Chapter 13 Observing Patterns in Inherited Traits (Sections 13.4 - 13.6)

1. Chapter 13Observing Patterns in Inherited Traits (Sections 13.4 - 13.6)

2. 13.4 Mendel’s Theory of Independent Assortment • When homologous chromosomes separate during meiosis, either one of the pair can end up in a particular nucleus • Thus, gene pairs on one chromosome get sorted into gametes independently of gene pairs on other chromosomes • Punnett squares can be used to predict inheritance patterns of two or more genes simultaneously

3. Dihybrid Cross • In a dihybrid cross, individuals identically heterozygous for alleles of two genes (dihybrids) are crossed, and the traits of the offspring are observed • dihybrid cross • Breeding experiment in which individuals identically heterozygous for two genes are crossed • The frequency of traits among the offspring offers information about the dominance relationships between the paired alleles

4. A Dihybrid Cross • Start with one parent plant that breeds true for purple flowers and tall stems (PPTT ) and one that breeds true for white flowers and short stems (pptt) • Each plant makes only one type of gamete (PT or pt) • All F1 offspring will be dihybrids (PpTt) and have purple flowers and tall stems

5. A Dihybrid Cross (cont.) • Then cross two F1 plants: a dihybrid cross (PpTt X PpTt) • Four types of gametes can combine in sixteen possible ways • In F2 plants, four phenotypes result in a ratio of 9:3:3:1 • 9 tall with purple flowers • 3 short with purple flowers • 3 tall with white flowers • 1 short with white flowers

6. Law of Independent Assortment • Mendel discovered the 9:3:3:1 ratio in his dihybrid experiments – and noted that each trait still kept its individual 3:1 ratio • Each trait (gene pair) sorted into gametes independently of other traits (gene pairs) • law of independent assortment • During meiosis, members of a pair of genes on homologous chromosomes get distributed into gametes independently of other gene pairs

7. Independent Assortment

8. Independent Assortment A This example shows just two pairs of homologous chromosomes in the nucleus of a diploid (2n) reproductive cell. Maternal and paternal chromosomes, shown in pink and blue, have already been duplicated. B Either chromosome of a pair may get attached to either spindle pole during meiosis I. With two pairs of homologous chromosomes, there are two different ways that the maternal and paternal chromosomes can get attached to opposite spindle poles. or meiosis I meiosis I C Two nuclei form with each scenario, so there are a total of four possible combinations of parental chromosomes in the nuclei that form after meiosis I. meiosis II meiosis II D Thus, when sister chromatids separate during meiosis II, the gametes that result have one of four possible combinations of maternal and paternal chromosomes. gamete genotype: pt PT pT Pt Fig. 13.7, p. 194

9. A This example shows just two pairs of homologous chromosomes in the nucleus of a diploid (2n) reproductive cell. Maternal and paternal chromosomes, shown in pink and blue, have already been duplicated. B Either chromosome of a pair may get attached to either spindle pole during meiosis I. With two pairs of homologous chromosomes, there are two different ways that the maternal and paternal chromosomes can get attached to opposite spindle poles. or meiosis I meiosis I C Two nuclei form with each scenario, so there are a total of four possible combinations of parental chromosomes in the nuclei that form after meiosis I. meiosis II meiosis II D Thus, when sister chromatids separate during meiosis II, the gametes that result have one of four possible combinations of maternal and paternal chromosomes. gamete genotype: pt PT pT Pt Independent Assortment Stepped Art Fig. 13.7, p. 194

10. A Dihybrid Cross

11. A Dihybrid Cross PT Pt pT pt 4 PP PPTT PPTt PpTT PpTt parent plant homozygous for purple flowers and long stems parent plant homozygous for white flowers and short stems Pt PPTt PPtt PpTt Pptt pptt PPTT pT PpTT PpTt ppTT ppTt pt PT 1 PpTt 2 dihybrid pt PpTt Pptt ppTt pptt PT Pt pT pt four types of gametes 3 Fig. 13.8, p. 195

12. A Dihybrid Cross Fig. 13.8.1-3, p. 195

13. A Dihybrid Cross Fig. 13.8.4, p. 195

14. A Dihybrid Cross PT Pt pT pt 4 PP PPTT PPTt PpTT PpTt parent plant homozygous for purple flowers and long stems parent plant homozygous for white flowers and short stems Pt PPTt PPtt PpTt Pptt pptt PPTT pT PpTT PpTt ppTT ppTt pt PT 1 PpTt 2 dihybrid pt PpTt Pptt ppTt pptt PT Pt pT pt four types of gametes 3 Stepped Art Fig. 13.8, p. 195

15. ANIMATION: Dihybrid cross To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

16. The Contribution of Crossovers • Genes that are far apart on a chromosome tend to assort into gametes independently because crossing over occurs between them very frequently • Genes that are very close together on a chromosome are linked, they do not assort independently because crossing over rarely happens between them • linkage group • All genes on a chromosome

17. Key Concepts • Insights From Dihybrid Crosses • Pairs of genes on different chromosomes are typically distributed into gametes independently of how other gene pairs are distributed • Breeding experiments with alternative forms of two unrelated traits can be used as evidence of such independent assortment

