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Observable Patterns of Inheritance

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Observable Patterns of Inheritance

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  1. Observable Patterns of Inheritance Chapter 11

  2. Earlobe Variation • Whether a person has attached or detached earlobes depends on a single gene • Attached earlobes: two copies of the recessive allele for this gene • Detached earlobes: either one or two copies of the dominant allele

  3. Early Ideas about Heredity • People knew that sperm and eggs transmitted information about traits • Blending theory • Problem: • Would expect variation to disappear • What would you expect to happen with trait such as height? • Variation in traits persists

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

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

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

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

  8. Genetic Terms A pair of homologous chromosomes Figure 11.4Page 179 A gene locus A pair of alleles Three pairs of genes Figure 11.4Page 179

  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 Experimental intercross between two F1 heterozygotes AA X aa Aa (F1 monohybrids) Aa X Aa ?

  12. 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 Figure 11.5Page 180

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

  14. True-breeding homozygous recessive parent plant F1PHENOTYPES aa True-breeding homozygous dominant parent plant Aa Aa a a Aa Aa A AA A Aa Aa Aa Aa An F1 plant self-fertilizes and produces gametes: F2PHENOTYPES Aa AA Aa A a A AA Aa a Aa aa Aa aa Monohybrid CrossIllustrated Figure 11.7Page 181

  15. 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

  16. 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

  17. 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

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

  19. 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 Figure 11.9 (1)Page 183

  20. Dihybrid Cross: F2 Results X AaBb AaBb 1/4 AB 1/4 Ab 1/4 aB 1/4 ab 9/16 purple-flowered, tall 1/4 AB 1/16 AABB 1/16 AABb 1/16 AaBB 1/16 AaBb 3/16 purple-flowered, dwarf 3/16 white-flowered, tall 1/16 AaBb 1/16 AAbb 1/16 Aabb 1/4 Ab 1/16 AABb 1/16 white-flowered, dwarf 1/16 AaBB 1/16 aaBB 1/16 aaBb 1/4 aB 1/16 AaBb 1/16 Aabb 1/16 aaBb 1/16 aabb 1/16 AaBb 1/4 ab Figure 11.9(2)Page 183

  21. 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

  22. 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

  23. 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

  24. Dominance Relations Complete dominance Incomplete dominance Codominance

  25. Incomplete Dominance Incomplete Dominance X Homozygous parent Homozygous parent All F1 are heterozygous X Figure 11.10Page 184 F2 shows three phenotypes in 1:2:1 ratio

  26. Codominance: ABO Blood Types • 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

  27. ABO Blood Type:Allele Combinations Range of genotypes: IA IA IB IB or or IA i IA IB IB i ii Blood types: A AB B O Figure 11.11Page 184

  28. 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

  29. Pleiotropy • Alleles at a single locus may have effects on two or more traits • Marfan syndrome - Mutation in gene for fibrillin affects skeleton, cardiovascular system, lungs, eyes, and skin

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

  31. Coat Color inRetrievers BBEE X bbee F1puppies are all BbEe F2puppies BE Be bE be BE black BBEE BBEe BbEE BbEe Be BBee BbEe Bbee BBEe brown bE BbEe bbEE bbEe BbEE yellow Figure 11.13Page 186 be Bbee bbEe bbee BbEe

  32. Comb Shape in Poultry RRpp (rose comb) rrPP (pea comb) P: X RrPp (all walnut comb) F1: F2: 9/16 walnut 3/16 rose 3/16 pea 1/16 single rrpp rrPP rrPp RRpp Rrpp RRPP RRPp RrPP RrPp Figure 11.15Page 187

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

  34. 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

  35. 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

  36. (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 Describing Continuous Variation

  37. Temperature Effects on Phenotype • Rabbit is 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 Figure 11.18Page 190

  38. Environmental Effects on Plant Phenotype • Hydrangea macrophylla • Action of gene responsible for floral color is influenced by soil acidity • Flower color ranges from pink to blue