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Genetic Probabilities. Learning Objectives. By the end of this class you should understand: The purpose and nature of dihybrid crosses How to calculate the probability that an unaffected person may be a carrier for a disorder What a rare-allele assumption is for

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Presentation Transcript
learning objectives
Learning Objectives

By the end of this class you should understand:

  • The purpose and nature of dihybrid crosses
  • How to calculate the probability that an unaffected person may be a carrier for a disorder
  • What a rare-allele assumption is for
  • Identify examples of chromosomal linkage
probability
Probability
  • A probability is a number that represents the number of outcomes that fit a certain definition
    • All probabilities are between 0 and 1
    • 0 = never happens, 1 = always happens
  • Probabilities may be derived from Punnett Squares
    • Number of particular outcomes divided by total number of outcomes
independent probabilities
Independent Probabilities
  • When two effects do not interact, they are said to be independent
  • The assortment of chromosomes during meiosis is independent and follow\'s Mendel\'s Law of Independent Assortment
  • Two genes on the same chromosome are not independent
    • Chromosomal linkage
probability of carrier
Probability of Carrier
  • If an individual has a family history of a recessive allele, that individual may be a carrier even if they are healthy
  • If we make the rare allele assumption we can assume it has not been introduced by any other pairings
  • Probabilities can be influenced by additional knowledge
multiple punnett squares
Multiple Punnett Squares
  • If someone\'s genotype is unknown, you may use each genotype to make a separate Punnett Square
    • Assume “Aa” and “AA” for that individual
    • Draw separate Punnett Squares for each crossing

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rare allele assumption
Rare Allele Assumption
  • If an unknown person has no family history of the disorder, you may instead assume they are homozygous dominant
    • This is the rare-allele assumption

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actual example of probability
Actual Example of Probability
  • Individual #1 has brown eyes
  • Individual 1\'s father has brown eyes, as does his entire family
  • Individual 1\'s mother has light blue eyes
  • Individual #2 has brown eyes
  • Individual #2\'s parents both had brown eyes
  • Individual #2\'s maternal grandfather had blue eyes
  • Using the rare allele assumption, what is the probability that #1 x #2 can produce blue eyes?
dihybrid crosses
Dihybrid Crosses
  • A dihybrid cross should have the same probabilities as each individual cross separately
    • Independence
  • Chromosomal linkage violates the independence pattern
    • Closely resembles a single Punnett Square for both alleles
    • Why not exact?
crossing over
Crossing Over
  • Imagine an X chromosome with both hemophilia and red-green colorblindness
  • Use this X chromosome as X\' in the following cross:
    • XY x X\'X
  • With crossing over in Meiosis Prophase I, the X woman\'s X chromosomes trade some genes
    • May then become XY x XHXC for hemophilia and colorblindness separately
dihybrid practice
Dihybrid Practice
  • Perform a dihybrid cross: AaX\'Y x AaX\'X
  • Assume X\' is a recessive defect. What is the probability that a boy will have the disorder? What is the probability that a girl will have the disorder?
  • What is the probability that a child will have both?
is this necessary
Is This Necessary?
  • The answers were obtainable by using individual Punnett Squares!
  • The rules may get more complicated:
    • Perform a AaZz x AaZz cross with the following phenotype rules:
      • If zz, individual is black
      • If has a dominant Z, individual phenotype depends on A:
        • If AA, individual is red
        • If Aa, individual is brown
        • If aa, individual dies at birth
    • Will see more polygenic traits in later chapters
pedigree practice
Pedigree Practice
  • Draw the pedigree for the following information:
    • Mother healthy, father afflicted, four children
    • 1st child: Boy, healthy, married, two healthy sons
    • 2nd child: Girl, healthy, married, one afflicted son, one healthy daughter, one healthy son
    • 3rd child: Girl, healthy, married, one afflicted son, two healthy daughters
    • 4th child: Boy, healthy, married, one healthy daughter
  • What is the pattern of inheritance?
pedigree practice1
Pedigree Practice
  • Everyone choose one of the five patterns and draw your own pedigree chart!
  • Be sure it has at least 3 generations and there should be at least five crosses of interest
  • Trade with a partner and analyze which pattern(s) it matches!
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