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
?
A a
a Aa aa
a Aa aa
A A
a Aa Aa
a Aa Aa
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?
A a
A AA Aa
A AA Aa
A A
A AA AA
A AA AA
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 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!
See you for the exam tomorrow!