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Chapter 13 Observing Patterns in Inherited Traits

Chapter 13 Observing Patterns in Inherited Traits. 13.1 Menacing Mucus. Cystic fibrosis (CF) is the most common fatal genetic disorder in the United States The CFTR gene encodes CFTR protein which maintains a thin film of water on the surface of the epithelial sheets

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Chapter 13 Observing Patterns in Inherited Traits

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  1. Chapter 13Observing Patterns in Inherited Traits

  2. 13.1 Menacing Mucus • Cystic fibrosis (CF) is the most common fatal genetic disorder in the United States • The CFTR gene encodes CFTR protein which maintains a thin film of water on the surface of the epithelial sheets • A deletion of three base pairs (ΔF508, deletion) prevents proper membrane trafficking of CFTR so it can’t do its job • Mucus obstructs the airways and bacteria infect the intestine and lungs – most CF patients live no longer than thirty years

  3. ATP ΔF508 Figure 13-1a p203

  4. 13.2 Mendel, Pea Plants, and Inheritance Patterns • Recurring inheritance patterns are observable evidence of how heredity works • Before the discovery of genes, it was thought that inherited traits resulted from a blend of parental characters

  5. Mendel’s Experimental Approach • Gregor 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 • Mendel discovered that traits of offspring of cross-fertilized pea plants often appear in predictable patterns • Mendel’s work led him to conclude that hereditary information passes from one generation to the next in discrete units

  6. Gregor Mendel

  7. Garden Pea Plant: Self Fertilization and Cross-Fertilization B C A carpel anther D E

  8. ANIMATED FIGURE: Crossing garden pea plants 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

  9. Terms Used in Modern Genetics • Genes are heritable units of information about traits • Each gene has a specificlocuson a chromosome • Diploid cells (chromosome number 2n) have pairs of genes on homologous chromosomes • Alleles are different molecular forms of a gene

  10. Loci of Some Human Genes

  11. ribosomal RNA skin pigmentation fibrillin 1 (Marfan syndrome) (Tay–Sachs disease) 15 Figure 13-3a p205

  12. (Canavan disease) p53 tumor antigen NF1 (neurofibromatosis) serotonin transporter BRCA1 (breast, ovarian cancer) Growth hormone 17 Figure 13-3b p205

  13. LDL receptor (coronary artery disease) insulin receptor brown hair color green/blue eye color (Warfarin resistance) HCG, β chain 19 LH, β chain Figure 13-3c p205

  14. prion protein (Creutzfeldt– Jakob disease) oxytocin GHRH (acromegaly) 20 Figure 13-3d p205

  15. dystrophin (muscular dystrophy) (anhidrotic ectodermal dysplasia) IL2RG (SCID-X1) XIST X chromosome inactivation control (hemophilia B) (hemophilia A) (red-deficient color blind) X (green-deficient color blind) Figure 13-3e p205

  16. Terms Used in Modern Genetics • The particular set of alleles that an individual carries is the individual’s genotype • An individual with two identical alleles of a gene is homozygous for that gene • An individual with nonidentical alleles of a gene is heterozygous for that gene

  17. Terms Used in Modern Genetics • A hybrid is the heterozygote offspring of a cross between two individuals that breed true for different forms of a trait • An individual’s genotype determines its phenotype, which refers to an individual’s observable traits • Any mutated gene is a new allele, whether or not it affects phenotype

  18. Terms Used in Modern Genetics • An allele is dominantif its effect masks the effect of a recessive allele paired with it • Capital letters (P) signify dominant alleles; lowercase letters (p) signify recessive alleles • Homozygous dominant(PP) • Homozygous recessive(pp) • Heterozygous(Pp)

  19. Genotypes Give Rise to Phenotypes PP (homozygous for dominant allele P) pp (homozygous for recessive allele p) Pp (heterozygous at the P gene locus) genotype: phenotype:

