Inheritance, Genes, and Chromosomes

1. Inheritance, Genes, and Chromosomes

2. 12 Inheritance, Genes, and Chromosomes 12.1 What Are the Mendelian Laws of Inheritance? 12.2 How Do Alleles Interact? 12.3 How Do Genes Interact? 12.4 What Is the Relationship between Genes and Chromosomes? 12.5 What Are the Effects of Genes Outside the Nucleus? 12.6 How Do Prokaryotes Transmit Genes?

3. 12 Inheritance, Genes, and Chromosomes The population of Tasmanian devils was reduced by hunting and diseases, and the remaining individuals are closely related. Now a type of cancer threatens the population, spreading due to the genetic relatedness. Opening Question: How can knowledge of genetics be used to save the Tasmanian devil?

4. 12.1 What Are the Mendelian Laws of Inheritance? Humans have been deliberately breeding plants and animals for thousands of years. Two theories emerged to explain breeding experiments: 1. Blending inheritance—gametes contain hereditary determinants that blend in the zygote. Offspring phenotypes are intermediate.

5. 12.1 What Are the Mendelian Laws of Inheritance? 2. Particulate inheritance—hereditary determinants are distinct and remain intact at fertilization. Experiments performed by the monk, Gregor Mendel, supported the particulate theory.

6. Figure 12.1 Gregor Mendel and His Garden

7. 12.1 What Are the Mendelian Laws of Inheritance? Mendel’s theory of inheritance was published in 1866 but was largely ignored until 1900. By that time chromosomes had been discovered and biologists realized that genes (hereditary determinants) might be carried on chromosomes.

8. 12.1 What Are the Mendelian Laws of Inheritance? Mendel worked with the garden pea, which has both male and female sex organs and normally self-fertilizes.

9. 12.1 What Are the Mendelian Laws of Inheritance? Mendel could control pollination and fertilization by removing the male organs and manually pollinating the flowers.

10. 12.1 What Are the Mendelian Laws of Inheritance? Pea plants have many varieties with easily recognized characteristics. Character: observable physical feature (e.g., seed shape) Trait: form of a character (e.g., round or wrinkled seeds) Mendel worked with true-breeding varieties.

11. 12.1 What Are the Mendelian Laws of Inheritance? Mendel developed hypotheses to explain inheritance of different traits, then designed crossing experiments to test them. • He transferred pollen from one plant to another: the parental generation, P • The seeds and offspring were the first filial generation, F1

12. 12.1 What Are the Mendelian Laws of Inheritance? • In some experiments the F1 plants were allowed to self-pollinate and produce a second filial generation, F2

13. 12.1 What Are the Mendelian Laws of Inheritance? Mendel first performed monohybrid crosses: crossing parental varieties with contrasting traits for a single character. • The F1 offspring were not a blend of the two parental traits. Only one of the traits was present (e.g., round seeds). • Some F2 had wrinkled seeds. The trait had not disappeared because of blending. These results supported the particulate theory.

14. Figure 12.2 Mendel’s Monohybrid Experiments (Part 1)

15. Figure 12.2 Mendel’s Monohybrid Experiments (Part 2)

16. 12.1 What Are the Mendelian Laws of Inheritance? Mendel made monohybrid crosses for seven traits; all gave similar results. The trait that occurred in the F1 and was more abundant in the F2 was called dominant, the other recessive. In the F2 the ratioof dominant to recessive traits was about 3:1.

17. Figure 12.2 Mendel’s Monohybrid Experiments (Part 1)

18. Figure 12.2 Mendel’s Monohybrid Experiments (Part 2)

19. 12.1 What Are the Mendelian Laws of Inheritance? Mendel proposed that hereditary determinants (genes) occur in pairs and segregate from one another during formation of gametes. He also proposed that each pea plant has two genes for each character, one inherited from each parent.

20. 12.1 What Are the Mendelian Laws of Inheritance? Diploid: the state of having two copies of each gene Haploid: having just a single copy

21. 12.1 What Are the Mendelian Laws of Inheritance? Different traits arise from different forms of a gene (now called alleles). • An organism that is homozygous for a gene has two alleles that are the same. • An organism that is heterozygous for a gene has two different alleles. One may be dominant, (e.g., round [R]), and the other recessive, (e.g., wrinkled [r]).

22. 12.1 What Are the Mendelian Laws of Inheritance? Phenotype is the physical appearance of an organism. Genotype is the genetic constitution of the organism. Mendel proposed that the phenotype is the result of the genotype.

23. 12.1 What Are the Mendelian Laws of Inheritance? Mendel’s first law— Thelaw of segregation: the two copies of a gene separate during gamete formation; each gamete receives only one copy.

