Genetic probabilities
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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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2/3

1/3


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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2/3

1/3


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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