1. Mendelian Genetics Chapter Four

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

3. Moravian Sheep Breeders Association (1837) 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? Why do valued traits sometimes disappear and then reappear in some offspring?

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

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 Can follow discrete traits – no intermediate forms Used pure-breeding lines (8 generations) to form hybrid lines: offspring of dissimilar parents

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 was careful: many controls reciprocal crosses Mendel’s Experiments

9. x P parental F 1 first filial recessive – a trait that disappears in the hybrids (but may re-appear in subsequent generations) dominant – a trait “unchanged” in the hybrids Mendel’s Experiments Monohybrid Cross

10. x P parental F 1 first filial F2F2 smooth : wrinkled 2.96 : 1 5474 smooth, 1850 wrinkled Mendel’s Experiments Monohybrid Cross

11. 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 Mendel’s Experiments Monohybrid Cross

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

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

15. 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) Genetic Language:

16. YY yy Y Yy Homozygous Heterozygous Parental “Pure-breeding Lines” F 1 “Hybrid” Genetic Language:

17. Yy F 1 monohybrid self-fertilization Yy YYYy yy Yy Y y F2F2 Phenotype 3 : 1 BUT Genotype 1 : 2 : 1 Punnett Squares:

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

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

20. 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. Principle of segregation Discrete factors explained how a characteristic could persist through generations without blending and why it could “disappear and reappear” in subsequent generations Mendel’s 1 st Law:

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

23. 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 The probability of rolling two 2’s with a pair of dice: Mendel Understood Probability

24. 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 Mendel Understood Probability 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 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) The probability of rolling a 2 or a 5 = 1/6 + 1/6 = 1/3 Mendel Understood Probability

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

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

28. Practice Probability What is the probability that you will roll one dice and see: 1.A 3? 2.A odd number? 3.A 3 or a 4? Rolling two dice what is the probability to see: 1.Two 3’s (one on each dice)? 2.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 YYRRyyrr Homozygous Parental Pure-breeding lines for two traits Yellow or Green Seed Color (Y or y) Round or Wrinkled Shape (R or r)

34. YYRR yyrr P X F1F1 YR yr Were the two traits transmitted together or independently? Let’s check the F 2 Mendel’s Next Experiment: Dihybrid cross

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

36. 315 108 101 32 Four Phenotypes: In Reality F 2 Looks Like: 9 : 3 : 3 : 1 Mendel’s Next Experiment: Dihybrid cross

37. Four Phenotypes: F 2 offspring of Dihybrid cross new phenotypes recombinants Mendel’s Next Experiment: Dihybrid cross original phenotypes parental or non-recombinant

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

39. YR Therefore traits must be transmitted independently YryRyr(¼) YR Yr yR yr (¼) YYRRYyRRYYRr YyRr YYRrYYrrYyRrYyrr YyRR YyRr yyRRyyRr YyrryyRryyrr 12 round : 4 wrinkled 12 yellow : 4 green 9:3:3:1 3 : 1 Mendel’s Next Experiment: Dihybrid cross

40. Independent Assortment Mendel’s 2nd Law: 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 yyrrtt Tall plants Parental Short plants X Mendel’s 3rd Experiment: Trihybrid cross YyRrTt Tall plants F1F1 F2F2 27:9:9:9:3:3:3:1 Independent Assortment

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

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 11 th