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Mendelian Genetics
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Gregor Mendel(1822-1884)
• Father of genetics• Austrian Monk• Between 1856 – 1863 he
studied ~28,000 pea plants
• Importance of his work not realized until 20th century, ~20 years after death
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Mendel’s Experiments
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The Peas
• Advantages of Pea Plants:– Grown in small areas– distinct heritable
features – Each feature has two
variants– Lots of offspring– Can self-fertilize or
cross fertilize
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Reproduction in Flowering Plants
• Pollen (produced by stamen) contain sperm
• Ovary contains egg• Pollen grows tube down
style to carry sperm to egg• Self-fertilization - sperm
and egg from same flower• Cross-fertilization - sperm
and egg from different flower
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His Experiments
• First created true-breeding peas by allowing them to self-fertilize
• True breeding = only produces offspring with one specific trait
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His Experiments• Then, he cross-fertilize (hybridized) two contrasting, true-breeding varieties (P1 generation - parent)
• Offspring is the F1
Generation – first filial• Then allowed the F1 hybrids
to self-pollinate to produce an F2 generation – 2nd filial
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His Experiments
P1 Generation
F1 GenerationAll tall
F1 Generation
F2
Generation3 tall, one short
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His Experiments• Results:• F1 generation: all dominant phenotypes• F2 generation: 3:1 ratio of dominant to recessive phenotypes
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His Experiments
• Created three Laws of Inheritance– Law of Dominance– Law of Segregation– Law of Independent Assortment
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Mendel’s Laws of Inheritance
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Law of Dominance• In a cross of true-breeding parents, only one form
of the trait will appear in the next generation
• The apparent trait is dominant, the other is recessive
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Law of Segregation• The pair of factors (alleles) is segregated, or
separated, during formation of gametes (sperm and egg)
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Law of Independent Assortment
• Factors (alleles) for different traits are distributed in gametes independently of each other
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Applying Mendel’s Laws – Punnet Squares
• Two types – used to show expected genotype of offspring
• Monohybrid– 2 gametes from each parent– 2x2 grid
• Dihybrid– 4 gametes from each parents– 4x4 grid
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Applying Mendel’s Laws – Test Cross
• A mating between an organism with unknown genotype and a homozygous recessive organism
• Example:– Mice can either be brown (B) or white (b). You
have a mouse with a brown coat. – Can be BB or Bb– If Bb: offspring half brown/half white– If BB: offspring all brown