24. Working with Data 12.1: Mendel’s Monohybrid Experiments Mendel’s monohybrid crosses were key to rejecting the blending theory of inheritance. Mendel calculated ratios in the F2 generation, but did not do statistical analyses to determine whether the observed patterns might be due to chance alone.

25. Working with Data 12.1: Mendel’s Monohybrid Experiments Mendel’s data from the F2 generation after crossing green- and yellow-seeded plants:

26. Working with Data 12.1: Mendel’s Monohybrid Experiments Use the hypothesis that the ratio of yellow to green seeds in the F2 generation, 3:1, and perform a chi-square test to analyze the results for each plant in the table. Question 1: What can you conclude about this hypothesis from the individual plants? How many crosses have P-values > 0.05?

27. Working with Data 12.1: Mendel’s Monohybrid Experiments Now total the data from all the plants and rerun the chi-square analysis. Question 2: What can you conclude? What does your analysis indicate about the need for using a large number of organisms in studies of genetics?

28. Figure 12.3 Mendel’s Explanation of Inheritance (Part 1)

29. 12.1 What Are the Mendelian Laws of Inheritance? In the F2 generation, half of the gametes will have the R allele and the other half will have the r allele. Allele combinations can be predicted using a Punnett square.

30. In-Text Art, Ch. 12, p. 236

31. Figure 12.3 Mendel’s Explanation of Inheritance (Part 2)

32. 12.1 What Are the Mendelian Laws of Inheritance? There are four possible combinations of alleles in the F2 generation: RR, Rr, rR, and rr. If R is dominant, there are three ways to get round seeds, and only one way to get wrinkled seeds, resulting in the 3:1 phenotype ratio.

33. 12.1 What Are the Mendelian Laws of Inheritance? Genes are now known to be short sequences of DNA; a DNA molecule makes up a chromosome. Alleles of a gene can separate during meiosis I.

34. Figure 12.4 Meiosis Accounts for the Segregation of Alleles (Part 1)

35. Figure 12.4 Meiosis Accounts for the Segregation of Alleles (Part 2)

36. 12.1 What Are the Mendelian Laws of Inheritance? Genes determine phenotypes through the proteins they encode. Dominant genes are expressed; recessive genes may be mutated and no longer expressed, or encode non-functional proteins. Wrinkled seed phenotype is due to absence of starch branching enzyme (SBE1).

37. 12.1 What Are the Mendelian Laws of Inheritance? One of Mendel’s hypotheses: there are two possible allele combinations (RR or Rr) for seeds with the round phenotype. He tested this hypothesis by doing test crosses: F1 individuals are crossed with homozygous recessive individuals (rr). His hypothesis accurately predicted the results of his test crosses.

38. Figure 12.5 Homozygous or Heterozygous?

39. 12.1 What Are the Mendelian Laws of Inheritance? Mendel’s second law— Independent assortment: copies of different genes assort independently. To test this he crossed true-breeding peas that differed in 2 characteristics: seed shape and color. Round, yellow seeds (RRYY) Wrinkled, green seeds (rryy)

40. 12.1 What Are the Mendelian Laws of Inheritance? F1 generation is RrYy—all round yellow. Crossing the F1 generation (double heterozygotes) is a dihybrid cross. Mendel asked whether, in the gametes produced by RrYy, the traits would be linked, or segregate independently.

41. 12.1 What Are the Mendelian Laws of Inheritance? • If linked, gametes would be RY or ry; F2 would have three times more round yellow than wrinkled green. • If independent, gametes could be RY, ry, Ry, or rY. F2 would have nine different genotypes; phenotypes would be in 9:3:3:1 ratio.

42. Figure 12.6 Independent Assortment (Part 1)

43. Figure 12.6 Independent Assortment (Part 2)

44. 12.1 What Are the Mendelian Laws of Inheritance? The experiments supported the hypothesis of independent assortment. It doesn’t always apply to genes located on the same chromosome. But it is correct to say that chromosomes segregate independently during formation of gametes, and so do any two genes located on separate chromosome pairs.

45. Figure 12.7 Meiosis Accounts for Independent Assortment of Alleles (Part 1)

46. Figure 12.7 Meiosis Accounts for Independent Assortment of Alleles (Part 2)

47. 12.1 What Are the Mendelian Laws of Inheritance? One key to Mendel’s success was large sample sizes. By counting many progeny, he was able to see clear patterns. Later, geneticists began using probability calculations to predict ratios of genotypes and phenotypes, and statistical techniques to determine whether actual results matched predictions.

48. 12.1 What Are the Mendelian Laws of Inheritance? 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.

49. 12.1 What Are the Mendelian Laws of Inheritance? The multiplication rule— Probability of two independent events happening together: multiply the probabilities of the individual events. Tossing two coins: probability that both will come up heads = ½ × ½ = ¼

50. Figure 12.8 Using Probability Calculations in Genetics