Mendelian Patterns of Inheritance

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Mendelian Patterns of Inheritance
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Outline

• Blending Inheritance
• Monohybrid Cross
• Law of Segregation
• Modern Genetics
• Genotype vs. Phenotype
• Punnett Square
• Dihybrid Cross
• Law of Independent Assortment

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Gregor Mendel

• Austrian monk who formulated fundamental laws of
  heredity in early 1860s.
• Studied science and mathematics at University of
  Vienna.
• Conducted breeding experiments with the garden
  pea Pisum sativum.
• Carefully gathered and documented mathematical
  data from his experiments.

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Blending Inheritance

• At the time, most breeders believed parents of
  contrasting appearance always produce offspring
  of intermediate appearance.
• Mendels experiments helped him formulate the
  particulate theory of inheritance.
• Inheritance involves reshuffling of genes from
  generation to generation.

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Garden Peas
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One-Trait Inheritance

• Mendel performed cross-breeding experiments
  between true-breeding plants.
• Chose varieties that differed in only one trait
  (monohybrid cross)
• Performed reciprocal crosses
• Parental generation P
• First generation offspring F1
• Second generation offspring F2
• Formulated law of segregation

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Mendels Monohybrid Cross
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Mendels Monohybrid Cross
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Law of Segregation

• Each individual has two factors for each trait.
• The factors segregate during gamete formation.
• Each gamete contains only one factor from each
  pair of factors.
• Fertilization gives each new individual two
  factors for each trait.

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Modern Genetics View

• Each trait in a pea plant is controlled by two
  alleles (alternate forms of a gene).
• Dominant allele (capital letter) masks the
  expression of the recessive allele (lower-case).
• Alleles occur on a homologous pair of chromosomes
  at a particular gene locus.
• Homozygous identical alleles
• Heterozygous different alleles

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Homologous Chromosomes
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Genotype Versus Phenotype

• Genotype
• Refers to the alleles an individual receives at
  fertilization.
• Phenotype
• Refers to the physical appearance of the
  individual.

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Punnett Square

• All possible kinds of sperm are lined up on one
  axis, and all possible kinds of eggs are lined up
  on the other axis.
• Every possible combination of alleles is placed
  within the squares.

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PUNNETT SQUARE
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Testcross

• A monohybrid testcross is used to determine if an
  individual with the dominant phenotype is
  homozygous dominant or heterozygous for a
  particular trait.
• The individual with the dominant trait in
  question is crossed with a homozygous recessive

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Testcross
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Two-Trait Inheritance

• Mendel performed cross using true-breeding plants
  differing in two traits.
• Dihybrid Cross
• Observed phenotypes among F2 plants.
• Formulated law of independent assortment

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Law of Independent Assortment

• Each pair of factors segregates independently of
  the other pairs.
• All possible combinations of factors can occur in
  the gametes.

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Dihybrid Cross
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Dihybrid Cross
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Two-Trait Testcross

• A two-trait testcross is used to determine if an
  individual is homozygous dominant or heterozygous
  for either of the two traits.

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Two-Trait Testcross
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Incomplete Dominance

• Exhibited when the heterozygote has an
  intermediate phenotype between that of either
  homozygote.

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Incomplete Dominance
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Polygenic Inheritance

• Occurs when a trait is governed by two or more
  sets of alleles.
• Each dominant allele has a quantitative effect on
  the phenotype, and these effects are additive.
• Result in continuous variation of phenotypes.

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Height in Human Beings
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Terminology

• Pleiotropy - A gene that affects more than one
  characteristic of an individual.
• Sickle-cell anemia (incomplete dominance)
• Codominance - More than one allele is fully
  expressed.
• ABO blood type (multiple allelic traits)
• Epistasis - A gene at one locus interferes with
  the expression of a gene at a different locus.
• Human skin color (polygenic inheritance)

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Environment and Phenotype

• Himalayan Rabbits - Enzyme coding for black fur
  is active only at low temperatures.
• Black fur only occurs on extremities.

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Review

• Blending Inheritance
• Monohybrid Cross
• Law of Segregation
• Modern Genetics
• Genotype vs. Phenotype
• Punnett Square
• Dihybrid Cross
• Law of Independent Assortment

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