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Foundations of Genetics Observable Patterns of Inheritance

Foundations of Genetics Observable Patterns of Inheritance. Chapter 8. Earlobe Variation . Whether a person is born with attached or detached earlobes depends on a single gene Gene has two molecular forms (alleles). Earlobe Variation.

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Foundations of Genetics Observable Patterns of Inheritance

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  1. Foundations of GeneticsObservable Patterns of Inheritance Chapter 8

  2. Earlobe Variation • Whether a person is born with attached or detached earlobes depends on a single gene • Gene has two molecular forms (alleles)

  3. Earlobe Variation • You inherited one allele for this gene from each parent • Dominant allele specifies detached earlobes • Recessive allele specifies attached lobes

  4. Dominant & Recessive Alleles • If you have attached earlobes, you inherited two copies of the recessive allele • If you have detached earlobes, you may have either one or two copies of the dominant allele

  5. 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

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

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

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

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

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

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

  12. Genotype & Phenotype • Genotype refers to particular genes an individual carries • Phenotype refers to an individual’s observable traits (that are expressed) • Cannot always determine genotype by observing phenotype

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

  14. F1 Results of One Monohybrid Cross

  15. F2 Results of Monohybrid Cross

  16. Mendel’s Monohybrid Cross Results 5,474 round 1,850 wrinkled 6,022 yellow 2,001 green 882 inflated 299 wrinkled 428 green 152 yellow F2 plants showed dominant-to-recessive ratio that averaged 3:1 705 purple 224 white 651 long stem 207 at tip 787 tall 277 dwarf

  17. 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

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

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

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

  21. Homozygous recessive Homozygous recessive a a a a A A Aa Aa Aa Aa a A aa Aa aa Aa Punnett Squares of Test Crosses Two phenotypes All dominant phenotype

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

  23. A Dihybrid Cross - F1 Results purple flowers, tall white flowers, dwarf TRUE- BREEDING PARENTS: AABB x aabb GAMETES: AB AB ab ab AaBb F1 HYBRID OFFSPRING: All purple-flowered, tall

  24. 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)

  25. 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

  26. ab ab aB AB AB Ab Ab aB Explanation of Mendel’s Dihybrid Results If the two traits are coded for by genes on separate chromosomes, sixteen gamete combinations are possible 1/4 1/4 1/4 1/4 1/4 1/16 1/16 1/16 1/16 AABB AABb AaBB AaBb 1/4 1/16 1/16 1/16 1/16 AABb AAbb AaBb Aabb 1/4 1/16 1/16 1/16 1/16 AaBB AaBb aaBB aaBb 1/4 1/16 1/16 1/16 1/16 AaBb Aabb aaBb aabb

  27. ab ab aB AB AB Ab Ab aB 16 Allele Combinations in F2 1/4 1/4 1/4 1/4 1/4 1/16 1/16 1/16 1/16 AABB AABb AaBB AaBb 1/4 1/16 1/16 1/16 1/16 AABb AAbb AaBb Aabb 1/4 1/16 1/16 1/16 1/16 AaBB AaBb aaBB aaBb 1/4 1/16 1/16 1/16 1/16 AaBb Aabb aaBb aabb

  28. 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

  29. Independent Assortment Metaphase I OR A A a a A A a a B B b b b b B B Metaphase II: A A a a A A a a B B b b b b B B Gametes: B B b b b b B B A A a a A A a a 1/4 AB 1/4 ab 1/4 Ab 1/4 aB

  30. 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)

  31. 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

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

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

  34. 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)

  35. 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

  36. 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

  37. ABO Blood Type:Allele Combinations • Type A - IAIA or Iai (core glycolipid H + N-acetyl-D-galactosamine) • Type B - IBIBor IBi (core glycolipid H + D-galactose) • Type AB - IAIB (both glycolipids) • Type O - ii (core glycolipid H only)

  38. 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 or B, but H shared by both Types A and B

  39. 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, but a core glycolipid H common to both A and B.

  40. 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

  41. 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

  42. Pleiotrophic 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

  43. Epistasis • Interaction between the products of gene pairs • Common among genes for hair color in mammals

  44. 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

  45. 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

  46. 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

  47. 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

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

  49. 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

  50. 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

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