Genetic Probabilities

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# Genetic Probabilities - PowerPoint PPT Presentation

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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## PowerPoint Slideshow about ' Genetic Probabilities' - steven-meyers

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Presentation Transcript
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
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

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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
• 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!