Observing Patterns in Inherited Traits

1. Observing Patterns in Inherited Traits Chapter 13 Biology Concepts and Applications, Eight Edition, by Starr, Evers, Starr. Brooks/Cole, Cengage Learning 2011.

2. Cystic Fibrosis • Most common fatal genetic disorder in the US • Gene  CFTR (encodes protein that moves chloride ions out of the epithelial cells) • Water follows chloride ions  osmosis • Goal  thin film of water on surface of epithelial sheets that allow mucus to slide easily over the wet sheets of cells • Mutation in CFTR gene  deletion of three base pairs • Disrupts membrane trafficking of CFTR • Protein is made by never released from the cell • Prevents binding with bacteria at the cell surface  decrease immune response

3. Menacing Mucus • Outcome  transport of chloride ions and water out of the epithelial cells are interrupted • Thick globs of mucus accumulate and clog passageways • Breathing becomes difficult due to mucus obstruction • Treatment  • posture change, thumps on chest and back to clear the lungs • Antibiotics to control infections • Lifespan  30 yrs • Genetics  must have 2 copies of the inherited mutated gene (one from each parent) • 1:3,300 births

4. Mendel, Pea Plants, and Inheritance Patterns • By experimenting with pea plants, Mendel was the first to gather evidence of patterns by which parents transmit genes to offspring

5. Mendel’s Experiments

6. carpel stamen a Garden pea flower, cut in half. Sperm form in pollen grains, which originate in male floral parts (stamens). 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 stamens snipped off. This is one way to assure cross-fertilization of plants. c Later, seeds develop inside pods of the cross-fertilized plant. An embryo within 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. 10.3, p.154

7. Producing Hybrids • Hybrids Heterozygous individual • Offspring of a cross between two individuals that breed true for different forms of a trait • Each inherits nonidentical alleles for a trait being studied

8. Mendel’s Experimental Approach • Control over reproduction • Removing a flower’s pollen-bearing anthers • “Bred true” all offspring have the same form of the traits as the parent(s), generation after generation, mutations aside • “Cross-fertilize”  transfer pollen among individuals that have different traits • Mendel discovered  traits of the offspring from cross-fertilized pea plants appear in predictable patterns • Outcome  Hereditary information is passed from one generation to the next in discrete units

9. Producing Hybrid Offspring

10. Homozygous dominant parent Homozygous recessive parent (chromosomes duplicated before meiosis) meiosis I meiosis II (gametes) (gametes) fertilization produces heterozygous offspring Fig. 10.5, p.156

11. Heritable Units of Information • Genes • Heritable units of information about traits • Each has its own locus on the chromosome • Alleles • Different molecular forms of the same gene • Locus location of a gene on a chromosome • Figure 13.3 and 13.4 in text • Mutation • Permanent change in a gene’s information

12. Heritable Units of Information

13. a A pair of homologous chromosomes, both unduplicated. In most species, one is inherited from a female parent and its partner from a male parent. b A gene locus (plural, loci), the location for a specific gene on a chromosome. Alleles are at corresponding loci on a pair of homologous chromosomes c A pair of alleles may be identical or not. Alleles are represented in the text by letters such as D or d. d Three pairs of genes (at three loci on this pair of homologous chromosomes); same thing as three pairs of alleles. Fig. 10.4, p.155

14. Modern Genetic Terms • Genotype • An individual’s alleles at any or all gene loci • A set of alleles carried by an individual • Phenotype • An individual’s observable traits • Dominant allele may mask effect of a recessive allele paired with it • Recessive allele is masked by a dominant allele

15. Modern Genetic Terms • Homozygous  having identical alleles of a gene • Heterozygous  having 2 different alleles of a gene • Homozygous dominant • Has two dominant alleles for a trait (AA) • Homozygous recessive • Has two recessive alleles (aa) • Heterozygote • Has two nonidentical alleles (Aa)

16. Key Concepts:MODERN GENETICS • Gregor Mendel gathered the first indirect, experimental evidence of the genetic basis of inheritance • His meticulous work tracking traits in many generations of pea plants gave him clues that heritable traits are specified in units • The units, distributed into gametes in predictable patterns, were later identified as genes

17. Mendel’s Theory of Segregation • Mendel’s Theory of Segregation: • Diploid organisms have pairs of genes, on pairs of homologous chromosomes • Based on monohybrid experiments • Law of Segregation  During meiosis • Genes of each pair separate • Each gamete gets one or the other gene

18. Producing Hybrid Offspring • Crossing two true-breeding parents of different genotypes yields hybrid offspring • All F1 offspring are heterozygous for a gene, and can be used in monohybrid experiments • All F1 offspring of parental cross AA x aa are Aa • Monohybrid cross • Breeding experiment Aa x Aa (heterozygote's) are crossed • Frequency of traits among the offspring offers information about the dominance relationship between the alleles • Punnett square  predict the genetic and phenotypic outcome of a cross

19. A Monohybrid Cross • Crosses between F1 monohybrids resulted in these allelic combinations among F2 offspring • Phenotype ratio 3:1 • Evidence of dominant and recessive traits

20. F2 Offspring: Dominant and Recessive Traits

21. Trait Studied Dominant Form Recessive Form F2 Dominant-to-Recessive Ratio Seed shape 5,474 round 1,850 wrinkled 2.98:1 Seed color 6,022 yellow 2,001 green 3.01:1 Pod shape 2.95:1 882 inflated 299 wrinkled Pod color 428 green 152 yellow 2.82:1 Flower color 705 purple 224 white 3.15:1 Flower position 3.14:1 651 long stem 207 at tip Stem length 2.84:1 277 dwarf 787 tall Fig. 10.6, p.156

22. Predicting Probability: Punnett Squares

23. female gametes male gametes Aa aa a A a A A a A a Aa AA Aa A A A A a aa Aa aa Aa aa a a a Stepped Art Fig. 10-7a, p.157

24. Predicting F1 Offspring

25. Predicting F2 Offspring (3:1)

26. TESTCROSS • Testcross  • Method of determining genotype in which an individual of unknown genotype is crossed with one that is known to be homozygous recessive • Ex. Offspring is homozygous recessive for a trait • One parent is homozygous dominant for that trait, what is the genotype for the other parent??? • SOLVE and WRITE here.

