Chapter 14

1. Chapter 14 • Overview: Drawing from the Deck of Genes • What genetic principles account for the transmission of traits from parents to offspring?

2. One possible explanation of heredity is a “blending” hypothesis

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

4. Figure 14.1 • Gregor Mendel • Documented a particulate mechanism of inheritance through his experiments with garden peas

5. Identification of 2 Laws of Inheritance • Concept 14.1: • Mendel discovered the basic principles of heredity • By breeding garden peas in carefully planned experiments • available in many varieties • Could control mating

6. Some genetic vocabulary: • Character: a heritable feature, such as flower color • Trait: a variant of a character, such as purple or white flowers

7. Mendel chose to track • Only those characters that varied in an “either-or” manner • Mendel also made sure that • He started his experiments with varieties that were “true-breeding” • True breeding plants self-pollinate, all offspring are of the same variety

8. In a typical breeding experiment • Mendel mated two contrasting, true-breeding varieties, a process called hybridization • The true-breeding parents • Are called the P generation

9. The hybrid offspring of the P generation • Are called the F1 generation • When F1 individuals self-pollinate • The F2 generation is produced

10. The Law of Segregation • When Mendel crossed contrasting, true-breeding white and purple flowered pea plants • All of the offspring were purple • When Mendel crossed the F1 plants • Many of the plants had purple flowers, but some had white flowers

11. 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

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

13. Mendel’s Model • Mendel developed a hypothesis • To explain the 3:1 inheritance pattern that he observed among the F2 offspring • Four related concepts make up this model

14. Allele for purple flowers Homologous pair of chromosomes Locus for flower-color gene Allele for white flowers Figure 14.4 Mendel’s Model • First, alternative versions of genes • Account for variations in inherited characters, which are now called alleles

15. Second, for each character • An organism inherits two alleles, one from each parent • A genetic locus is actually represented twice

16. Mendel’s Model • Third, if the two alleles at a locus differ • Then one, the dominant allele, determines the organism’s appearance • The other allele, the recessive allele, has no noticeable effect on the organism’s appearance

17. Mendel’s Model • Fourth, the law of segregation • The two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes

18. Punnett Square • Does Mendel’s segregation model account for the 3:1 ratio he observed in the F2 generation of his numerous crosses? • We can answer this question using a Punnett square

19. 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 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. F2 Generation 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 Punnett Square • Mendel’s law of segregation, probability and the Punnett square

20. Useful Genetic Vocabulary • An organism that is homozygous for a particular gene • Has a pair of identical alleles for that gene • Exhibits true-breeding • An organism that is heterozygous for a particular gene • Has a pair of alleles that are different for that gene

21. An organism’s phenotype • Is its physical appearance • An organism’s genotype • Is its genetic makeup

22. 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 versus genotype

23. The Testcross • In pea plants with purple flowers • The genotype is not immediately obvious

24. A testcross • Determines the genotype of an organism with the dominant phenotype, but unknown genotype • Crosses an individual of the dominant phenotype with an individual that is homozygous recessive for a trait

25.  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 • The testcross Figure 14.7

26. The Law of Independent Assortment • Mendel derived the law of segregation • By following a single trait • The F1 offspring produced in this cross • Were monohybrids, heterozygous for one character

27. Mendel identified his second law of inheritance • By following two characters at the same time • Crossing two, true-breeding parents differing in two characters • Produces dihybrids in the F1 generation, heterozygous for both characters

