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Chapter 14

Chapter 14. Mendel and the gene idea. Drawing from the Deck of Genes What genetic principles account for the transmission of traits from parents to offspring?. One possible explanation  “blending” hypothesis.

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Chapter 14

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  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

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