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Observing Patterns in Inherited Traits

Observing Patterns in Inherited Traits. Chapter 10. Early Ideas about Heredity. People knew that sperm and eggs transmitted information about traits Blending theory Problem: Would expect variation to disappear Variation in traits persists. Gregor Mendel.

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Observing Patterns in Inherited Traits

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  1. Observing Patterns in Inherited Traits Chapter 10

  2. Early Ideas about Heredity • People knew that sperm and eggs transmitted information about traits • Blending theory • Problem: • Would expect variation to disappear • Variation in traits persists

  3. Gregor Mendel • Strong background in plant breeding and mathematics • Using pea plants, found indirect but observable evidence of how parents transmit genes to offspring

  4. The Garden Pea Plant • Self-pollinating • True breeding (different alleles not normally introduced) • Can be experimentally cross-pollinated

  5. Genetic Terms A pair of homologous chromosomes A gene locus A pair of alleles Three pairs of genes

  6. Genes • Units of information about specific traits • Passed from parents to offspring • Each has a specific location (locus) on a chromosome

  7. Alleles • Different molecular forms of a gene • Arise by mutation • Dominant allele masks a recessive allele that is paired with it

  8. Allele Combinations • Homozygous • having two identical alleles at a locus • AA or aa • Heterozygous • having two different alleles at a locus • Aa

  9. Genotype & Phenotype • Genotype refers to particular genes an individual carries • Phenotype refers to an individual’s observable traits • Cannot always determine genotype by observing phenotype

  10. Tracking Generations • Parental generation P mates to produce • First-generation offspring F1 mate to produce • Second-generation offspring F2

  11. Monohybrid Crosses • Use F1 offspring of parents that breed true for different forms of a trait:(AA x aa = Aa) • The experiment itself is a cross between two identical F1 heterozygotes, which are the “monohybrids” (Aa x Aa)

  12. Monohybrid Crosses Homozygous Homozygous dominant parent recessive parent (chromosomes duplicated before meiosis) meiosis I meiosis II (gametes) (gametes) fertilization produces heterozygous offspring

  13. Mendel’s Monohybrid Cross Results F2 plants showed dominant-to-recessive ratio that averaged 3:1

  14. Probability The chance that each outcome of a given event will occur is proportional to the number of ways that event can be reached

  15. A a A AA Aa a Aa aa Punnett Square of a Monohybrid Cross Female gametes Dominant phenotype can arise 3 ways, recessive only 1 Male gametes

  16. F1 Results of One Monohybrid Cross

  17. F2 Results of Monohybrid Cross

  18. Testcross • Individual that shows dominant phenotype is crossed with individual with recessive phenotype • Examining offspring allows you to determine the genotype of the dominant individual

  19. Mendel’s Theory of Segregation • An individual inherits a unit of information (allele) about a trait from each parent • During gamete formation, the alleles segregate from each other

  20. Dihybrid Cross Experimental cross between individuals that are homozygous for different versions of two traits

  21. A Dihybrid Cross - F1 Results

  22. F1 Results of Mendel’s Dihybrid Crosses • All plants displayed the dominant form of both traits • We now know: • All plants inherited one allele for each trait from each parent • All plants were heterozygous (AaBb)

  23. Phenotypic Ratios in F2 Four Phenotypes: • Tall, purple-flowered (9/16) • Tall, white-flowered (3/16) • Dwarf, purple-flowered (3/16) • Dwarf, white-flowered (1/16) AaBbX AaBb

  24. Explanation of Mendel’s Dihybrid Results If the two traits are coded for by genes on separate chromosomes, sixteen gamete combinations are possible

  25. Independent Assortment • Mendel concluded that the two “units” for the first trait were to be assorted into gametes independently of the two “units” for the other trait • Members of each pair of homologous chromosomes are sorted into gametes at random during meiosis

  26. Independent Assortment

  27. Tremendous Variation Number of genotypes possible in offspring as a result of independent assortment and hybrid crossing is 3n (n is the number of gene loci at which the parents differ)

  28. Impact of Mendel’s Work • Mendel presented his results in 1865 • Paper received little notice • Mendel discontinued his experiments in 1871 • Paper rediscovered in 1900 and finally appreciated

