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!