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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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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 • 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 • 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 • 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 • 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 ? 2/3 1/3
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 ? 2/3 1/3
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 • 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 • 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 • 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? • 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 • 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 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!
