Genetics: Incomplete Dominance

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Genetics Incomplete Dominance Codominance

• Biology 12

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Joke of the day
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Review of Mendels Principles

• Genes are passed parents ? offspring get one
  allele from each parent
• During Meiosis, the alleles for a gene segregate
  from each other.
• During Meiosis, genes independently assort with
  each other.

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Review Dominant/Recessive

• One allele is dominant over the other (capable of
  masking the recessive allele)

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Review Problem

• In pea plants, purple flowers (P) are dominant
  over white flowers (p) show the cross between two
  heterozygous plants.

P p
GENOTYPES

P p
- PP (1) Pp (2) pp (1) - ratio 121
PHENOTYPES
- purple (3) white (1) - ratio 31
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Exceptions to Mendels principles

• Some alleles are neither dominant nor recessive.
• Many traits are controlled by more than one gene
  (polygenic traits)

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Incomplete dominance

• A situation in which neither allele is dominant.
• When both alleles are present a new phenotype
  appears that is a blend of each allele.
• The heterozygous phenotype is somewhere in
  between homozygous phenotype.
• 2 alleles produce 3 phenotypes

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Incomplete Dominance
Rr pink


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Problem Incomplete Dominance

• Show the cross between a pink and a white flower.

GENOTYPES

- Rr (2) rr (2) - ratio 11

PHENOTYPES
- pink (2) white (2) - ratio 11
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Codominance

• Neither allele are dominant both are expressed.
• A cross between organisms with two different
  phenotypes produces offspring with has both
  phenotypes of the parental traits shown.

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Example codominance

• In some chickens
• Black Chicken x White ? Speckled Chicken

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Example

• Roan coat color in horses and cattle are
  codominant for the alleles R and r.
• Homozygous R individuals are red (or bay)
• Homozygous r individuals are white
• While heterozygous Rr individuals are roan (which
  means they have both white and red hair, giving
  them a lightened appearance.

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What about the F2 generation?

• Phenotype?
• 1 red, 2 roan and 1 white
• 121
• Genotype?
• 1Hr Hr, 2 HrHw and 1Hw Hw

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Which type of dominance
Codominance!
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To do

• Page 145 questions 2, 3, 4, 5
• Practise sheet Codominance and Incomplete
  dominance
• Colouring sheet