Mendelian Genetics

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Mendelian Genetics
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Do You Know These Words

• Heredity
• Passing on of characteristics from parent to
  offspring
• Genetics
• Part of biology that studies heredity
• Gene
• Piece of DNA that codes for a polypeptide
  (specify certain trait)
• Trait
• Characteristic that is inherited
• Homologous Chromosomes
• Chromosome of the same type (pairs)
• Locus
• Specific location of a gene on a chromosome
• Diploid
• 2 copies of each chromosome 2n
• Haploid
• 1 copy of each chromosome 1n

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Gregor Mendel

• Austrian monk
• A mathematician aware of probability
• The father of genetics
• Worked with pea plants

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Why Peas?

• Peas have many traits with 2 different forms that
  are easy to distinguish (ex green or yellow
  seeds tall or short plants)
• Could easily control mating of the pea plants
  through cross-pollination
• Pea plants are small, they grow quickly and
  easily, and they produce a lot of offspring.

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Mendels Crosses

• In one cross, Mendel crossed tall pea plants with
  short pea plants
• All of the offspring were tall!
• He then crossed two of the offspring tall plants
• The next offspring were ¾ tall and ¼ short
• Mendel said that 2 factors control each trait
  (these are genes found on chromosomes)
• Each form of a gene is called an allele.
• The two alleles are found on homologous
  chromosomes.

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Mendels Crosses

• Parent x Parent P1 generation
• Offspring x Offspring F1 generation
• Offspring F2 generation

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Mendels Law of Dominance

• Mendel called the observable trait (all tall
  plants) dominant and the trait that disappeared
  (short plants) recessive
• The allele for tall plants is dominant
• The allele for short plants is recessive
• If T tall and t short
• TT tall plant, Tt tall plant, tt short plant

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Mendels Theory of Segregation

• Each parent passes on (at random) to offspring
  only one allele for each trait!
• A plant that is Tt can pass either T or t in a
  gamete.
• Ex Humans have ____ chromosomes.
• When eggs and sperm are made, only half of these
  chromosomes are passedone of each kind from mom
  and one of each kind from dad
• That way a fertilized egg (and the baby) will
  have the normal of chromosomes

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Mendels Theory of Segregation

• Diploid cells have pairs of genes on pairs of
  homologous chromosomes.
• The 2 genes of each pair are separated during
  meiosis and end up in different gametes.

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More vocabulary!

• Phenotype individuals observable traits
• Ex (tall, short)
• Genotype genes an individual carries
• Ex TT, Tt
• Homozygous has 2 of the same alleles
• Purebred, true-breeding
• TT, tt
• Heterozygous has 2 different alleles
• Hybrid
• Tt

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Monohybrid Cross

• Cross a single trait
• Cross 2 parents that are homozygous for different
  alleles of a single trait
• Saw purple allele is dominant to (masks) white
  allele

• Purple flower x White flower
• All purple flower plants
• 705 purple
• 224 white
• About a 31 ratio

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Now lets investigate the heredity seen in Reebops!
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Punnett Squares

• Can be used to show all possible offspring!
• Ex. R round seed, r wrinkled seed
• Homozygous round x homozygous wrinkled
• What F1 offspring can be produced? What are the
  genotypes and phenotypes?

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F2 Generation

• Now lets cross two of the heterozygous offspring
  from the F1 generation.

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Dihybrid Cross

• Cross two traits
• Start with 2 parents that are homozygous for
  different alleles for 2 traits
• Round Yellow Seed x Wrinkled Green Seed
• R round Y yellow
• r wrinkled y green

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Punnett Square

• Example RRYY x rryy

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Mendels Theory of Independent Assortment

• Gene pairs for different traits are inherited
  independently of each other
• Ex Genes for round seed shape are inherited
  independently of genes for yellow seed color!
• Note We will see later that this does not
  always apply

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Test Cross

• I have a guinea pig that is black.
• I know that the black allele (B) is dominant to
  the brown allele (b).
• What are the possible genotypes for my guinea
  pig?
• I want to breed guinea pigs, and I want to have
  pure-bred guinea pigs!
• How do I know if he is pure-bred?

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Test Cross

• My guinea pig may be BB or Bb. Both would show
  the black phenotype!
• To see if a dominant phenotype is due to a
  homozygous dominant or a heterozygous phenotype,
  I can
• Cross it with one that is homozygous recessive!
  B B B b
• b b
• b b