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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
- 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

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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

- Perform a AaZz x AaZz cross with the following phenotype rules:

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!

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