Mendel and Heredity (Chapter 8)

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Mendel and Heredity(Chapter 8)

• D. Blanck
• PLHS Biology

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I. Orgins of Genetics

• Heredity the passing of traits from parents to
  offspring
• (CharacteristicTrait)
• ? before DNA and chromosomes were discovered,
  heredity was a great mystery

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• B. Gregor Johann Mendel
• 1) Austrian monk that
• is credited as the father
• of the scientific study of heredity
• 2) Experimented with different varieties of
  garden peas
• a) 1st to develop rules to predict
• patterns of heredity

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• b) heredity provides the basis for
• Genetics the study of genes
• c) repeated experiments of T.A. Knight
• Mendel extended Knights work by
  applying math!
• (ratios and proportions)

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• 3. Why did Mendel use garden peas?
• a) Peas have clear traits - easy to tell
  apart (see table 8-1, pg 163)
• b) Easy to control pollination (male
• and female parts are in same flower)
• 1) self-fertilization flower
• fertilizes itself
• 2) cross-pollination
• transfer of pollen
• between plants
• c) Easy to grow

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• C) Mendels work with ratios
• 1. Mendels Experiment
• monohybrid cross only 1 trait
• Step 1 Make sure plant is true-breeding
• ? allow plant line to self-pollinate for
  many generations
• ? results in no variation in traits

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• Step 2 Cross two P generation (parental
• generations) plants with contrasting traits
• ? observe offspring (F1 generation)
• ? record of F1 plants with each trait

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• Step 3 Allow F1 plants to self-pollinate
• ? observe and count this second
• generation of offspring
• F2 generation

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• 2. Mendels results
• a) F1 gen. showed only one form of trait
• (ex purple flowers)
• b) F2 gen. showed both forms of trait
• (ex 705 purple 224 white)
• c) For each of the 7 traits, he found the
• same 31 ratio!

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Mendels results for other traits
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results2
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• II. Gene Theory
• A. Mendels Hypothesis - foundation of
  genetics
• 1. For each trait, an individual has 2 copies
  of the gene, one from each parent
• 2. There are alternative versions of genes

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• Alleles alternative forms of a gene
• (green seed vs yellow seed)
• ? 1 allele for each gene comes from
  each parent
• Genotype set of 2 alleles
• ex GG or gg
• (G green and g yellow)
• Phenotype observable characteristic
• ex pea appears green or yellow

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• Homozygous 2 identical alleles for a trait
• (ex GG and gg)
• Heterozygous 2 different alleles for a trait
• (ex Gg or gG)

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• 3. When 2 different alleles occur together, one
  may be completely expressed. The other may have
  no observable effect on phenotype
• a) Dominant allele exclusively
• expressed
• PP purple Pp purple
• b) Recessive allele NOT expressed
• when dominant form is present
• Pp Purple pp white

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III. Studying Heredity

• A. Punnett Square predicts the expected
  genotypes of a cross.

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• Punnett squares can also deal with multiple or
  complex traits.
• A Green
• a absence of Green
• (blue)
• B Brown
• b absence of Brown
• (blue)

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Color Blindness

• An X-Linked Trait

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Codominance or incomplete dominance
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