Genetic Crosses

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Genetic Crosses
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Genetics and Probability

• Mendel saw that the results he obtained from
  crosses were similar and followed a pattern.
• He determined that probability could be applied
  to the data.

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Probability

• Refers to the likelihood that an event will
  happen. (heads or tails)
• Probability of times expected
• of opportunities

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Rules

• Each event is independent of the previous one.
  (past outcomes do not affect future outcomes).
• The probability of a certain outcome can be
  predicted.

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Punnett Squares

• Used to help predict the gene combinations
  possible from a cross.
• A B
• A
• B

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Rules

• Alleles indicated by letters
• Capital dominant alleles
• Lower case recessive alleles

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Homo vs. Hetero

• Homozygous individuals have 2 of the same alleles
  for a trait. (purebred)
• Heterozygous individuals have one of each allele
  for a trait. (hybrid)

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How do you tell?

• Phenotype physical characteristics of an
  individual.
• Genotype genetic makeup of an individual (what
  alleles they have).

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Probability and Segregation

• Mendels results consistently followed the
  predictions of probablilty.
• This supported Mendels theory of segregation.

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Probability and Prediction

• Probability can predict the average outcome of a
  large number of independent events.
• Cannot predict the outcome of an individual
  event.
• As the of individuals increases, the closer the
  results will be to expected outcomes.

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What Letters to Use?

• Each parent has 2 possible alleles.
• Use the letter that starts the word of the
  dominant characteristic Use B if Black hair is
  dominant, and use b if Blonde hair is recessive.

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Steps for Solving Genetic Problems

• Step 1 Figure out the genotypes of the parents.
• Step 2 Figure out what kinds of gametes the
  parents can produce.
• Step 3 Set up a Punnett Square for your mating.
• Step 4 Fill in the babies inside the table by
  matching the egg allele at the top of the column
  with the sperm allele at the head of the row.

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Steps for Solving Genetic Problems

• Step 5 Figure out the genotypic ratio for your
  predicted babies.
• Step 6 Figure out the phenotypic ratio for your
  predicted babies.
• Step 7 Answer the question you've been asked.

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Problem1

• Create a punnett square for a cross of a
  heterozygous male and female for freckles.
• F Freckles
• f No Freckles
• List the genotypic and phenotypic ratios of the
  offspring.

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Problem 2

• Black fur (B) is dominant to brown fur (b) in
  rabbits. Create a Punnett square which crosses 2
  heterozygous rabbits.
• List the genotypic and phenotypic ratios of the
  offspring.

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Problem 3

• Multiple trait crosses or dihybrid crosses
  involve more than 1 trait and therefore more than
  1 pair of alleles.
• Make a dihybrid Punnett square using the
  following data
• Green seeds is dominant over yellow
• Tall plants are dominant over short
• Cross 2 heterozygous parents and list the
  genotypic and phenotypic ratios.