seen in homozygous
  recessive organisms. (tt)

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Law of Segregation

• During the formation of gametes (eggs or sperm),
  the two alleles responsible for a trait separate
  from each other
• Tt can make two gametes
• T gamete
• t gamete

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Applying the Law of Segregation
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Law of Independent Assortment

• Alleles for different traits are distributed to
  gametes independently of one another.
• This law can be illustrated using dihybrid
  crosses.

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Dihybrid Cross

• Traits Seed shape Seed color
• Alleles R round Y yellow r
  wrinkled y green
• What gametes are possible in an individual that
  is heterozygous for both traits?

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Dihybrid Cross

• Traits Seed shape Seed color
• Alleles R round Y yellow r
  wrinkled y green

FOIL
RrYy x RrYy
RY
Ry
rY
ry
RY
Ry
ry
rY
All possible gamete combinations
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Dihybrid Cross
RRYy
RrYy
RrYY
RRYY
RRYy
RRyy
Rryy
RrYy
rrYy
RrYY
RrYy
rrYY
RrYy
Rryy
rrYy
rryy
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Dihybrid Cross
Round/Yellow 9Round/green
3wrinkled/Yellow 3wrinkled/green 1 9331