  20. Take-Home Message:How do alleles contribute to traits? • Gregor Mendel discovered the role of alleles in inheritance by breeding pea plants and tracking traits of their offspring • Genotype refers to the particular set of alleles carried by an individual’s somatic cell; phenotype refers to the individual’s set of observable traits; genotype is the basis of phenotype • A homozygous individual has two identical alleles at a particular locus; a heterozygous individual has nonidentical alleles at the locus • Dominant alleles mask the effects of recessive ones in heterozygous individuals

  21. 13.3 Mendel’s Law of Segregation • Pairs of genes on homologous chromosomes separate during meiosis, so they end up in different gametes • Mendel showed that garden pea plants inherit two “units” of information for a trait, one from each parent

  22. Gene Segregation • Homologous chromosomes (and all the alleles they carry) segregate into separate gametes during meiosis • Plants homozygous for the dominant allele (PP) can only make gametes that carry the allele P • Plants homozygous for the recessive allele (pp) can only make gametes that carry the allele p • Heterozygous plants produce both type of gametes

  23. 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

  24. DNA replication meiosis I 2 1 meiosis II 3 gametes (P) gametes (p) zygote (Pp) female gametes 4 male gametes Stepped Art Figure 13-5 p206

  25. female gametes male gametes Figure 13-5b p206

  26. Testcrosses • A testcross is 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 (PP x pp) or (Pp x pp)

  27. Monohybrid Crosses • A monohybrid crossis a testcross that checks for a dominance relationship between two alleles at a single locus • May be a cross between true breeding (homozygous) individuals (PP x pp), or between identical heterozygotes (Pp x Pp)

  28. Generations in a Monohybrid Cross • P stands for parents,Ffor filial (offspring) • F1: First generation offspring of parents • F2: Second generation offspring of parents

  29. Mendel’s Monohybrid Crosses • Mendel used monohybrid crosses 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

  30. Table 13-1 p207

  31. Mendel’s 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

  32. Offspring of Mendel’s Monohybrid Cross

  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 pp genotype in the offspring of a heterozygous cross (Pp x Pp) • This is the basis of Mendel’s law ofsegregation • 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. Take-Home Message:What is Mendel’s 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 • Mendel discovered patterns of inheritance in pea plants by tracking the results of many monohybrid crosses

  35. ANIMATED FIGURE: Monohybrid 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

  36. 13.4 Mendel’s Law of Independent Assortment • Mendel’s law of independent assortment • During meiosis, members of a pair of genes on homologous chromosomes get distributed into gametes independently of other gene pairs

  37. Dihybrid Crosses • Dihybrid crossestest for dominance relationships between alleles at two loci • Individuals that breed true for two different traits are crossed (PPTT x pptt) • 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

  38. The Contribution of Crossovers • Independent assortment also occurs when the genes are on the same chromosome, but far enough apart that crossing over occurs between them very frequently • Genes that have loci very close to one another on a chromosome tend to stay together during meiosis and not assort independently

  39. Linkage Groups • All genes on one chromosome are called a linkage group • The farther apart two genes are on a chromosome, the more often crossing over occurs between them • Linked genes are very close together; crossing over rarely occurs between them • The probability that a crossover will separate alleles of two genes is proportional to the distance between those genes

  40. Take-Home Message: What is Mendel’s law of independent assortment? • Each member of a pair of genes on homologous chromosomes tends to be distributed into gametes independently of how other genes are distributed during meiosis

  41. ANIMATED FIGURE: Independent assortment 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. ANIMATED FIGURE: 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

  43. 13.5 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

  44. Codominance • Codominance • 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 • Genes with three or more alleles in a population • Example: ABO blood types

  45. Codominance in ABO Blood Types AA or AO BB or BO Genotypes: AB OO Phenotypes (blood type): A AB B O

  46. 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

  47. heterozygous (Rr) homozygous (RR) homozygous (rr) Figure 13-10 p210

  48. Figure 13-10b p210

  49. INTERACTION: 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

  50. Epistasis • Epistasis • Two or more gene products influence a trait • Typically, one gene product suppresses the effect of another • Example: Coat color in dogs • Alleles B and b designate colors (black or brown) • Two recessive alleles ee suppress color

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