Observing Patterns in Inherited Traits

1. Observing Patterns in Inherited Traits Chapter 11

2. Impacts, Issues:The Color of Skin • Like most human traits, skin color has a genetic basis; more than 100 gene products affect the synthesis and deposition of melanins

3. 11.1 Mendel, Pea Plants, and Inheritance Patterns • Recurring inheritance patterns are observable outcomes of sexual reproduction • Before the discovery of genes, it was thought that inherited traits resulted from a blend of parental characters

4. Mendel’s Experimental Approach • Mendel was a monk with training in plant breeding and mathematics • He studied the garden pea (Pisum sativum), which breeds true for a number of traits

5. Garden Pea Plant: Self Fertilization and Cross-Fertilization

6. carpel anther A Garden pea flower, cut in half. Sperm form in pollen grains, which originate in male floral parts (anthers). Eggs develop, fertilization takes place, and seeds mature in female floral parts (carpels). B Pollen from a plant that breeds true for purple flowers is brushed onto a floral bud of a plant that breeds true for white flowers. The white flower had its anthers snipped off. Artificial pollination is one way to ensure that a plant will not self-fertilize. C Later, seeds develop inside pods of the cross-fertilized plant. An embryo in each seed develops into a mature pea plant. D Each new plant’s flower color is indirect but observable evidence that hereditary material has been transmitted from the parent plants. Fig. 11-3, p. 170

7. Animation: Crossing garden pea plants

8. Terms Used in Modern Genetics • Genes • Heritable units of information about traits • Parents transmit genes to offspring • Each gene has a specific locus on a chromosome • Diploid cells (chromosome number 2n) have pairs of genes on homologous chromosomes

9. Terms Used in Modern Genetics • A mutation is a permanent change in a gene • May cause a trait to change • Alleles are different molecular forms of a gene • A hybrid has nonidentical alleles for a trait • Offspring of a cross between two individuals that breed true for different forms of a trait are hybrids

10. Terms Used in Modern Genetics • An individual with nonidentical alleles of a gene is heterozygous for that gene • An individual with identical alleles of a gene is homozygous for that gene

11. Terms Used in Modern Genetics • An allele is dominant if its effect masks the effect of a recessive allele paired with it • Capital letters (A) signify dominant alleles; lowercase letters (a) signify recessive alleles • Homozygous dominant (AA) • Homozygous recessive (aa) • Heterozygous (Aa)

12. Terms Used in Modern Genetics • Gene expression • The process by which information in a gene is converted to a structural or functional part of a cell or body • Expressed genes determine traits

13. Terms Used in Modern Genetics • Genotype • The particular alleles an individual carries • Phenotype • An individual’s observable traits

14. Terms Used in Modern Genetics • P stands for parents, F for filial (offspring) • F1: First generation offspring of parents • F2: Second generation offspring of parents

15. 11.1 Key ConceptsWhere Modern Genetics Started • Gregor Mendel gathered the first experimental evidence of the genetic basis of inheritance • His meticulous work gave him clues that heritable traits are specified in units • The units, which are distributed into gametes in predictable patterns, were later identified as genes

16. 11.2 Mendel’s Law of Segregation • Garden pea plants inherit two “units” of information for a trait, one from each parent

17. Testcrosses • Testcross • A method of determining if an individual is heterozygous or homozygous dominant • An individual with unknown genotype is crossed with one that is homozygous recessive (AA x aa) or (Aa x aa)

18. Monohybrid Experiments • Monohybrid experiments • Testcrosses that check for a dominance relationship between two alleles at a single locus • May be crosses between true breeding (homozygous) individuals (AA x aa), or between identical heterozygotes (Aa x Aa)

19. Mendel’s Monohybrid Experiments • Mendel used monohybrid experiments to find dominance relationships among pea plant traits • When he crossed plants that bred true for white flowers with plants that bred true for purple flowers, all F1 plants had purple flowers • When he crossed two F1 plants, ¾ of the F2 plants had purple flowers, ¼ had white flowers

20. Segregation of Alleles at a Gene Locus

21. homozygous dominant parent homozygous recessive parent (chromosomes duplicated before meiosis) meiosis I meiosis II (gametes) (gametes) fertilization produces heterozygous offspring Fig. 11-5, p. 172

22. homozygous dominant parent homozygous recessive parent (chromosomes duplicated before meiosis) meiosis I meiosis II (gametes) (gametes) fertilization produces heterozygous offspring Stepped Art Fig. 11-5, p. 172

23. Mendel’s Monohybrid Experiments

24. Trait Studied Dominant Form Recessive Form F2 Dominant-to- Recessive Ratio Seed shape 5,474 round 1,850 wrinkled 2.98 to 1 Seed color 6,022 yellow 2,001 green 3.01 to 1 Pod shape 882 inflated 299 wrinkled 2.95 to 1 Pod color 428 green 152 yellow 2.82 to 1 Flower color 705 purple 224 white 3.15 to 1 Flower position 651 along stem 3.14 to 1 207 at tip Stem length 787 tall 277 dwarf 2.84 to 1 Fig. 11-6, p. 172