  29. Dominance Relations • Complete dominance • Incomplete dominance • Heterozygote phenotype is somewhere between that of two homozyotes • Codominance • Non-identical alleles specify two phenotypes that are both expressed in heterozygotes

  30. Genetics of ABO Blood Types: Three Alleles • Gene that controls ABO type codes for enzyme that dictates structure of a glycolipid on blood cells • Two alleles (IA and IB) are codominant when paired • Third allele (i) is recessive to others

  31. ABO Blood Type:Allele Combinations • Type A - IAIA or IAi • Type B - IBIBor IBi • Type AB - IAIB • Type O - ii

  32. ABO Blood Type: Glycolipids on Red Cells • Type A - Glycolipid A on cell surface • Type B - Glycolipid B on cell surface • Type AB - Both glyocolipids A & B • Type O - Neither glyocolipid A nor B

  33. ABO and Transfusions • Recipient’s immune system will attack blood cells that have an unfamiliar glycolipid on surface • Type O is universal donor because it has neither type A nor type B glycolipid

  34. Flower Color in Snapdragons: Incomplete Dominance Red-flowered plant X White-flowered plant Pink-flowered F1 plants (homozygote) (homozygote) (heterozygotes)

  35. Flower Color in Snapdragons: Incomplete Dominance Pink-flowered plant X Pink-flowered plant White-, pink-, and red-flowered plants in a 1:2:1 ratio (heterozygote) (heterozygote)

  36. Flower Color in Snapdragons: Incomplete Dominance • Red flowers - two alleles allow them to make a red pigment • White flowers - two mutant alleles; can’t make red pigment • Pink flowers have one normal and one mutant allele; make a smaller amount of red pigment

  37. Comb Shape in Poultry Alleles at two loci (R and P) interact • Walnut comb - RRPP, RRPp, RrPP, RrPp • Rose comb - RRpp, Rrpp • Pea comb - rrPP, rrPp • Single comb - rrpp

  38. Pleiotropy • Alleles at a single locus may have effects on two or more traits • Classic example is the effects of the mutant allele at the beta-globin locus that gives rise to sickle-cell anemia

  39. Genetics of Sickle-Cell Anemia • Two alleles 1) HbA Encodes normal beta hemoglobin chain 2) HbS Mutant allele encodes defective chain • HbS homozygotes produce only the defective hemoglobin; suffer from sickle-cell anemia

  40. Pleiotropic Effects of HbS/HbS • At low oxygen levels, cells with only HbS hemoglobin “sickle” and stick together • This impedes oxygen delivery and blood flow • Over time, it causes damage throughout the body

  41. Genetics of Coat Color in Labrador Retrievers • Two genes involved - One gene influences melanin production • Two alleles - B (black) is dominant over b (brown) - Other gene influences melanin deposition • Two alleles - E promotes pigment deposition and is dominant over e

  42. Allele Combinations and Coat Color • Black coat - Must have at least one dominant allele at both loci • BBEE, BbEe, BBEe, or BbEE • Brown coat - bbEE, bbEe • Yellow coat - Bbee, BbEE, bbee

  43. Albinism • Phenotype results when pathway for melanin production is completely blocked • Genotype - Homozygous recessive at the gene locus that codes for tyrosinase, an enzyme in the melanin-synthesizing pathway

  44. Campodactyly: Unexpected Phenotypes • Effect of allele varies: • Bent fingers on both hands • Bent fingers on one hand • No effect • Many factors affect gene expression

  45. Continuous Variation • A more or less continuous range of small differences in a given trait among individuals • The greater the number of genes and environmental factors that affect a trait, the more continuous the variation in versions of that trait

  46. Human Variation • Some human traits occur as a few discrete types • Attached or detached earlobes • Many genetic disorders • Other traits show continuous variation • Height • Weight • Eye color

  47. Describing Continuous Variation (line of bell-shaped curve indicates continuous variation in population) Number of individuals with some value of the trait Number of individuals with some value of the trait Range of values for the trait Range of values for the trait

  48. Temperature Effects on Phenotype • Himalayan rabbits are Homozygous for an allele that specifies a heat-sensitive version of an enzyme in melanin-producing pathway • Melanin is produced in cooler areas of body

  49. Environmental Effects on Yarrow Plants

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