Mendelian Genetics

1. Mendelian Genetics Chapter Four

2. Theories of Inheritance Homunculus (Ancient Greeks – 17th ce) sperm caries a miniature human that uses egg as a growth medium (spermists) Problem: Doesn’t explain why kids sometimes look like their mom Blending Descendents possess traits that are intermediate between those of parents, become mixed and forever changed in the offspring. Problem: Over time, a population would become uniform in appearance Once blended, traits should not reappear in subsequent generations • Pangenesis • Heredity units (pangenes) are formed in all organs, spread • through blood to genitals • Problem: Blood transfusions into experimental animals • did not change characteristics of progeny.

3. Moravian Sheep Breeders Association (1837) Why do valued traits sometimes disappear and then reappear in some offspring? • Breeders could predict the traits of offspring if they could answer the basic questions: • What is inherited? • How is it inherited? • What is the role of chance in heredity?

4. Gregor J. Mendel (1822-1884) Monastery of St. Thomas Brno, Czech Republic Versuche über Pflanzen-Hybriden "Experiments in Plant Hybridization" Society for the Study of the Natural Sciences Proceedings (1866)

5. Pisum sativum Mendel chose a great “model organism” • Self fertilizing plants that can be cross-fertilized • Relatively quick generation time • Can grow large numbers of plants in limited space • Used pure-breeding lines (8 generations) to form • hybrid lines: offspring of dissimilar parents • Can follow discrete traits – no intermediate forms

6. Mendel’s Experiments • Studied 7 characteristics of • pure-breeding lines: • Seed color (yellow vs. green) • Seed shape (round vs. wrinkled) • Flower color (purple vs. white) • Pod color (green vs. yellow) • Pod shape (round vs. pinched) • Stem length (long vs. short) • Flower position (along stem vs. at the tip) “either - or” phenotypes with no intermediates

7. Mendel’s Experiments • Pure Breeding Lines: • Crossing two of same phenotype always produces one phenotype • Hybrids: • Crossing two of same phenotype can lead to offspring of two phenotypes • Example – cross two tall plants, offspring are a combination of tall and short plants

8. Cross fertilization emasculation Mendel’s Experiments • Mendel was careful: • many controls • reciprocal crosses

9. P parental x F1 first filial dominant – a trait “unchanged” in the hybrids recessive – a trait that disappears in the hybrids (but may re-appear in subsequent generations) Mendel’s Experiments Monohybrid Cross

10. P parental x F1 first filial F2 Mendel’s Experiments Monohybrid Cross 5474 smooth, 1850 wrinkled smooth : wrinkled 2.96 : 1

11. Mendel’s Experiments Monohybrid Cross Seed shape 5474 smooth, 1850 wrinkled 2.96 : 1 Seed color 6022 yellow, 2001 green 3.01 : 1 Flower color 705 purple, 224 white 3.15 : 1 Pod color 428 green, 152 yellow 2.82 : 1 Pod shape 882 round, 299 pinched 2.95 : 1 Stem length 787 long, 277 short 2.84 : 1 Flower position 858 stem, 651 tip 3.14 : 1 3 : 1 Dominant : Recessive

12. Mendel’s Deductions Proposed that “unit factors” exist in pairs to explain these results Each parent has two unit factors but contributes only one to every progeny in the form of gametes Designated upper-case as Dominant and lower-case as Recessive

13. yy YY Y y Mendel’s Deductions Seed coat color Dominant Recessive All offspring will be yellow and will be heterozygotes

14. Genetic Language: Gene - Discrete “unit factors” of inheritance Allele - Different forms of a gene (e.g. Y or y) Genotype - Allelic composition of a trait (e.g. YY, Yy, or yy) Phenotype - Physical manifestation of a trait (e.g. Yellow or green seed)

15. Genetic Language: Homozygous – Individuals with two identical copies of a gene Same allele (yy) Heterozygous - Individuals with two different copies of a gene Two different alleles (Yy)

16. yy YY Genetic Language: Parental “Pure-breeding Lines” Y Homozygous Homozygous Yy F1 “Hybrid” Heterozygous

17. Yy YY Yy Y y Yy yy Punnett Squares: F1 monohybrid self-fertilization Y y F2 Yy Phenotype 3 : 1 BUT Genotype 1 : 2 : 1

18. Genotype vs. Phenotype YY Yy yy Heterozygous hybrid Homozygous recessive Homozygous dominant Same phenotype How do you distinguish between the two?

19. y y y y Y Yy Yy YY yy yy Yy Yy Yy Y Y y Yy yy Yy yy Test Cross: If homozygous, all progeny are Yellow If heterozygous, progeny 1 : 1 Yellow : Green

20. Mendel’s 1st Law: • Principle of segregation • Hereditary traits are determined by discrete factors (now • called genes) that appear in pairs. During sexual development, • these pairs are separated (segregated) into gametes and only • one factor from each parent is passed to the offspring. Discrete factors explained how a characteristic could persist through generations without blending and why it could “disappear and reappear” in subsequent generations

21. Practice Your Punnetts! • Draw the punnett squares • Calculate # of each genotype and phenotype • Yy cross yy (Y = yellow, y = green) • Yy cross Yy • Rr cross rr (R = round, r = wrinkled) • RR cross rr • BB cross Bb (B = brown, b = blue) • bb cross bb

22. The probability of rolling a 2 with one roll of one die: Mendel Understood Probability Probability:The number of times an event is expected to occur divided by the number of trials during which that event could have happened 1 event / 6 possible outcomes = 1/6