28. How are two characters transmitted from parents to offspring? • As a package? • Independently?

29. EXPERIMENT Two true-breeding pea plants—one with yellow-round seeds and the other with green-wrinkled seeds—were crossed, producing dihybrid F1 plants. Self-pollination of the F1 dihybrids, which are heterozygous for both characters, produced the F2 generation. The two hypotheses predict different phenotypic ratios. Note that yellow color (Y) and round shape (R) are dominant. P Generation YYRR yyrr  Gametes YR yr F1 Generation YyRr Hypothesis of independent assortment Hypothesis of dependent assortment Sperm Yr 1 ⁄4 1 ⁄4 YR 1 ⁄4 yR yr 1 ⁄4 Sperm Eggs 1⁄2 yr 1⁄2 YR RESULTS 1 ⁄4 YR Eggs YyRr YYRR YYRr YyRR 1 ⁄2 YR F2 Generation (predicted offspring) YYRR YyRr 1 ⁄4 Yr YYrr YyRr Yyrr YYrr yr 1 ⁄2 yyrr YyRr 1 ⁄4 yR YyRR YyRr yyRR yyRr CONCLUSION The results support the hypothesis of independent assortment. The alleles for seed color and seed shape sort into gametes independently of each other. 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 • A dihybrid cross • Illustrates the inheritance of two characters • Produces four phenotypes in the F2 generation Figure 14.8

30. Using the information from a dihybrid cross, Mendel developed the law of independent assortment • Each pair of alleles segregates independently during gamete formation

31. Complex Patterns of Inheritance • Concept 14.3: • The relationship between genotype and phenotype is rarely simple

32. Extending Mendelian Genetics for a Single Gene • The inheritance of characters by a single gene • May deviate from simple Mendelian patterns

33. The Spectrum of Dominance • Complete dominance • Occurs when the phenotypes of the heterozygote and dominant homozygote are identical

34. 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 • In incomplete dominance • The phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties Figure 14.10

35. In codominance • Two dominant alleles affect the phenotype in separate, distinguishable ways • The human blood group AB • Is an example of codominance

36. Multiple Alleles • Most genes exist in populations • In more than two allelic forms

37. Table 14.2 • The ABO blood group in humans • Is determined by multiple alleles

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

39. Frequency of Dominant Alleles • Dominant alleles • Are not necessarily more common in populations than recessive alleles

40. Pleiotropy • In pleiotropy • A gene has multiple phenotypic effects • That is, one gene controls many physical things, like multiple symptoms of a disease

41. Extending Mendelian Genetics for Two or More Genes • Some traits • May be determined by two or more genes

42. Epistasis 1) In epistasis • A gene at one locus alters the phenotypic expression of a gene at a second locus

43. 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 • An example of epistasis Figure 14.11

44. Polygenic Inheritance 2) Many human characters • Vary in the population along a continuum and are called quantitative characters • Usually indicates ploygenic inheritance – additive effect of two or more genes on a single phenotype character

45.  AaBbCc AaBbCc aabbcc Aabbcc AABBCc AABBCC AaBbcc AaBbCc AABbCc 20⁄64 15⁄64 Fraction of progeny 6⁄64 1⁄64 • Quantitative variation usually indicates polygenic inheritance • An additive effect of two or more genes on a single phenotype Figure 14.12

46. Nature and Nurture: The Environmental Impact on Phenotype • Another departure from simple Mendelian genetics arises • When the phenotype for a character depends on environment as well as on genotype

47. Multifactorial characters • Are those that are influenced by both genetic and environmental factors • Examples: height, build, skin color • NOT things like blood type

48. Human Traits and Mendelian Genetics • Concept 14.4: • Humans are not convenient subjects for genetic research

49. Pedigree Analysis • A pedigree • Is a family tree that describes the interrelationships of parents and children across generations

50. First generation (grandparents) ww ww Ww Ww Ff Ff ff Ff Second generation (parents plus aunts and uncles) ww Ww Ww ww Ww Ff ww ff Ff FF or Ff Ff ff Third generation (two sisters) ff FF or Ff WW or Ww ww No Widow’s peak Free earlobe Widow’s peak Attached earlobe (a) Dominant trait (widow’s peak) (b) Recessive trait (attached earlobe) • Inheritance patterns of particular traits • Can be traced and described using pedigrees Figure 14.14 A, B