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 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 ? Aa aAaaa aAaaa AA aAa a 2/31/3

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 ? Aa AAAAa AAAAa AA AAA A 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?

Probability Level: Expert

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 X H X C 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  1 st child: Boy, healthy, married, two healthy sons  2 nd child: Girl, healthy, married, one afflicted son, one healthy daughter, one healthy son  3 rd child: Girl, healthy, married, one afflicted son, two healthy daughters  4 th 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!

See you for the exam tomorrow!