Polygenic Traits

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Polygenic Traits
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Definition

• Some traits are determined by the combined effect
  of two or more pairs of alleles. These traits
  are called polygenic traits.
• Each gene contributes a small but additive effect
  to the trait.
• Other names for polygenic traits are
  multi-factorial traits, or quantitative traits.

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Polygenic traits are continuous

• Because so many alleles contribute to the final
  phenotype, a variety of phenotypes can occur!
• For example, height is a polygenic trait. If you
  look around, you will notice there are not two
  set heights, but rather a continuum of height
  among your classmates.

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Polygenic Traits are Continuos

• However, when dealing with polygenic traits that
  are only controlled by two or three pairs of
  alleles, we can complete Punnett squares to
  determine the genotypes and phenotypes of the F1
  generation.

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Examples of Polygenic Traits

• Pepper ColorGene 1 Rred ryellowGene
  2 Yabsence of chlorophyll (no green)
  ypresence of chlorophyll (green)

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Pepper Color

• Possible genotypes
• R-/Y- red (red/no chlorophyll)
• R-/yy brown/orange (red/chlorophyll)
• rr/Y- yellow (yellow/no chlorophyll)
• rr/yy green (yellow/chlorophyll)

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Pepper Color

• Try crossing a brown pepper (RRyy) with a yellow
  pepper (rrYY).
• Which trait will your offspring (F1 generation)
  produce?
• What traits are produced when you cross two of
  the peppers found in the F1 generation?

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More Polygenic Traits

• Chicken Combs
• There are 4 different comb shapes that can appear
  in a chicken.
• Comb shape is controlled by two genes found on
  two different pairs of chromosomes.
• Gene 1 R
• Gene 2 P

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Chicken Comb Example

• 4 different phenotypes result
• Rose Combs (R-pp)
• Walnut Combs (R-P-)
• Pea Combs (rrP-)
• Single Combs (rrpp)

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Try these questions

• Indicate the phenotypes of the parents and give
  the genotypic and phenotypic ratios of their
  offspring for the following crosses (Refer to
  your phenotype chart for chickens)
• rrPP x RRpp
• RrPp x RrPp

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Another example of a polygenic trait

• Hair Color
• Hair color is controlled by alleles on
  chromosomes 3, 6, 10, and 18.
• The more dominant alleles that appear in the
  genotype, the darker the hair!

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Polygenics Problem

• The height of plants is controlled by 4 pairs of
  alleles. Alleles A, B, and C contribute 3 cm to
  the plant's height. Alleles that are recessive do
  not contribute to the height. Gene L is always
  found in a homozygous dominant condition and
  always contributes 40 cm to the height.
• a) What would be the height of a plant with the
  genotype AABBCCLL?
• b) What would be the height of a plant with a
  genotype aabbccLL?
• c) What would be the height of the offspring
  produced from a cross between the plants in a)
  and b)?
• d) What would be the heights of the offspring
  produced from a cross between AaBbCcLL and
  AaBbCcLL? (now there is a challenge!)

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Next Topic.

• Epistasis

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Lets Review

• Sex linked traits
• - X linked recessive
• - X linked dominant
• - y linked
• Sex influenced traits
• Polygenics
• Epistasis
• Pick up a piece of flip chart paper and a marker
  for your table. Summarize the new genetics
  topics.