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Lab Exam

Lab Exam. Genotype & Phenotype. Genetics Problem-Solving Secrets!. Known Genotype can be used to infer unknown Phenotype (but not always, due to complications, e.g., penetrance) Known Phenotype can be used to infer unknown Genotype

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Lab Exam

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  1. Lab Exam

  2. Genotype & Phenotype

  3. Genetics Problem-Solving Secrets! • Known Genotype can be used to infer unknown Phenotype • (but not always, due to complications, e.g., penetrance) • Known Phenotype can be used to infer unknown Genotype • (but not always due to lack of 1:1 correspondence: more than one genotype can give rise to a given phenotype) • Genotype (diploid) gives rise to Gametes (haploid) via Meiosis • Gametes (haploid) give rise to “Progeny” (diploid) via Fertilization • Fertilization (syngamy) always results in Diploidy (I.e., >ploidy than haploid) • Meiosis always results in Haploidy (I.e., anaphase I reduction division from diploidy to haploidy)

  4. Characters & Traits

  5. Mendel’s Protocol Controlled breeding, with specific characters scored for specific traits (e.g., character = flower color, trait = purple vs. white)

  6. True Breeding True breeding results when both parents are homozygous for the same trait, e.g., a purple purple x purple purple cross can result only in purple purple purple-flowered progeny; similarly ww x wwonly ww progeny

  7. Dominant Phenotype Recessive Phenotype Dominant & Recessive Alleles

  8. Dominant Homozygote Heterozygote (note color) Recessive Homozygote (note color) genotype? genotype? Recessive Homozygote (note color) genotype? Homo- & Heterozygotic

  9. Dominant Homozygote Recessive Homozygote (note color) Segregation occurs here Heterozygote (note color) Dominant Homozygote Heterozygotes Recessive Homozygote Segregation of Alleles

  10. Meiosis and Segregation

  11. Generations

  12. Phenotype Genotype (diploid) Meiosis Genotype (haploid/ gametes) Fertilization Genotype (diploid) Phenotype Genotype (haploid/ gametes) Fertilization Meiosis Punnett Square And Don’t Forget…

  13. 1:2:1 Genotypic Ratio

  14. 3:1 Phenotypic Ratio dominant recessive

  15. = monohybrid Monohybrid Cross

  16. = monohybrid Monohybrid Cross

  17. Test Cross • What is the genotype of the purple-flowered plant, PP or Pp? • To find out we can cross the plant with a “Blank Slate”, I.e., a homozygous recessive plant (pp) • In that case, the phenotypes of all progeny will (in a sense) be dependent only on the genotype of the purple-flowered parent • We call this kind of test of genotype a “Test Cross”

  18. Test Cross

  19. Test Cross

  20. Beyond 1 locus, 2 alleles, & Complete Dominance

  21. = dihybrid 9:3:3:1 phenotypic ratio  Dihybrid Cross (2 loci, 2 alleles)

  22. Dihybrid Cross (2 loci, 2 alleles) • Note that 9:3:3:1 ratio is dependent on: • Two loci, two alleles per locus • Independent assortment between loci (genotypic independence) • Dominance-recessive relationships betwee the alleles found at each locus • One locus does not affect the phenotype of the other locus (phenotypic independence)

  23. = dihybrids  9:3:3:1 phenotypic ratio Dihybrid Cross (2 loci, 2 alleles) Dominant-Dominant Recessive-Dominant Dominant-Recessive Recessive-Recessive

  24. = dihybrids Dihybrid Cross (2 loci, 2 alleles)

  25. Independent Assortment

  26. = trihybrids Many Loci, Many Alleles

  27. Probability Theory • Statistical Independence • Range of Probabilities (0..1) • Law of Multiplication • Calculation for Events not Happening • The Law of Addition

  28. Genotype Probabilities AaBbCcDdEe x AABbCcDDEc p[A] = 0.5 p[AX] = 0.5 + 0.5 = 1.0 p[a] = 0.5 p[A] = 0.0 p[A] = 0.5 X p[A] = 0.5 p[Aa] = 0.5 x 1.0 = 0.5 p[Xa] = 0.0 + 0.5 = 0.5 What Fraction AaBbCcDcEe?

  29. Incomplete Dominance Note 1:1 correspondance between genotype & phenotype!

  30. Incomplete Dominance

  31. Incomplete Dominance

  32. 1 Locus, >2 Alleles

  33. 1 Locus, >2 Alleles

  34. 1 Locus, >2 Alleles

  35. Codominance

  36. Codominance • Generally, at the molecular level & to the extent that proteins are made at all, most alleles are codominant • In the heterozygote more than one type of protein product is produced per locus per chromosome • Aa and AA have different molecular phenotypes even if A is dominant to a at the organismal level

  37. Note codominant at molecular level Codominance

  38. Genes that exert effects on multiple aspects of physiology or anatomy are pleiotropic • This is a common feature of human genes • Marfan syndrome: Affects the eye, the skeleton and the cardiovascular system • Albinism: Affects skin, eyes, and even hearing • White eye in Drosophila: flight muscles also affected • What all of the this means is that individual genes typically are active within numerous tissues, and that a character often may be modified via different pathways and routes • e.g., more than one gene may be involved in a character’s expression, some with more-generally acting and others with more-specific effects Pleitropy

  39. Pleitropy (but, in fact, probably did not suffer from Marfan Syndrome)

  40. Pleitropy

  41. B  Black b  brown Bx  Black bb  brown C  color c  no color CX  color cc  no color Epistasis (interacting loci) • Lack of 9:3:3:1 ratio is due to lack of phenotypic indepence • In the next chapter (15) we’ll see the consequence of lack of genotypic independence, a.k.a., linkage Note not 9:3:3:1 ratios

  42. Polygenic Inheritance • Means Many Genes,One Character • Influenced by alleles at two or more loci, with each making a contribution to the phenotype • Polygenic traits account for most of the observable variations seen in humans • Example: skin color, hair color, eye color, stature, weight, height, shape of face, behavior, etc.

  43. Genotype 1 Genotype 1 Phenotype Genotype 2 Genotype 2 Environment Environment Norms of Reaction (Reaction Norms) Many Characters, One Gene Reaction Norms are the measure of how organisms (or genoyptes) respond, phenotypically, to their environments

  44. phenotype environment environment environment • Nature = Genetics (Genotype) • Nurture = the Environment • Phenotype = Genotype + Environment + (the Interaction of Genotype & Environment) • “Nature vs. Nurture” is a shorthand for asking whether or not a Reaction Norm (phenotype as a function of environment) is a Horizontal Line • Often Nature vs. Nurture debates center around phenomenon for which we don’t have a strong mechanistic understanding, e.g., human psychology Nature vs. Nurture

  45. Human Genetics (Pedigree Analysis)

  46. Human Dominant & Recessive Traits Table is from http://207.233.44.253/wms/reynolmj/lifesciences/lecturenote/bio3/Chap09.ppt • Most genetic diseases are recessive traits • In other words, there is an absence of a protein function

  47. Pedigree Analysis This is Skipping of Generations

  48. Pedigree Analysis

  49. Autosomal Recessive Inheritance • Heterozygotes carry the recessive allele but exhibit the wildtype phenotype • Males and females are equally affected and may transmit the trait • May skip generations • Note that with rare recessive traits we usually assume that people from outside of a family do not possess the affecting allele

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