Observing Patterns in Inherited Traits

1. Observing Patterns in Inherited Traits Chapter 11 Hsueh-Fen Juan Oct. 16, 2012

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. Video: Genetics of skin color

4. 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

5. 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

6. Garden Pea Plant: Self Fertilization and Cross-Fertilization 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 (等位基因) 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 (異型合子) • An individual with identical alleles of a gene is homozygous for that gene (同型合子)

11. Terms Used in Modern Genetics • An allele is dominant (顯性) if its effect masks the effect of a recessive (隱性) 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 (基因表現) • 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(基因型) • The particular alleles an individual carries • Phenotype (表型) • An individual’s observabletraits

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 (試交) • 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 (單性雜交) • Testcrosses that check for a dominance relationship between two allelesat a single locus (某基因位上，等位基因僅兩種才能用) • 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. Mendel’s Monohybrid Experiments

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

23. Construction of a Punnett Square

24. Phenotype Ratios in a Monohybrid Experiment

25. Phenotype Ratios in a Monohybrid Experiment

26. Mendel’s Law of Segregation (分離律) • 重點：單性雜交實驗驗證了分離律 • 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 in modern terms (當時尚未知染色體的存在) • 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

27. 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

28. 11.3 Mendel’s Law of Independent Assortment • Mendel’s law of independent assortment • Many genes are sorted into gametes independently of other genes

29. Dihybrid Experiments • Dihybrid experiments (兩性雜交) • 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

30. Independent Assortment at Meiosis 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

31. Mendel’s Dihybrid Experiments 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

32. 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

33. 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

34. 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(亦即一對等位基因之分離，與另一對等位基因之分離毫不相干) • 重點：兩性雜交實驗驗證了自由配合律 • Exception: Genes that have loci very close to one another on a chromosome tend to stay together during meiosis (基因連鎖則不成立)

35. 11.3 Key ConceptsInsights from Dihybrid Experiments • Some experiments yielded evidence of independent assortment: Genes are typically distributed into gametes independently of other genes

36. 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

37. Codominance in ABO Blood Types • Codominance (等顯性) • Two nonidentical alleles of a gene are both fully expressed in heterozygotes, so neither is dominant or recessive • May occur in multiple allele systems • Multiple allele systems (複對偶基因) • Genes with three or more alleles in a population • Example: ABO blood types

38. Codominance in ABO Blood Types

39. Incomplete Dominance • Incomplete dominance • One allele is not fully dominant over its partner • The heterozygote’s phenotype is somewhere between the two homozygotes, resulting in a 1:2:1 phenotype ratio in F2 offspring • Example: Snapdragon color • RR is red • Rr is pink • rr is white

40. Incomplete Dominance in Snapdragons

41. Fig. 11-11a, p. 176

42. Fig. 11-11b, p. 176

43. Epistasis • Epistasis (上位基因) • Two or more gene products influence a trait • Typically, one gene product suppresses the effect of another, so the resulting phenotype is somewhat unexpected. • 通常會有一個基因在眾基因的交互作用中佔優勢，稱此基因為上位基因(例：藍眼瑪莉花色基因) • Example: Coat color in dogs • Alleles B and b designate colors (black or brown) • Two recessive alleles ee suppress color • Allele E promotes the deposition of melanin in fur, but two recessive alleles (ee) reduce it.

44. Epistasis in Chicken Combs Dramatic variations in their combs

45. Animation: Comb shape in chickens

46. Epistasis in Coat Colors

47. EE/Ee/ee為上位基因，決定是否有黑色素，因此只要出現基因型ee，不論是BB, Bb或bb，皆因無黑色素而呈表型yellow eb eB EB Eb EeBb EeBB EEBB EEBb EB black black black black EEbb EeBb Eebb EEBb Eb chocolate black chocolate black EeBB EeBb eeBB eeBb eB black yellow yellow black eebb Eebb eeBb EeBb eb chocolate yellow yellow black Allele B (black) is dominant to b (brown). Allele E promotes the deposition of melanin in fur, but two recessive alleles (ee) reduce it. Fig. 11-13a, p. 177

48. Pleiotropy • Pleiotropy (基因多效性) • One gene product influences two or more traits • Example: Some tall, thin athletes have Marfan syndrome, a potentially fatal genetic disorder • 單基因(產物)多表型 FBN1, which encodes a connective protein called fibrillin-1 Long fibers of fibrillin impart elasticity to the tissues of the heart, skin, blood vessels, tendons and other body parts.

49. 11.5 Linkage Groups • The farther apart two genes are on a chromosome, the more often crossing over (互換) occurs between them • Linkage group (基因連鎖群) • All genes on one chromosome are called a linkage group • Linked genes are very close together; crossing over rarely occurs between them

50. Linkage and Crossing Over