Patterns of Inheritance

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Patterns of Inheritance

• geneticsthe branch of biology that studies
  heredity.

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MENDELS PRINCIPLES

• Gregor Mendel, an Austrian monk, carried out
  important studies of hereditythe passing on of
  characteristics from parents to offspring.
• Characteristics that are inherited are called
  traits.

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Mendel chose his subject carefully

• Modern genetics began with Gregor Mendels
  quantitative experiments with pea plants
• Mendel crossed pea plants that differed in
  certain characteristics and traced the traits
  from generation to generation
• This illustration shows his technique for
  cross-fertilization

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Mendel was a careful researcher

• He studied only one trait at a time to control
  variables, and he analyzed his data
  mathematically.
• The tall pea plants he worked with were from
  populations of plants that had been tall for many
  generations and had always produced tall
  offspring.
• Such plants are said to be true breeding
• A hybrid is the offspring of parents that have
  different forms of a trait, such as tall and
  short height.

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• The original parents, the true-breeding plants,
  are known as the P1 generation.
• The offspring of the parent plants are known as
  the F1 generation
• When you cross two F1 plants with each other,
  their offspring are the F2 generation

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Mendels principle of segregation describes the
inheritance of a single characteristic

• From his experimental data, Mendel deduced that
  an organism has two genes (alleles) for each
  inherited characteristic
• One characteristic comes from each parent
• A sperm or egg carries only one allele of each
  pair
• The pairs of alleles separate when gametes form
• This process describes Mendels law of
  segregation

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The rule of dominance I

• If two alleles differ, then one, the dominant
  allele, is fully expressed in the organisms
  appearance.
• The other, the recessive allele, has no
  noticeable effect on the organisms appearance.
• Purple color is dominant over white

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The rule of dominance II

• An uppercase letter is used for the dominant
  allele and a lowercase letter for the recessive
  allele.
• The dominant allele is always written first.
• An organism is homozygous for a trait if its two
  alleles for the trait are the same. PP (dominant)
  or pp (recessive)
• An organism is heterozygous for a trait if its
  two alleles for the trait differ from each other.
  (Pp) (dominant)

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Phenotypes and Genotypes

• The way an organism looks and behaves is called
  its phenotype.
• The allele combination an organism contains is
  known as its genotype. PP, Pp, or pp

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Punnett Squares

• In 1905, Reginald Punnett, an English biologist,
  devised a shorthand way of finding the expected
  proportions of possible genotypes in the
  offspring of a cross
• A Punnett square predicts the results of a
  genetic cross between individuals of known
  genotype

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Homologous chromosomes bear the two alleles for
each characteristic

• Alternative forms of a gene (alleles) reside at
  the same locus on homologous chromosomes

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The principle of independent assortment is
revealed by tracking two characteristics at once

• By looking at two characteristics at once, Mendel
  found that the alleles of a pair segregate
  independently of other allele pairs during gamete
  formation
• This is known as the principle of independent
  assortment