Other Patterns of Inheritance

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Section 11.3

• Other Patterns of Inheritance

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THINK ABOUT IT

• Mendels principles offer a set of rules with
  which to predict various patterns of inheritance.
• There are exceptions to every rule, and
  exceptions to the exceptions.
• What happens if one allele is not completely
  dominant over another? What if a gene has several
  alleles?

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Beyond Dominant and Recessive Alleles

• What are some exceptions to Mendels principles?
• Some alleles are neither dominant nor recessive
  (incomplete dominance)
• Many traits are produced by the interaction of
  several genes (co-dominance)
• Many genes exist in several different forms, and
  are therefore said to have multiple alleles.
  (multiple alleles)
• .

4
Incomplete Dominance

• A cross between two four oclock plants shows a
  common exception to Mendels principles.
• neither allele is completely dominant over the
  other.
• The phenotype is a blend of the two homozygous
  phenotypes.

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Incomplete Dominance
R W
R
W

• In 4 Oclock flowers, alleles Rred Wwhite
  RWpink.
• When R self pollinates red
• When W self pollinated white
• When R W cross pollinates pink (R does not
  dominate W. They share results in incomplete
  dominance)
• RWXRW

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Incomplete Dominance
R W
R RR RW
W RW WW

• In 4 Oclock flowers, alleles Rred Wwhite
  RWpink.
• When R self pollinates red
• When W self pollinated white
• When R W cross pollinates pink (R does not
  dominate W. They share results in incomplete
  dominance)
• RWXRW

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Incomplete Dominance
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Codominance

• Another exception to Mendelian rules
• both alleles in the heterozygous genotype are
  expressed in the phenotypes.
• R red color W white color RW roan coat color

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Codominance

• R red coat color W white coat color RW roan
  coat color

R R
W RW RW
W RW RW
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Codominance

• For example, in certain varieties of chicken, the
  allele for black feathers is codominant with the
  allele for white feathers.
• Heterozygous chickens have a color described as
  erminette, speckled with black and white
  feathers.

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multiple alleles

• A single gene can have many possible alleles.
• A gene with more than two alleles is said to have
  multiple alleles.
• Ex. Is the human genes for blood type.

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Multiple Alleles
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Polygenic traits

• Controlled by the interaction of two or more
  genes and exhibit a wide range of phenotypes.
• Ex. variety of skin color in humans comes about
  partly because more than four different genes
  probably control this trait.

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Polygenic Traits
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Genes and the Environment

• Does the environment have a role in how genes
  determine traits?
• Environmental conditions can affect gene
  expression and influence genetically determined
  traits.

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Genes and the Environment

• The phenotype of an organism results only partly
  from its genotype.
• Environmental conditions can affect how genes are
  expressed.

Snowshoe hare-Summer
Snowshoe hare- Winter