Mendel and Heredity

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Mendel and Heredity

• Chapter 10 and 12

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Key Terms

• Heredity The passing of traits from parents to
  offspring
• Genetics the branch of science that studies
  heredity
• Gene the unit of heredity codes for a
  specific trait
• Gregor Mendel the father of modern genetics
• Gametes male and female sex cells
• Zygote forms when the sperm and egg meet

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Summary of Mendels Experiments

• Why peas?
• Characters exist in two clear forms (tall or
  short, purple or white flowers, etc.)
• Self-fertilization is possible
• Small plant
• Grows easily
• Matures quickly
• Many offspring

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What did Mendel do?

• He self pollinated the pea plants until they were
  true-breeding
• Had only one version of a trait in their genes
• Only genes for purple flowers, or for being tall
• This made the P-generation (parent generation)
• He then crossed 2 different true-breeding plants
  and recording the results. This made the F1
  generation (filial 1). These are hybrids.
• Finally, he crossed 2 members of the F1
  generation making the F2 generation (filial 2)

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What was the result of Mendels experiment?

• All members of the F1 generation displayed the
  dominant traits
• Seemed as if one trait had disappeared!
• The members of the F2 generation displayed a 31
  ratio of dominant to recessive
• The trait reappeared!

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The Result of Mendels Experiments.

• Mendels Hypotheses
• For each inherited character, a person has 2
  copies of the gene one from each parent
• There are alternative versions of genes
  (tall/short, wrinkled/round, etc.)
• When 2 alternative versions occur in the same
  individual, one is expressed and the other is
  hidden

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Genetic Rules

• Rule of Unit Factor
• There are genes on chromosomes that control
  traits and at least 2 versions of each trait
  exists
• Rule of Dominance
• One trait is dominant to the other and masks the
  other gene when it is present
• Law of Segregation
• Traits are inherited independent of each other
• Law of Independent Assortment
• Homologous chromosomes sort into the gametes
  independently of each other

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Some more terms

• Alleles different versions of a gene
• Dominant the gene that is always expressed if
  it is present capital letter
• Recessive must have 2 copies to be expressed,
  otherwise it is hidden by the dominant gene
  lower-case letter
• Homozygous person has 2 of the same allele TT
  or BB or tt or bb
• Heterozygous person has 2 different alleles
  Tt or Bb

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• Phenotype
• The way an organism looks
• Only reveals the genotype if the recessive trait
  is displayed

• Genotype
• The genes that an organism possesses
• Homozygous
• Heterozygous

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How do we determine the genotype and phenotype of
the offspring
Punnett Squares!

• Used to determine probability
• Monohybrid
• Dihybrid
• Trihybrid

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More Complex Inheritance

• Incomplete Dominance the heterozygote has a
  blend of the 2 traits
• Codominance both traits occur equally
• Multiple Alleles more than two alleles for a
  trait
• Polygenic Inheritance more than one gene
  controls the trait usually shows a wide range
  of variation skin color and height

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Nature vs. NurtureThe effect of the environment

• Environmental effects can determine if a gene is
  expressed
• Chemicals, stress, sunlight, nutrition, etc
• Internal environment can also determine if a gene
  is expressed
• Gender, weight, hormone imbalances

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Sex-linked Genes

• On the X or Y Chromosome
• Examples Color-blindness, hemophilia, duchenes
  muscular dystrophy
• Disorders occur more frequently in males

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Codominance
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Incomplete Dominance
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Multiple alleles
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Semi Dominance