Mendel and the Gene Idea: Experimenting with Peas for Genetic Traits

1. Chapter 14 • Mendel and the gene idea

2. Drawing from the Deck of Genes • What genetic principles account for the transmission of traits from parents to offspring?

3. One possible explanation  “blending” hypothesis

4. Alternative to the blending model is “particulate” hypothesis of inheritance: the gene idea • Parents pass on discrete heritable units, genes

5. Figure 14.1 • Gregor Mendel • Particulate mechanism of inheritance, experimented with garden peas

6. Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments

7. Used a quantitative approach • (from Christian Doppler)

8. Mendel chose to work with peas because: • Available in many varieties • He could strictly control mating

9. Removed stamens from purple flower 1 APPLICATION By crossing (mating) two true-breeding varieties of an organism, scientists can study patterns of inheritance. In this example, Mendel crossed pea plants that varied in flower color. 2 Transferred sperm- bearing pollen from stamens of white flower to egg- bearing carpel of purple flower TECHNIQUE TECHNIQUE RESULTS TECHNIQUE Parental generation (P) Stamens (male) Carpel (female) Pollinated carpel matured into pod 3 Planted seeds from pod 4 When pollen from a white flower fertilizes eggs of a purple flower, the first-generation hybrids all have purple flowers. The result is the same for the reciprocal cross, the transfer of pollen from purple flowers to white flowers. Examined offspring: all purple flowers 5 First generation offspring (F1) • Crossing pea plants Figure 14.2

10. Genetic vocabulary • Character: a heritable feature, e.g. flower color • Trait: a variant of a character, e.g. purple or white flowers

11. Mendel chose to track • Characters that varied in an “either-or” manner • Varieties that were “true-breeding”

12. Mendel mated two contrasting, true-breeding varieties (hybridization) • The true-breeding parents called P generation

13. Hybrid offspring • F1 generation • When F1 individuals self-pollinate • F2 generation produced

14. The Law of Segregation • Crossed contrasting, true-breeding white and purple flowered pea plants • All offspring were purple • When F1 plants crossed • Many offspring plants were purple, but some were white

15. P Generation (true-breeding parents)  Purple flowers White flowers EXPERIMENT True-breeding purple-flowered pea plants and white-flowered pea plants were crossed (symbolized by ). The resulting F1 hybrids were allowed to self-pollinate or were cross- pollinated with other F1 hybrids. Flower color was then observed in the F2 generation. F1 Generation (hybrids) All plants had purple flowers F2 Generation RESULTS Both purple-flowered plants and white- flowered plants appeared in the F2 generation. In Mendel’s experiment, 705 plants had purple flowers, and 224 had white flowers, a ratio of about 3 purple : 1 white. • Mendel discovered • A ratio of about three to one, purple to white flowers, in the F2 generation Figure 14.3

16. Mendel reasoned that • In the F1, only the purple flower factor was affecting flower color in these hybrids • Purple flower color was dominant, and white flower color was recessive

17. Table 14.1 • Mendel observed same pattern in other pea plant characters

18. Mendel’s hypothesis: • Explains the 3:1 inheritance pattern in F2 4 concepts make up model 

19. Allele for purple flowers Homologous pair of chromosomes Locus for flower-color gene Allele for white flowers Figure 14.4 • First, alternative versions of genes • Variations in inherited characters, called alleles

20. Second, for each character • Organism inherits two alleles, one f/ each parent • A genetic locus is actually represented twice

21. Third, if the two alleles at a locus differ • One, the dominant allele*, determines the organism’s appearance • The other, the recessive allele*, has no noticeable effect on appearance * The expression of the allele is dominant or recessive not the allele itself

22. Fourth, the law of segregation • The two alleles for a heritable character separate (segregate) during gamete formation different gametes

23. P Generation Each true-breeding plant of the parental generation has identical alleles, PP or pp. Gametes (circles) each contain only one allele for the flower-color gene. In this case, every gamete produced by one parent has the same allele.  Appearance:Genetic makeup: Purple flowersPP White flowerspp Gametes: p P F1 Generation Union of the parental gametes produces F1 hybrids having a Pp combination. Because the purple- flower allele is dominant, all these hybrids have purple flowers. When the hybrid plants produce gametes, the two alleles segregate, half the gametes receiving the P allele and the other half the p allele. Appearance:Genetic makeup: Purple flowersPp 1/2 Gametes: 1/2 p P F1 sperm F2 Generation This box, a Punnett square, shows all possible combinations of alleles in offspring that result from an F1 F1 (PpPp) cross. Each square represents an equally probable product of fertilization. For example, the bottom left box shows the genetic combination resulting from a p egg fertilized by a P sperm. p P P Pp PP F1 eggs p pp Pp Random combination of the gametes results in the 3:1 ratio that Mendel observed in the F2 generation. 3 : 1 Figure 14.5 Mendel’s law of segregation, probability and the Punnett square

24. If homozygous f/ a particular gene • Pair of identical alleles • True-breeding • If heterozygous f/ a particular gene • Pair of alleles that are different

25. Phenotype • Physical appearance • Genotype • Genetic makeup

26. Phenotype Genotype Purple PP (homozygous) 1 Pp (heterozygous) 3 Purple 2 Pp (heterozygous) Purple pp (homozygous) White 1 1 Ratio 3:1 Ratio 1:2:1 Figure 14.6 • Phenotype v. genotype

