Inheritance, Genes, and Chromosomes

1. 8 Inheritance, Genes, and Chromosomes

2. Chapter 8 Inheritance, Genes, and Chromosomes • Key Concepts • 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • 8.2 Alleles and Genes Interact to Produce Phenotypes • 8.3 Genes Are Carried on Chromosomes • 8.4 Prokaryotes Can Exchange Genetic Material

3. Chapter 8 Opening Question How is hemophilia inherited through the mother, and why is it more frequent in males?

4. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • Early experiments with genetics yielded two theories: • Blending inheritance—gametes contained determinants (genes) that blended when gametes fused during fertilization • Particulate inheritance—each determinant was physically distinct and remained intact during fertilization

5. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • Mendel used the scientific method and studied garden peas. • Their flowers have both male and female sex organs, pistils, and stamens, to produce gametes. • Male organs can be removed to allow fertilization by another flower.

6. In-Text Art, Ch. 8, p. 145

7. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • Character—observable physical feature (e.g., flower color, seed shape) • Trait—form of a character (e.g., purple flowers or white flowers, wrinkled seeds) • Mendel worked with true-breeding varieties—when plants of the same variety were crossed, all offspring plants produced the same seeds and flowers.

8. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • Mendel’s crosses: • Pollen from one parent was transferred to the stigma of the other parent. • Parental generation = P • Resulting offspring = first filial generation or F1 • If F1 plants self-pollinate, they produce second filial generation or F2.

9. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • In Mendel’s first experiment, he crossed plants differing in just one character (P). • This produced monohybrids in the F1 generation. • The monohybrids were then allowed to self-pollinate to form the F2 generation—a monohybrid cross. • Mendel repeated this for seven characters.

10. Figure 8.1 Mendel’s Monohybrid Experiments (Part 1)

11. Figure 8.1 Mendel’s Monohybrid Experiments (Part 2)

12. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • One trait of each pair disappeared in the F1 generation and reappeared in F2 —these traits are recessive. • The trait that appears in the F1 is the dominanttrait. • The ratio of dominant traits to recessive traits in the F2 was about 3:1.

13. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • Mendel’s observations rejected the blending theory of inheritance and supported the particulate theory. • He proposed that the determinants occur in pairs and are segregated in the gametes. • Each plant has two genes for each character, one from each parent. • Diploid—two copies of a gene • Haploid—one copy of a gene

14. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • Alleles are different forms of a gene, such as smooth or wrinkled seeds. • True-breeding individuals have two copies of the same allele—they are homozygous for the allele (e.g., ss). • Heterozygous individuals have two different alleles (e.g., Ss).

15. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • Phenotype—physical appearance of an organism (e.g., spherical seeds) • Genotype—the genetic makeup (e.g., Ss) • Spherical seeds can be the result of two different genotypes—SS or Ss.

16. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • Mendel’s first law: • Thelaw of segregation states that the two copies of a gene separate when an individual makes gametes. • Each gamete receives only one copy.

17. Figure 8.2 Mendel’s Explanation of Inheritance (Part 1)

18. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • When the F1 self-pollinates, there are three ways to get the dominant trait (e.g., spherical), but only one way to get the recessive (wrinkled)—resulting in the 3:1 ratio. • Allele combinations can be predicted using a Punnett square.

19. Figure 8.2 Mendel’s Explanation of Inheritance (Part 2)

20. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • A gene is a short sequence on a longer DNA molecule. • DNA molecules make up the chromosomes. • Different alleles of a gene segregate when chromosomes separate during meiosis I.

21. Figure 8.3 Meiosis Accounts for the Segregation of Alleles (Part 1)

22. Figure 8.3 Meiosis Accounts for the Segregation of Alleles (Part 2)

23. Figure 8.3 Meiosis Accounts for the Segregation of Alleles (Part 3)

24. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • Mendel tested his hypothesis by doing test crosses: • He did this to determine whether an individual is homozygous or heterozygous for a trait by crossing it with a homozygous recessive individual. • Mendel crossed the F1 with known homozygotes (e.g., wrinkled or ss).

25. Figure 8.4 Homozygous or Heterozygous? (Part 1)

26. Figure 8.4 Homozygous or Heterozygous?

27. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • Mendel’s next experiment involved: • Crossing peas that differed in two characters—seed shape and seed color • True-breeding parents: • SSYY—spherical yellow seeds • ssyy—wrinkled green seeds

28. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • F1 generation is SsYy—all spherical yellow. • Crossing the F1 generation (all identical double heterozygotes) is a dihybrid cross. • Mendel asked whether, in the gametes produced by SsYy, the traits would be linked, or segregate independently.

29. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • Two possibilities included: • Alleles could maintain associations seen in parental generation—they could be linked • If linked, gametes would be SY or sy; F2 would have three times more spherical yellow than wrinkled green. • If independent, gametes could be SY, sy, Sy, or sY. F2 would have nine different genotypes; phenotypes would be in 9:3:3:1 ratio.

30. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • Or: • The segregation of S from s could be independent of Y from y—the two genes could be unlinked • If independent, gametes could be SY, sy, Sy, or sY in equal numbers. • The F2 generation would have nine different genotypes; and four phenotypes in a 9:3:3:1 ratio. • This prediction was supported.

31. Figure 8.5 Independent Assortment (Part 1)

32. Figure 8.5 Independent Assortment (Part 2)

33. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • Mendel’s second law: • Thelaw of independent assortment states that alleles of different genes assort independently during gamete formation. • This law doesn’t always apply to genes on the same chromosome, but chromosomes do segregate independently.

34. Figure 8.6 Meiosis Accounts for Independent Assortment of Alleles

35. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • One of Mendel’s contributions to genetics was the use of mathematical analyses—the rules of statistics and probability. • His analyses revealed patterns that allowed him to formulate his hypotheses. • Probability calculations and Punnett squares give the same results.

36. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • Probability • If an event is certain to happen, probability = 1 • If an event cannot possibly happen, probability = 0 • All other events have a probability between 0 and 1

37. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • Two coin tosses are independent events, each will come up heads ½ the time. • The probability that both will come up heads is: • ½ x ½ = ¼ • To get the joint probability, multiply the individual probabilities (multiplication rule).

38. Figure 8.7 Using Probability Calculations in Genetics

39. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • Probability in a monohybrid cross • After self-pollination of an F1 Ss, the probability that the F2 offspring will have the genotype SS is ½ x ½ = ¼; the same for ss offspring. • There are two ways to get a heterozygote Ss; the probability is the sum of the individual probabilities (addition rule): • ¼ + ¼ = ½

40. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • Human pedigrees can show Mendel’s laws. • Humans have few offspring; pedigrees do not show the clear proportions that the pea plants showed. • Geneticists use pedigrees to determine whether a rare allele is dominant or recessive.

41. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • Pattern of inheritance for a rare dominant allele: • Every person with the abnormal phenotype has an affected parent. • Either all (if homozygous parent) or half (if heterozygous parent) of offspring in an affected family are affected.

42. Figure 8.8 Pedigree Analysis and Inheritance (Part 1)

43. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • Pattern of inheritance for a rare recessive allele: • Affected people often have two unaffected parents. • In an affected family, one-fourth of children of unaffected parents are affected.

44. Figure 8.8 Pedigree Analysis and Inheritance (Part 2)

45. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • The great “chestnut vs. black” debate • Shortly after the rediscovery of Mendel’s pea plant experiments, biologists began investigating whether Mendel’s laws applied to other species besides peas. In England, a debate broke out about whether the color of racehorses might be governed by Mendel’s laws. Using the British Jockey Club registry of racehorse pedigrees, biologists noted: • A chestnut (red) stallion, when bred to chestnut mares, always produced chestnut foals. • Certain black stallions, when bred to chestnut mares, produced only black foals. • Other black stallions, when bred to chestnut mares, produced a mix of black foals and chestnut foals. • Discuss with a partner: • 1. Which is dominant: black, chestnut, or neither? Why? • 2. Would you expect horses with the dominant allele to be superior in some way? (For example, would they be faster, or healthier, or produce more offspring?) • 3. Consider a cross of black × chestnut. Is this a monohybrid cross, dihybrid cross, or a test cross? What will the results of this cross tell you? Can one cross (one foal) be informative?

46. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • Which is dominant, black or chestnut? • a. Black • b. Chestnut • c. Neither; black and chestnut are incompletely dominant. • d. I don’t know.

47. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • Coin tossing for fun and (not much) profit • Working in small groups, make predictions about the results of tossing a dime and a penny: • What is the probability that both coins will come up heads? • What is the probability that one coin (either coin) will come up heads, and the other will come up tails? • Are your predictions the same in each case, or different? Why? • Now each member of your group will flip a real dime and real penny once. Each member of your team should record his or her own data for the next question. Then tally up the group data. • Look at the overall data for your group. Did the results match your predictions?

48. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • What were your personal results? (Report just the results of your own dime & penny toss - not the results of your whole group.) • a. Both were heads. • b. Dime heads, penny tails. • c. Dime tails, penny heads. • d. Both were tails. • Do the overall class data match your predictions? Do they differ from the results of your small group? Why or why not? • If your predictions were not supported by the class data, explain why not. • If your predictions were supported by the class data, now think about how you derived those predictions. What mathematical procedures were you using when you calculated your two predictions? Why?

49. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • Testing your probability intuition: The two-child puzzle • Working in pairs or small groups, consider the following brainteaser: • A family has two children: a newborn and a toddler. • At least one of the children is a boy. • What is the probability that the other child is a girl? • Discuss with your classmates, and be prepared to explain your reasoning to the rest of the class.

50. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • A family has two children, at least one of which is a boy. What is the probability that the other is a girl? • a. 1/4 • b. 1/3 • c. 1/2 • d. 2/3 • e. 3/4