Inheritance, Genes, and Chromosomes

1. 8 Inheritance, Genes, and Chromosomes

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

3. 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

4. Concept 8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws • Mendel used the scientific method and studied garden peas.

5. 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.

6. 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.

7. 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.

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

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

10. 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.

11. 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

12. 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).

13. 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.

14. 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.

15. 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.

16. 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).

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

18. Figure 8.4 Homozygous or Heterozygous?

19. 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

20. 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.

21. 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.

22. Figure 8.5 Independent Assortment (Part 1)

23. Figure 8.5 Independent Assortment (Part 2)

24. 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.

25. 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.

26. 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

27. 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).

28. Figure 8.7 Using Probability Calculations in Genetics

29. 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

30. 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.

31. 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.

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

33. 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.

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

35. Concept 8.2 Alleles and Genes Interact to Produce Phenotypes • Different alleles arise through mutation— rare, stable, inherited changes in the genetic material. • The wild type is the allele present in most of the population. Other alleles are mutant alleles. • A gene with a wild-type allele that is present less than 99 percent of the time is called polymorphic.

36. Concept 8.2 Alleles and Genes Interact to Produce Phenotypes • A given gene may have more than two alleles. • Multiple alleles increase the number of possible phenotypes and may show a hierarchy of dominance in heterozygotes. • One example is the coat color in rabbits.

37. Concept 8.2 Alleles and Genes Interact to Produce Phenotypes • Some alleles are neither dominant nor recessive. • A heterozygote has an intermediate phenotype in incomplete dominance. • Red + white snapdragons = pink in F1 • Red and white colors reappear in F2 as well as pink.

38. Figure 8.10 Incomplete Dominance Follows Mendel’s Laws (Part 1)

39. Figure 8.10 Incomplete Dominance Follows Mendel’s Laws (Part 2)

40. Concept 8.2 Alleles and Genes Interact to Produce Phenotypes • Codominance—two alleles of a gene produce phenotypes that are both present in the heterozygote. • Example: • ABO blood group system has three alleles of the gene: IA, IB, and IO.

41. Figure 8.11 ABO Blood Reactions Are Important in Transfusions

42. Concept 8.2 Alleles and Genes Interact to Produce Phenotypes • Epistasis—phenotypic expression of one gene is influenced by another gene • Example: • Coat color in Labrador retrievers: • Allele B (black) dominant to b (brown) • Allele E (pigment deposition) is dominant to e (no pigment deposition—yellow)

43. Figure 8.12 Genes Interact Epistatically

44. Concept 8.2 Alleles and Genes Interact to Produce Phenotypes • Hybrid vigor, orheterosis, is a cross between two different true-breeding homozygotes. • It can result in offspring with stronger, larger phenotypes. • Most complex phenotypes are determined by multiple genes. • Quantitative traits conferred by multiple genes are measured, rather than assessed qualitatively.

45. In-Text Art, Ch. 8, p. 154

46. Concept 8.2 Alleles and Genes Interact to Produce Phenotypes • Genotype and environment interact to determine the phenotype of an organism. • Two parameters describe the effects: • Penetrance is the proportion of individuals with a certain genotype that show the phenotype. • Expressivity is the degree to which genotype is expressed in an individual.

47. Concept 8.3 Genes Are Carried on Chromosomes • Genes are sequences of DNA that reside at a particular site on a chromosome—a locus (plural loci). • The genetic linkage of genes on a single chromosome can alter their patterns of inheritance.

48. Concept 8.3 Genes Are Carried on Chromosomes • Genetic linkage was discovered by Thomas Hunt Morgan and students at Columbia University using the fruit fly Drosophila melanogaster. • Much genetic research has been done with Drosophila, which is considered a model organism because of its size, ease of breeding, and short generation time.

49. Concept 8.3 Genes Are Carried on Chromosomes • Some crosses performed with Drosophila did not yield expected ratios according to the law of independent assortment. • Instead, some genes for body color and wing shape were inherited together. • Morgan theorized that the two loci were linked on the same chromosome and could not assort independently.

50. Figure 8.13 Some Alleles Do Not Assort Independently (Part 1)