Patterns of Inheritance: From Mendel to Modern Understanding

1. Chapter 12 Patterns of Inheritance

2. BACK IN THE DAY…. • Theories on Inheritance • Bloodlines • Heridity occurs within species • Traits are transmitted directly from parents to offspring

3. Josef Kolreuter • Hybridizations • First generation looked like neither parents • Second generation was variable

4. T.A. Knight • Worked with yellow and green peas • Observed all F1 generation were yellow, but F2 generation varied between green and yellow

5. Mendel • Worked with pea plants • Round vs wrinkled • Yellow vs green seed • White vs purple flower • Green vs yellow pod • Inflated vs constricted pod • Terminal vs axial flower • Tall vs short height

6. Mendel’s experimental design • Allowed plants to self cross for many generations to ensure purity • Crossed pollen of plants true breeding for specific characteristics • Permitted hybrid offspring to self fertilize for several generations

7. Monohybrid Crosses

8. Always showed similar results • F1 generation exhibited only one of the two traits without blending….what theory did this disprove? • F2 generation exhibits both traits in a 3:1 ratio • Actually a 1:2:1 ratio…why? • Genotype vs Phenotype

9. What was Mendel able to understand based on his observations? • 1. Plants did not follow blended inheritance, traits were discrete characteristics • 2. For each pair of alternatives for a trait the one that was not shown in the F1 generation was still present, but it was latent • 3. Pairs of alternative traits were segregated among the offspring • 4. Mendelian Ratio ALWAYS seen

10. Mendel’s Model • 1. Parents transmit discrete information to offspring not direct physiological traits now called genes • 2. Each individual receives two genes that encode each trait. ( where are the genes found?) • 3. Not all genes are identical- alternative forms called alleles • Homozygous • Heterozygous • 4. Two alleles remain discrete • 5. The presence of an allele does not ensure that the trait it encodes will be expressed.

11. Principle of Segregation • Two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent during fertilization

12. Punnett Squares

13. Dominant Traits in Humans • Middigital hair • Brachydactyly • Huntington Disease • PTC sensitivity • Polydactyly • Hypercholesterolemia

14. Recessive Traits • Albinism • Red-green color blindness • Cystic fibrosis • Duchenne muscular dystrophy • Hemophilia • Sickle cell anemia

15. Pedigree

16. Independent Assortment • Alleles of each gene assort independantly • What does this mean? • Pea color does not necessarily connected to plant height • Demonstrated in Dihybrid Crosses

17. Dihybrid Cross • You are looking at 4 different traits • F1 generation only shows 2 of the 4 traits…Why? • F2 generation 9:3:3:1 ratio

19. Probability • Rule of addition • Probability of tow mutually exclusive events • This OR that • = Probability of event 1 + probability of even 2 • Suppose a high school consists of 25% juniors, 15% seniors, and the remaining 60% is students of other grades. The relative frequency of students who are either juniors and seniors is 40%. We can add the relative frequencies of juniors and seniors because no student can be both junior and senior. • P(J or S) = 0.25 + 0.15which equals 0.40

20. Probability • Rule of Multiplication • Probability of This AND that occuring • =Probability of Event A * probability of Event B • Suppose we have two dice. A is the event that 4 shows on the first die, and B is the event that 4 shows on the second die. If both dice are rolled at once, what is the probability that two 4s occur? • P(A) = 1/6P(B) = 1/6P(A and B) = P(A) . P(B) = 1/6 . 1/6 = 1/36

21. Rules can be applied to Dyhibrid cross • Example • Look at F1 generation • What are the odds of Wrinkled Seeds? • What are the odds of Green Seeds? • Multiply them together to get the odds of wrinkled green seeds.

22. Test Cross • If you do not know what genotype an organism has you can cross it with an individual who is homozygous recessive for the trait to find out.

23. Breaking the Rules • Instances when Medel’s Laws may not be followed. • Polygenic Inheritance • Pleiotropy • Multiple alleles • Dominance is not always complete • Environmental Factors • Gene Interaction

24. Polygenic Inheritance • More than one gene affecting a single trait • Example Height or skin color

25. Pleiotropy • A single gene affects more than one trait • Example Cystic Fibrosis, albinism • Mouse Study- Yellow Mice Die

26. Multiple Alleles for One Gene • More than one allele controlling the phenotype • Example ABO blood types

27. Incomplete Dominance • Incomplete dominance is when the heterozygote is intermediate in appearance between the two homozygote.

28. Codominance • When the heterozygote shows some piece of the phenotype of both homozygotes • Ex. ABO blood types fur color in some animals

29. ABO Blood Types

30. Genes can be affected by the environment • Example Temperature effects on Siamese Cats • Temp above 33 degrees no pigment • Temp Below 33 degrees pigment

31. Epistasis • An interaction between genes in which one gene is altered by another gene • Example Labrador Retrievers • Black is dominant to chocolate B or bYellow is recessive epistatic (when present, it blocks the expression of the black and chocolate alleles) E or e