Today: Mendelian Genetics and Problems (In-Class 6) 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 1857- this monk (with extensive training in physics and botany) begins studying genetics Current dogma is Blended Inheritance 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? Mendel consistently observes this pattern of 3:1 ratios, and develops a 4 part HYPOTHESIS: 1. Alternate versions of different genes (alleles) account for variations in inherited characteristics. 2. For each character, an organism inherits two alleles, one from each parent. 1
Mendel consistently observes this pattern of 3:1 ratios, and develops a 4 part HYPOTHESIS: 3. If the two alleles differ, then one, the DOMINANT allele, controls the phenotype; the other, the RECESSEVE allele, has no noticeable effect on the organism s appearance. 4. The two alleles from each character segregate during gamete production (Mendel s Law of Segregation) For many traits, we can predict the genotypic frequencies of the offspring of two individuals using a PUNNETT SQUARE: Note that you could also do this mathematically! The PUNNETT Square constructed for Mendel s experiments predicts a 3:1 ratio. Simple Mendelian Inheritance- A Practice Problem (Part 1 A & B) Cystic Fibrosis is a Recessive Trait with Unusual Gene Frequencies Does this data support Mendel s hypotheses? 2
Understanding the predicted results of a PUNNETT SQUARE, allows for a TESTCROSS What s my phenotype? My genotype? Understanding the predicted results of a PUNNETT SQUARE, allows for a TESTCROSS! You Try! Part 1, Question 2 so All the examples considered far are MONOHYBRIDS Testing the Inheritance Patterns of Multiple Characters: Will Mendel s principles hold for the analysis of MULTIPLE CHARACTERS (DIHYBRID CROSSES)? 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 #1: (Mendel was lucky!) INCOMPLETE DOMINANCE 3
Complication #2: PLEIOTROPY (multiple effects) Example: Sickle-Cell Disease Complication #3: 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. Epistasis and Lab Pups Coat color in labradors is determined by 2 genes, a pigment gene (B), and a pigment delivery gene (E). Black is dominant to Brown, so Heterozygotes (Bb) are black. The delivery gene is also dominant, so EE or Ee individuals both express their pigments. Only ee individuals are yellow. Bonus Question: Epistasis and Lab Pups If I cross a Brown Lab (bbee) with a Black Lab (BbEe), can I expect any yellow puppies? If so, what proportion of the pups would I expect to be yellow? Complication #4: Codominance + Multiple Alleles Example: Human Blood Types Complication #4: Codominance + Multiple Alleles Example: Human Blood Types 4
Example: Paternity Testing Scenario : Suppose mother is Type A, baby is Type B. Other Issues: Individuals may display a range of small differences in traits, known as CONTINUOUS VARIATION Consider these three putative fathers: can any be the actual father? You Try! (Question 3) #1 (Type A): Yes or No? #2 (Type B): Yes or No? #3 (Type O): Yes or No? This usually indicates POLYGENIC INHERITANCE, where two or more genes create a single phenotypic character Other Issues: Environmental Effects on Phenotype Many factors, both genetic and environmental, influence the phenotype. Adding Chromosomes to the Inheritance Story! Sex Chromosomes & Autosomes Thomas Hunt Morgan s Contributions: Fruit Fly Genetics Single mating produces 100+ offspring A new generation can be bred every two weeks Only four pairs of chromosomes- 3 pairs of autosomes, 1 pair sex chromosomes (XX and XY) Unlike Mendel, Morgan does not have access to truebreeding strains. He breeds flies for a year, looking for distinct varieties. He discovers a male fly with white eyes, instead of red. In Drosophila, red eyes = Wild type (the most common phenotype in a natural population) white eyes = a Mutant Phenotype. 5
Morgan s Results: First Experiment: Morgan crosses a red-eyed female with a white-eyed male. ALL the offspring have red eyes. How would Mendel explain these results?? What would Mendel do next?? Morgan s Results: Next Experiment: Morgan crosses two of the red-eyed F1 flies with each other. What should he see if Mendel is correct?? He DOES find a 3:1 ratio, but ALL the whiteeyed flies are male!! Morgan s Results: Morgan Discovers Sex-Linked Genes! Was Mendel wrong?? What happened?!? The Transmission of SEX-LINKED Traits Practice Question: Sex-Linked Traits A = dominant allele carried on the X chromosome; a = the recessive allele. Note that both males and females are affected by sex-linked disorders! If you see the number 74, then you do not have redgreen 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. You Try! Question 4 & 5 6
More on Linked Genes: More on Linked Genes: We can use our understanding of Meiosis to think about WHY Mendel s Results were so predictable! So What happens if two genes are on the SAME chromosome?!? So What happens if two genes are on the SAME chromosome?!? So What happens if two genes are on the SAME chromosome?!? Cross-over! So why isn t this ratio what we expect for a dihybrid cross??? 7
Thought Question: Consider a chromosome containing 5 genes, A through E. Are genes A and B (or a and b) more or less likely to be inherited together than genes A and D? Explain your answer. 8