Today: Mendelian Genetics

Mendelian Genetics: Consider this…. 8 million possible chromosome combinations in each egg, and each sperm… = >70 trillion possibilities! How are we able to predict ANYTHING about inheritance??

With all these possibilities, how can we predict anything about inheritance? Gregor Mendel this monk (with extensive training in physics and botany) begins studying genetics Current dogma??

Mendel’s Technique: Studies peas- Typically Self- Fertilizing Multiple distinct CHARACTERS, with easy to identify TRAITS Several TRUE- BREEDING varieties available

What Mendel Observes: What does this data suggest about “blended inheritance”?

For many traits, we can predict the genotypic frequencies of the offspring of two individuals using a PUNNETT SQUARE:

The PUNNETT Square constructed for Mendel’s experiments predicts a 3:1 ratio. The data support Mendel’s Hypothesis!

Simple Mendelian Inheritance- A Practice Problem Cystic Fibrosis is a Recessive Trait with Unusual Gene Frequencies 1. If two carriers of the cystic fibrosis trait marry, what is the probability that their first child will be affected? 2. It they eventually have three children, what is the probability that all three will be affected?

Complication #1: (Mendel was lucky!) INCOMPLETE DOMINANCE Heterozygotes have a unique phenotype, between that of the homozygous dominant or recessive parents. Note: This is not blended inheritance!

Complication #2: PLEIOTROPY (multiple effects) Example: Sickle-Cell Disease

Complication #3: Codominance + Multiple Alleles Example: Human Blood Types

Example: Paternity testing Scenario : Suppose mother is Type A, baby is Type B. Consider these three putative fathers: can any be the actual father? #1 (Type A) #2 (Type B) #3 (Type O)

Complication #4: EPISTASIS Example: The “color gene”, C, allows pigment to be deposited in hair. When lacking, a mouse is albino, regardless of its genotype at the other locus.

This usually indicates POLYGENIC INHERITANCE, where two or more genes create a single phenotypic character Other Issues: Individuals may display a range of small differences in traits, known as CONTINUOUS VARIATION

Many factors, both genetic and environmental, influence the phenotype. Other Issues: Environmental Effects on Phenotype

A More Recent Complication… If you see the number 74, then you do not have red-green color blindness. If you see the number 21, you are color blind to some extent. A totally color-blind person will not be able to see any of the numbers. If you see the number 74, then you do not have red-green color blindness. If you see the number 21, you are color blind to some extent. A totally color-blind person will not be able to see any of the numbers.

Human males are more likely to be color-blind than human females. Why???

Chromosomes!

Is this what Mendel would expect?

If a color blind man marries a “wild-type” woman, what are the chances that a daughter of theirs will be colorblind? If a color blind man marries a “wild-type” woman, what are the chances that a daughter of theirs will be colorblind? What are the chances that their son will be colorblind? What are the chances that their son will be colorblind? Can females be colorblind? What would the genotype of the parents have to be? Can females be colorblind? What would the genotype of the parents have to be? Practice Question: Chromosomal Inheritance

Connecting Mendel to Lab: Reminder: One of the restriction enzymes (molecular scissors) we used can cut the normal hemoglobin gene, but not the mutant sickle version. normal Sickle cell allele