Mendelian Genetics

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Mendelian Genetics
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Epistasis a gene at one locus alters the
phenotypic expression of a gene at another locus
Epistasis in mouse hair color
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Some characters do not fit the either-or basis
that Mendel studied

• Quantitative characters vary in a population
  along a continuum.
• These are usually due to polygenic inheritance,
  the additive effects of two or more genes on a
  single phenotypic character.

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PolygenicTrait, Quantative Characters Human
height in 175 students at Connecticut
Agricultural College
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Human skin pigmentation as polygenic trait
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Genes and The Environment

• The product of a genotype is generally not a
  rigidly defined phenotype, but a range of
  phenotypic possibilities, the norm of reaction,
  that are determined by the environment.
• In some cases the norm of reaction has no breadth
  (for example, blood type).
• Norms of reactions are broadest for polygenic
  characters.
• For these multifactorial characters, environment
  contributes to their quantitative nature.

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Sun (smaller) and shade leaves in a hosta
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Environmental influence flower color in
hydrangea Blue color occurs in more acid soil
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Phenotype depends on the interaction of
environment and genes even identical twins
accumulate phenotypic differences
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Mendelian Inheritance in Humans Pedigree Analysis
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Key to reading genetic pedigrees
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Sex and Inheritance

• Sex-linked genes are carried on the sex
  chromosomes like hemophilia and color blindness
  that are carried on the X and thus show up more
  often in males or hairy ears that is carried on
  the Y and only shows up in males
• Sex-influenced genes are carried on regular
  autosomal chromosomes but the expression is
  influenced by the sex of the individual - The sex
  influence appears to be related to levels of male
  sex hormones in particular testosterone more
  testosterone leads to greater expression of the
  trait

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Pattern baldness in the Adams Family
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Cystic Fibrosis
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Sickle-cell anemia pleiotropy
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Distribution of Sickle-Cell Anemia
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Achondroplasia a dominant trait
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Woody Guthrie Huntingtons Disease Sufferer
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