27. Testcross • In pea plants with purple flowers, genotype is not obvious

28.  Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp APPLICATION An organism that exhibits a dominant trait, such as purple flowers in pea plants, can be either homozygous forthe dominant allele or heterozygous. To determine the organism’s genotype, geneticists can perform a testcross. TECHNIQUE In a testcross, the individual with the unknown genotype is crossed with a homozygous individual expressing the recessive trait (white flowers in this example). By observing the phenotypes of the offspring resulting from this cross, we can deduce the genotype of the purple-flowered parent. If PP, then all offspring purple: If Pp, then 1⁄2 offspring purple and 1⁄2 offspring white: p p p p RESULTS P P Pp Pp Pp Pp P p Pp pp Pp pp • Testcross Figure 14.7

29. Law of segregation • Determined by following a single trait • F1 were monohybrids, heterozygous for one character

30. The Law of Independent Assortment • Follow 2 characters at the same time • Crossing two, true-breeding parents differing in two characters dihybrids in the F1 generation, heterozygous for both characters

31. How are two characters transmitted f/ parents to offspring? • As a package? • Independently?

32. P Generation YYRR yyrr Gametes YR yr  F1 Generation YyRr Hypothesis of independent assortment Hypothesis of dependent assortment Sperm 1 ⁄4 Yr 1 ⁄4 YR 1 ⁄4 yR yr 1 ⁄4 Sperm Eggs 1⁄2 yr 1⁄2 YR 1 ⁄4 YR Eggs YyRr YYRR YYRr YyRR 1 ⁄2 YR F2Generation (predicted offspring) YYRR YyRr 1 ⁄4 Yr YYrr YyRr Yyrr YYrr yr 1 ⁄2 yyrr YyRr 1 ⁄4 yR YyRR YyRr yyRR yyRr 3 ⁄4 1 ⁄4 yr 1 ⁄4 Phenotypic ratio 3:1 YyRr Yyrr yyRr yyrr 1 ⁄16 3 ⁄16 3 ⁄16 9 ⁄16 Phenotypic ratio 9:3:3:1 315 108 101 Phenotypic ratio approximately 9:3:3:1 32 • Dihybrid cross • 4 phenotypes in the F2 generation

33. Law of independent assortment • Each pair of alleles segregates independently during gamete formation

34. The multiplication rule • Probability that two or more independent events will occur together is the product of their individual probabilities

35. Rr Segregation of alleles into sperm  Rr Segregation of alleles into eggs  Sperm r R 1⁄2 1⁄2 R R r R R 1⁄2 1⁄4 1⁄4 Eggs r r R r 1⁄2 r 1⁄4 1⁄4 Figure 14.9 • Probability in a monohybrid cross

36. Rule of addition • Probability that any one of two or more exclusive events will occur is sum of individual probabilities • e.g. probability of a heterozygote in a Tt xTt cross ¼ prob. of Tt and ¼ prob. Of tT  ¼ + ¼ = ½ probability of heterozygote

37. Dihybrid • Equivalent to two independent monohybrid crosses occurring simultaneously • When calculating: • Multiply individual probabilities

38. Inheritance patterns are often more complex than predicted by simple Mendelian genetics • Relationship between genotype and phenotype is rarely simple

39. The Spectrum of Dominance • Complete dominance • Phenotypes of the heterozygote and dominant homozygote are identical

40. Codominance • 2 dominant alleles affect the phenotype in separate, distinguishable ways • e.g. human blood group MN

41. P Generation White CWCW Red CRCR  Gametes CR CW Pink CRCW F1 Generation 1⁄2 1⁄2 Gametes CR CR 1⁄2 Sperm 1⁄2 CR CR Eggs F2 Generation 1⁄2 CR CR CR CR CW 1⁄2 Cw CW CW CR CW • Incomplete dominance • Phenotype of F1 hybrids between phenotypes of the two parental varieties Figure 14.10

42. Relation Between Dominance and Phenotype • Dominant and recessive alleles • Do not really “interact” • Lead to synthesis of different proteins that produce a phenotype

43. Dominant alleles • Not necessarily more common in populations than recessive alleles

44. Table 14.2 Multiple alleles • e.g. ABO blood group in humans • More than 2 alleles

45. Pleiotropy • Gene has multiple phenotypic effects • e.g. sickle-cell disease

46. e.g. Epistasis • Gene at one locus alters the phenotypic expression of a gene at second locus

47. BbCc BbCc  Sperm 1⁄4 bC 1⁄4 Bc 1⁄4 BC bc 1⁄4 Eggs 1⁄4 BBCC BbCC BbCc BC BBCc 1⁄4 bC BbCC bbCC bbCc BbCc 1⁄4 BBcc BBCc BbCc Bbcc Bc bbcc Bbcc BbCc 1⁄4 bbCc bc 4⁄16 9⁄16 3⁄16 • Example of epistasis Figure 14.11

48.  AaBbCc AaBbCc aabbcc Aabbcc AABBCc AABBCC AaBbcc AaBbCc AABbCc 20⁄64 15⁄64 Fraction of progeny 6⁄64 1⁄64 Polygenic inheritance • Quantitative variation  polygenic inheritance • Additive effect of two or more genes, continuum Figure 14.12

49. Nature and Nurture: Environmental Impact on Phenotype • Phenotype depends on environment & genotype

50. Figure 14.13 • Norm of reaction • Phenotypic range of a particular genotype that is influenced by the environment