25. Calculating Probabilities • Probability • A measure of the chance that a particular outcome will occur • Punnett square • A grid used to calculate the probability of genotypes and phenotypes in offspring

26. Construction of a Punnett Square

27. Phenotype Ratios in a Monohybrid Experiment

28. Phenotype Ratios in a Monohybrid Experiment

29. Fig. 11-7, p. 173

30. female gametes a A A a a a A A Aa A Aa A A A AA male gametes a a a a aa aa Aa aa aa Aa Aa A From left to right, step-by-step construction of a Punnett square. Circles signify gametes, and letters signify alleles: A is dominant; a is recessive. The genotypes of the resulting offspring are inside the squares. Fig. 11-7a, p. 173

31. F1 offspring aa True-breeding homozygous recessive parent plant a a Aa Aa A Aa Aa AA A Aa Aa True-breeding homozygous dominant parent plant Aa Aa B A cross between two plants that breed true for different forms of a trait produces F1 offspring that are identically heterozygous. Fig. 11-7b, p. 173

32. F2 offspring Aa Heterozygous F1 offspring A a AA Aa A Aa AA Aa aa Aa a Heterozygous F1 offspring Aa aa C A cross between the F1 offspring is the monohybrid experiment. The phenotype ratio of F2 offspring in this example is 3:1 (3 purple to 1 white). Fig. 11-7c, p. 173

33. Mendel’s Law of Segregation • Mendel observed a phenotype ratio of 3:1 in the F2 offspring of his monohybrid crosses • Consistent with the probability of the aa genotype in the offspring of a heterozygous cross (Aa x Aa) • This is the basis of Mendel’s law of segregation • Diploid cells have pairs of genes on pairs of homologous chromosomes • The two genes of each pair separate during meiosis, and end up in different gametes

34. 11.2 Key ConceptsInsights from Monohybrid Experiments • Some experiments yielded evidence of gene segregation: When one chromosome separates from its homologous partner during meiosis, the alleles on those chromosomes also separate and end up in different gametes

35. 11.3 Mendel’s Law of Independent Assortment • Mendel’s law of independent assortment • Many genes are sorted into gametes independently of other genes

36. Dihybrid Experiments • Dihybrid experiments • Tests for dominance relationships between alleles at two loci • Individuals that breed true for two different traits are crossed (AABB x aabb) • F2 phenotype ratio is 9:3:3:1 (four phenotypes) • Individually, each dominant trait has an F2 ratio of 3:1 – inheritance of one trait does not affect inheritance of the other

37. Independent Assortment at Meiosis

38. One of two possible alignments The only other possible alignment a Chromosome alignments at metaphase I: A A a a A A a a b B b B b b B B b The resulting alignments at metaphase II: a a A A A A a a b b b b B B B B a c Possible combinations of alleles in gametes: B A A B b a a b b A A b B a B AB ab Ab aB Fig. 11-8, p. 174

39. One of two possible alignments The only other possible alignment a Chromosome alignments at metaphase I: A A a a A A a a b b B b B b B B b The resulting alignments at metaphase II: a a A A A A a a b b b b B B B B a c Possible combinations of alleles in gametes: B A A B b a a b b A A b B a B AB ab Ab aB Stepped Art Fig. 11-8, p. 174

40. Mendel’s Dihybrid Experiments

41. Fig. 11-9a, p. 175

42. parent plant homozygous for purple flowers and long stems parent plant homozygous for white flowers and short stems P generation A Meiosis in homozygous individuals results in one kind of gamete. aabb AABB B A cross between plants homozygous for two different traits yields one possible combination of gametes: x ab AB Fig. 11-9a, p. 175

43. Fig. 11-9b, p. 175

44. AaBb AaBb AaBb All F1 offspring are AaBb, with purple flowers and tall stems. F1 generation C Meiosis in AaBb dihybrid plants results in four kinds of gametes: Ab ab AB aB F2 generation These gametes can meet up in one of 16 possible wayswhen the dihybrids are crossed (AaBb X AaBb): Fig. 11-9b, p. 175

45. Fig. 11-9c, p. 175

46. aB Ab ab AB AB AABB AABb AaBB AaBb AABb AAbb AaBb Aabb Ab aB aaBB AaBB AaBb aaBb AaBb Aabb aaBb aabb ab D Out of 16 possible genetic outcomes of this dihybrid cross, 9 will result in plants that are purple-flowered and tall; 3, purple-flowered and short; 3, white-flowered and tall; and 1, white-flowered and short. The ratio of phenotypes of this dihybrid cross is 9:3:3:1. Fig. 11-9c, p. 175

47. Animation: Dihybrid cross

48. Mendel’s Law of Independent Assortment • Mendel’s dihybrid experiments showed that “units” specifying one trait segregated into gametes separately from “units” for other traits • Exception: Genes that have loci very close to one another on a chromosome tend to stay together during meiosis

49. 11.3 Key ConceptsInsights from Dihybrid Experiments • Some experiments yielded evidence of independent assortment: Genes are typically distributed into gametes independently of other genes

50. 11.4 Beyond Simple Dominance • Mendel focused on traits based on clearly dominant and recessive alleles; however, the expression patterns of genes for some traits are not as straightforward