23. The probability of rolling two 2’s with a pair of dice: Mendel Understood Probability The Multiplication Rule:The probability of two or more independent events occurring simultaneously is the product of their individual probabilities. The probability of rolling a 2 = 1/6 So rolling two 2’s = 1/6 x 1/6 =1/36

24. Mendel Understood Probability In the cross Yy x Yy , what is the probability of yielding 3 yy offspring? The probability is ½ that a y will be contributed by one parent p(y) = ½ The probability is ½ that a y will be contributed by the other parent p(y) = ½ The probability of having one yy offspring ½ x ½ = ¼ p(yy) = ¼ The probability of having three yy offspring ¼ x ¼ x ¼ = 1/64

25. The probability of rolling a 2 or a 5 = 1/6 + 1/6 = 1/3 Mendel Understood Probability The Addition Rule: The probability that an event can occur in two or more alternative ways is the sum of the separate probabilities of the different ways. (Used to answer “either / or” questions only)

26. Mendel Understood Probability In the cross Yy x Yy , what is the probability of yielding yellow seeded offspring (Yy or YY)? The probability of being YY p(YY) = ¼ The probability of being Yy p(Yy) = ½ The probability of being either YY or Yy: ¼ + ½ = ¾

27. Probability One more thing to remember: p(a mutually exclusive event) = 1 – p(all the other events)

28. Practice Probability • What is the probability that you will roll one dice and see: • A 3? • A odd number? • A 3 or a 4? • Rolling two dice what is the probability to see: • Two 3’s (one on each dice)? • A 3 and a 4?

29. Mode of Inheritance The pattern that the trait follows in families: Four Mendelian: • Autosomal (non-sex chromosome) Recessive • Autosomal Dominant • X-linked Recessive • X-linked Dominant Also complex inheritance • will be covered later

30. Autosomal Traits: Both Males and Females affected, and both transmit to both sexes of offspring • Recessive – usually rare in population • Skips Generations • Inbreeding increases risk of recessive traits • Dominant – more common • Doesn’t skip generations • Complex

31. X-Linked Traits: Gene on X chromosome is carrying trait. • Recessive • Only males are affected • Passed from unaffected mothers to sons • Affected fathers will only transmit to heterozygous, unaffected daughters • Dominant • Males and females both affected • Can be passed to both offspring, however often see more females affected because of male lethality • Affected fathers to every single daughter

32. Two genes • Now lets examine what happens when we look at more than one gene at a time: • Two Traits • Two different genes • Two alleles per gene • Genes are each on separate chromosomes

33. Mendel’s Next Experiment: Dihybrid cross YYRR yyrr Parental Pure-breeding lines for two traits Homozygous Homozygous Yellow or Green Seed Color (Y or y) Round or Wrinkled Shape (R or r)

34. Y R y r F1 Mendel’s Next Experiment: Dihybrid cross yyrr YYRR X P Let’s check the F2 Were the two traits transmitted together or independently?

35. YR (½) yr (½) YR Two Phenotypes (½) ¼ ¼ YYRR YyRr YyRr 3 : 1 yr yellow round green wrinkled (½) ¼ ¼ yyrr Mendel’s Next Experiment: Dihybrid cross YyRr NOPE ! YyRr Traits transmitted together

36. Mendel’s Next Experiment: Dihybrid cross In Reality F2 Looks Like: Four Phenotypes: 315 108 101 32 9 : 3 : 3 : 1

37. new phenotypes recombinants original phenotypes parental or non-recombinant Mendel’s Next Experiment: Dihybrid cross F2 offspring of Dihybrid cross Four Phenotypes:

38. Y R Y r y R y r (¼) (¼) (¼) (¼) = 1 + + + Mendel’s Next Experiment: Dihybrid cross YyRr 4 different possible gametes

39. (¼) (¼) (¼) (¼) 12 yellow 12 round 4 wrinkled 4 green : : YyRr YYRR YYRr YyRR (¼) YYRr YYrr YyRr Yyrr (¼) YyRR YyRr yyRR yyRr (¼) YyRr Yyrr yyRr yyrr (¼) 3 : 1 Mendel’s Next Experiment: Dihybrid cross YR Yr yR yr YR Yr 9:3:3:1 yR yr Therefore traits must be transmitted independently

40. Mendel’s 2nd Law: Independent Assortment Inheritance of a pair of factors for one trait is independent of the simultaneous inheritance of factors for another trait Two genes will assort independently and randomly

41. YyRrTt F1 Tall plants F2 27:9:9:9:3:3:3:1 Mendel’s 3rd Experiment: Trihybrid cross yyrrtt YYRRTT Parental X Tall plants Short plants Independent Assortment

42. Mendel’s Laws 1. Principle of Segregation Two alleles segregate randomly during formation of gametes 2. Independent Assortment Two genes will assort independently and randomly from each other

43. Practice Your Punnetts! • Draw the punnett squares for two genes • Calculate # of each genotype and phenotype (Y = yellow, y = green) (R = round, r = wrinkled) • YyRr x YyRr • YYRr x Yyrr

44. Pedigree Analysis Pedigrees are visual ways to examine a family’s inheritance pattern for any trait of interest • Identify: • Relationships between family members • Who has trait of interest (phenotype) Mode of inheritance

45. Pedigree Analysis Insert Figure 4.13

46. Autosomal Recessive

47. Autosomal Dominant

48. Complex Inheritance

49. Next Class: • Read Chapter Five • Homework – Chapter Four Problems; • Review: 1,3,5, 7 • Applied: 1,2,4,5, 8, 9,10, 11,15 • Pedigree Assignment – Due October 11th