Variation

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Variation

• Section 10.3

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Variation in Inheritance

• Incomplete Dominance
• Codominance (multiple alleles)
• Polygenic inheritance

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Incomplete Dominance

• Both alleles contribute to a phenotype of a
  heterozygous individuals to produce a trait not
  like either parent.
• Phenotype intermediate between two pure traits
• Ex Snapdragons, Andalusian chick.

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Snapdragon Flower Color

• Alleles often written as capital letters with
  superscripts
• ex. CRCR (red) x CWCW (white)
• Incomplete Dominance revealed in Heterozygous
  Individual
• A cross between a white-flowered plant and a
  red-flowered plant will produce all pink F1
  offspring
• Self-pollination of the F1 offspring produces
  25 white, 25 red, and 50 pink offspring.

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Codominance (multiple alleles)

• Two dominant alleles are expressed at the same
  time
• Ex Roan coat color in horses CRCR, CWCW, CRCW

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CRCR (Red)
CWCW (White)
CRCW (Roan)
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• If a roan cow (RW) is mated to a roan bull (RW),
  what are the phenotypes of the offspring?

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Multiple Alleles

• Most genes have more than two alleles in a
  population
• The ABO blood groups in humans are determined by
  three alleles, IA, IB, and i.
• Both the IA and IB alleles are dominant to the i
  allele
• The IA and IB alleles are codominant to each
  other
• Because each individual carries two alleles,
  there are six possible genotypes and four
  possible blood types.

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ABO Blood Group System

• Type A IA IA or IA i
• Type B IB IB or IB i
• Type AB IA IB
• Type O ii

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Blood Groups
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Problems

• A man homozygous for type A blood marries a woman
  who is heterozygous for type B blood. What are
  the possible genotypes and phenotypes of their
  children?
• A couple has a child with type O blood. If one
  parent is type O, what are the possible genotypes
  of the other parent?

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Polygenic inheritance

• When two or more genes effect a single
  characteristic
• For example, skin color in humans is controlled
  by at least three different genes.
• Imagine that each gene has two alleles, one light
  and one dark, that demonstrate incomplete
  dominance.
• An AABBCC individual is dark and aabbcc is light

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• A cross between two AaBbCc individuals
  (intermediate skin shade) would produce offspring
  covering a wide range of shades
• The range of phenotypes forms a normal
  distribution.

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Importance of Environment

• In some cases environment plays an important part
  in the expression of genes
• Ex Temperature (Siameses cat fur), Nutrition
  (height growth of individual)
• However human blood type is not influenced by
  environment.

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Influence of Environment