27. Key Concepts:MONOHYBRID EXPERIMENTS • Some experiments yielded evidence of gene segregation • When one chromosome separates from its homologous partner during meiosis, the pairs of alleles on those chromosomes also separate and end up in different gametes • Law of segregation • Diploid cells carry pairs of genes, on pairs of homologous chromosomes • The two genes of each pair are separated from each other during meiosis, so they end up in different gametes

28. Mendel’s Theory of Independent Assortment • Mendel’sTheory of Independent Assortment: • Meiosis assorts gene pairs of homologous chromosomes independently of gene pairs on all other chromosomes • Based ondihybrid experiments • Pairs of homologous chromosomes align randomly at metaphase I

29. Independent Assortment in Meiosis I

30. One of two possible alignments The only other possible alignment a Chromosome alignments at metaphase I: 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 a A A b a a b A A b B B b B B c Possible combinations of alleles in gametes: AB ab Ab aB Fig. 10.8, p.158

31. Dihybrid Experiments • Start with a cross between true-breeding heterozygous parents that differ for alleles of two genes (AABB x aabb) • All F1 offspring are heterozygous for both genes (AaBb) • Frequency of traits among the offspring provides information about the dominance relationship between the paired alleles

32. Mendel’s Dihybrid Experiments • AaBb x AaBb • Phenotypes of the F2 offspring of F1 hybrids were close to a 9:3:3:1 ratio • 9 dominant for both traits • 3 dominant for A, recessive for b • 3 dominant for B, recessive for a • 1 recessive for both traits

33. Results of Mendel’s Dihybrid Experiments

34. Meiosis, gamete formation in true-breeding parent plants parent homozygous dominant for purple flowers, tall stems parent homozygous recessive for white flowers, short stems Gametes at fertilization Possible genotypes resulting from a cross between two F1 plants: meiosis, gamete formation meiosis, gamete formation All F1 plants are AaBb heterozygotes with purple flowers and tall stems. Fig. 10.9, p.159

35. Linkage Groups • All genes on the same chromosome are part of one linkage group • Crossing over between homologous chromosomes disrupts gene linkages

36. Crossover • Distance between gene • Genes far a parts on a chromosome  high frequency of crossing over occurs between them • Assort into gametes independently, just as if they were on different chromosomes • Genes close together on a chromosome  DO NOT assort independently and low frequency of crossing over • Tightly linked genes stay together during meiosis more frequently

37. Linkage and Crossing Over

38. Key Concepts:DIHYBRID EXPERIMENTS • Some experiments yielded evidence of independent assortment • During meiosis, the members of a pair of homologous chromosomes are distributed into gametes independently of all other pairs

39. Beyond Simple Dominance • Other types of gene expression • Codominant alleles • Incomplete dominance • Epistasis • Pleiotropy

40. Codominant Alleles • Both alleles are expressed at the same time in heterozygotes • Example: Multiple allele system (gene for which three or more alleles persist in a population) • ABO blood typing

41. AA BB or or Genotypes: AO AB BO OO Phenotypes (Blood type): A AB B O Fig. 10.10, p.160

42. ABO Gene • O is recessive when paired with A or B • Incorrect blood transfusion  Dangerous • Result: Red blood cells can clump or burst • O type = universal donor • AB type = universal recipient

43. Incomplete Dominance • An allele is not fully dominant over its partner on a homologous chromosome • Both are expressed • Produces a phenotype between the two homozygous conditions

44. Incomplete Dominance

45. homozygous parent (RR) homozygous parent (rr) heterozygous F1 offspring (Rr) x Cross two of the F1 plants, and the F2 offspring will show three phenotypes in a 1:2:1 ratio: RR Rr Rr rr Fig. 10.11, p.160

46. Epistasis • Interacting products of one or more genes affect the same trait • Trait is influenced by the products of multiple genes

47. RRpp (rose comb) X rrPP (pea comb) F1 of spring: RrPp (all walnut comb) F2 offspring: RrPp RrPp X RRPP, RRPp, RrPP, or RrPp RRpp or Rrpp rrPP or rrPp rrpp 9/16 walnut 3/16 rose 3/16 pea 1/16 single comb Fig. 10.12, p.161

48. More Epistasis – Skin color and Fur color

49. Color • Dominant B = black • Recessive b = brown • Dominant E = melanin to deposit in fur • Recessive e = reduces melanin deposition

50. EB Eb eB eb EEBB black EEBb black EeBB black EeBb black EB EEBb black EEbb chocolate EeBb black Eebb chocolate Eb EeBB black EeBb black eeBB yellow eeBb yellow eB eeBb yellow eebb yellow EeBb black Eebb chocolate eb Fig. 10.13, p.161