18. ANIMATION: Crossover review To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

19. 13.5 Beyond Simple Dominance • Mendel studied inheritance patterns that are examples of simple dominance, in which a dominant allele fully masks the expression of a recessive one • Other patterns of inheritance are not so simple: • Codominance • Incomplete dominance • Epistasis • Pleiotropy

20. Codominance • Codominant alleles are both expressed at the same time in heterozygotes, as in multiple allele systems suchas the one underlying ABO blood typing • codominant • Refers to two alleles that are both fully expressed in heterozygous individuals • multiple allele system • Gene for which three or more alleles persist in a population

21. Codominance: ABO Blood Types • Which two of the three alleles of the ABO gene you have determines your blood type • The A and the B allele are codominant when paired • Genotype AB = blood type AB • The O allele is recessive when paired with either A or B • Genotype AA or AO = blood type A • Genotype BB or BO= type B • Genotype OO = type O

22. Codominance: ABO Blood Types

23. Codominance: ABO Blood Types AA or AO BB or BO Genotypes: AB OO Phenotypes (blood type): A AB B O Fig. 13.9, p. 196

24. ANIMATION: Codominance: ABO blood types To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

25. Incomplete Dominance • With incomplete dominance,the heterozygous phenotype is between the two homozygous phenotypes • incomplete dominance • Condition in which one allele is not fully dominant over another, so the heterozygous phenotype is between the two homozygous phenotypes

26. Incomplete Dominance: Snapdragons • In snapdragons, one allele (R) encodes an enzyme that makes a red pigment, and allele (r) makes no pigment • RR = red; Rr = pink; rr = white • A cross between red and white (RR X rr) yields pink (Rr) • A cross between two pink (Rr X Rr) yields red, pink, and white in a 1:2:1 ratio

27. Incomplete Dominance: Snapdragons

28. Incomplete Dominance: Snapdragons homozygous (RR) x homozygous (rr) heterozygous (Rr) A Cross a red-flowered with a white-flowered plant, and all of the offspring will be pink heterozygotes. B If two of the pink heterozygotes are crossed, the phenotypes of the resulting offspring will occur in a 1:2:1 ratio. Fig. 13.10, p. 196

29. Incomplete Dominance: Snapdragons Fig. 13.10a, p. 196

30. Incomplete Dominance: Snapdragons homozygous (RR) x homozygous (rr) heterozygous (Rr) A Cross a red-flowered with a white-flowered plant, and all of the offspring will be pink heterozygotes. Fig. 13.10a, p. 196

31. Incomplete Dominance: Snapdragons Fig. 13.10b, p. 196

32. Incomplete Dominance: Snapdragons B If two of the pink heterozygotes are crossed, the phenotypes of the resulting offspring will occur in a 1:2:1 ratio. Fig. 13.10b, p. 196

33. ANIMATION: Incomplete dominance To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

34. Epistasis • Some traits are affected by multiple gene products, an effect called polygenic inheritance or epistasis • epistasis • Effect in which a trait is influenced by the products of multiple genes

35. Epistasis: Labrador Retriever • Labrador retriever coat color, can be black, brown, or yellow

36. Epistasis: Labrador Retriever • A dominant allele (B) specifies black fur, and its recessive partner (b) specifies brown fur • A dominant allele of a different gene (E ) causes color to be deposited in fur, and its recessive partner (e) reduces color • A dog with an E and a B allele has black fur • A dog with an E allele and homozygous for b is brown • A dog homozygous for the e allele has yellow fur regardless of its B or b alleles

37. Epistasis: Labrador Retriever

38. Animation: Coat Color in Labrador Retrievers

39. Pleiotropy • A pleiotropicgene influences multiple traits • Mutations in pleiotropicgenes are associated with complex genetic disorders such as sickle-cell anemia, cystic fibrosis, and Marfan syndrome • pleiotropic • Refers to a gene whose product influences multiple traits

40. Pleiotropy: Marfan Syndrome • In Marfan syndrome, mutations affect elasticity of tissues of the heart, skin, blood vessels, tendons, and other body parts • Haris Charalambous died when his aorta burst – he was 21

41. ANIMATION: Pleiotropic effects of Marfan syndrome To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE

42. Animation: Comb Shape in Chickens

43. 13.6 Complex Variation in Traits • Phenotype often results from complex interactions among gene products and the environment • Many traits show a continuous range of variation

44. Continuous Variation • Some traits appear in two or three forms; others occur in a range of small differences (continuous variation) • The more genes and environmental factors that influence a trait, the more continuous is its variation • continuous variation • In a population, a range of small differences in a shared trait

45. Continuous Variation (Cont.) • If a graph line drawn around the top of the bars showing the distribution of values for a trait is bell-shaped (a bell curve) the trait varies continuously • bell curve • Bell-shaped curve • Typically results from graphing frequency versus distribution for a trait that varies continuously

46. Continuous Variation (Cont.) • Human height and eye color are traits that vary continuously

47. Continuous Variation (Cont.)

48. Environmental Effects on Phenotype • Environmental factors often affect gene expression, which in turn affects phenotype: • Seasonal change in animal fur colors • Spines grow in presence of predators • Different plant heights when grown at different altitudes

49. Environmental Effects on Phenotype • In summer, the snowshoe hare’s fur is brown; in winter, white – offering seasonal camouflage from predators

50. Animation: Continuous Variation in Height