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Patterns of Inheritance 2 (Ch. 12)

Patterns of Inheritance 2 (Ch. 12). Curly the Curl-Eared Cat says Happy Halloween!. A Bit More Genetics. A bunch of “wacky and wild” fly geneticists having fun in the lab. Variations on Mendel’s Principles. Since Mendel: genetics more complicated Variations: Incomplete dominance

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Patterns of Inheritance 2 (Ch. 12)

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  1. Patterns of Inheritance 2 (Ch. 12) Curly the Curl-Eared Cat says Happy Halloween!

  2. A Bit More Genetics A bunch of “wacky and wild” fly geneticists having fun in the lab.

  3. Variations on Mendel’s Principles • Since Mendel: genetics more complicated • Variations: • Incomplete dominance • Codominance • Multiple alleles • Pleiotropy • Polygenic inheritance • Linked genes • Sex-linked genes.

  4. Incomplete dominance • Both alleles expressed in heterozygote (phenotype intermediate) • Ex, flower color in snapdragons

  5. Incomplete dominance • F1 cross has 1:2:1 genotypic (1 RR, 2 Rr, 1 rr) ratio • Phenotypic ratio same (1 red, 2 pink, 1 white) • Not 3:1 like dominant/recessive alleles!.

  6. Incomplete dominance • Ex, hypercholesterolemia mentioned before.... Receptor-mediated endocytosis in liver cells

  7. Incomplete dominance • Ex, hypercholesterolemia • Heterozygote with mild disease.

  8. Codominance and Multiple Alleles • Codominance: In heterozygote, each allele fully expressed • Multiple alleles: More than 2 alleles for a given gene • Ex, ABO blood groups. 3 alleles possible: i, IA, IB

  9. Codominance and Multiple Alleles • 3 alleles possible: i, IA, IB • Everone has 2 loci and 2 alleles: 6 genotypes possible! • ii, iIA, iIB, IAIB,IBIB, IAIA.

  10. Codominance and Multiple Alleles • Blood phenotypes (4) • Genotype Phenotype (Blood group) • ii O blood • iIB Bblood • IBIB B blood • iIA A blood • IAIA A blood • IAIB AB blood.

  11. Codominance and Multiple Alleles Genetics problem (blood type): Type O woman has Type B baby. But 2 boyfriends!!: one Type O and other Type AB. Which is the father??

  12. Pleiotropy • 1 gene affects many traits • Ex, sickle cell anemia • 1 wrong amino acid in hemoglobin, causes change in cell shape.

  13. Polygenic Inheritance • “Poly-” = “many” • “Genic” = “genes” • Trait affected by more than 1 gene • Ex, human skin color (melanin pigment) • At least 3 genes involved • For each, dominant allele makes some pigment, recessive doesn’t.

  14. Polygenic Inheritance • Skin color depends on how many copies of dominant alleles you have • Intelligence, height are other polygenic traits.

  15. Linked Genes • Linked genes: located on same chromosome • Mendel: genes for 2 traits on DIFFERENT sets of chromosomes • Assorted independently RrYy X RrYy example

  16. Linked Genes • Genes linked? On SAME pair homologous chromosomes?

  17. Linked Genes • Linked genes do not assort independently • Ex: flower color (P=purple and p=white) and pollen shape (L=long and l=short) in peas • Parents heterozygous both traits • Expect 9:3:3:1 ratio for F1 of dihybrid cross. Note: Change “Red” to “White” above

  18. Linked Genes • Actually get (close to) 3:1 ratio! • Why? Linked genes • Where one goes, other goes (P & L linked and p & l linked). Note change“Red” to read “White” in this chart!

  19. Flashback: Slide About Independent Assortment Genes Linked Genes NOT Linked

  20. Linked Genes Question • Individual organism has genotype AaBb. If genes A and B are linked (do NOT assort independently), how many different gamete genotypes can be produced by that individual organism? (Assume crossing-over does not occur).

  21. Linked Genes • Story complicated: crossing over during meiosis • Mixes genes: some chromosomes recombinant: have combination alleles not in parent (such as A and b here).

  22. Linked Genes • In real experiment, recombinants mix traits of parents • Ex, body color and wing shape linked Expect BOTH traits dominant or BOTH traits recessive in offspring (just like parents)

  23. Linked Genes • Here, body color and wing shape linked • Offspring: some recombinant traits • Why? Crossing over!. Expect BOTH traits dominant or BOTH traits recessive in offspring (just like parents) Recombinants!

  24. Linked Genes • Crossing over happens more the farther apart 2 genes are • Crossing over used to map genes on chromosomes.

  25. Sex-linked genes • Sex chromosomes: homologous pair #23 • X and Y • Females: XX • Males: XY • How sex inherited?

  26. Sex-linked genes • Female gives X • Male gives X or Y • Chance of boy or girl? 1/2 or 50%.

  27. Sex-linked genes • Chance of boy or girl? 1/2 or 50% • Chance of 2 boys in a row? • Multiplicative law: “Probability of 2 independent events happening simultaneously is the product of their probabilities” • Boy 1=1/2 • Boy 2=1/2 • Two in a row: 1/2 X 1/2 = 1/4.

  28. Sex-linked genes • Sex chromosomes: pair #23 • X and Y • Females XX • Males XY

  29. Sex-linked genes • X and Y different sizes (X big, Y short) • Sex-linked genes: present on one sex chromosome but not other

  30. Sex-linked genes • Most: on X but not Y • Females have 2 alleles (one on each X) but males only have 1 allele (on their X) • Inheritance differs for males and females!

  31. Sex-linked genes • Ex, eye color in fruit flies • Red dominant allele (R), white recessive (r)

  32. Sex-linked genes • Use XR for dominant allele and Xr for recessive. Y stands for Y chromosome • Some genotypes and phenotypes: • XRXR=red-eyed female • XrXR=red-eyed female • XrXr=white-eyed female • XRY=red-eyed male • XrY=white-eyed male.

  33. Sex-linked genes • Homozygous dominant red female with white male: all red

  34. Sex-linked genes • Heterozygous female with red male: all females red, but half males white

  35. Sex-linked genes • Heterozygous female with white male: half females red, half males red.

  36. Sex-linked genes • Human sex-linked traits • Red-green colorblindness

  37. Sex-linked genes • Human examples sex-linked traits • Hemophilia: bleeders disease • Recessive allele, locus X chromosome • XH is normal allele, Xh is hemophilia allele • Females can be carrier (heterozygous): Genotype XHXh • Can male be carrier? • No. Has disease (XhY) or doesn’t (XHY)

  38. Sex-linked genes • Hemophilia questions: • If boy has disease (XhY), did he get it from Mom or Dad? • Mom! Dad gave him the Y • A woman who is a carrier has children with a man who does not have hemophilia. Will any of their children have hemophilia? If so, boys or girls or both?

  39. Sex-linked genes • Hemophilia and history: Czar Nicholas II.

  40. Epistasis: interactions between genes • Sometimes, epistasis gives phenotypes not possible with each gene singly • Ex, labrador retrievers

  41. Epistasis: interactions between genes • Pigment production affected by “E” gene • Dominant allele (E) allows pigment, recessive (e) prevents it • ee genotype prevents pigment in hairs: gives no dark color at all (yellow lab) BBEE X bbee yellow lab black lab

  42. F2puppies BE Be bE be BE black BBEE BBEe BbEE BbEe Be BBee BbEe Bbee BBEe chocolate bE BbEe bbEE bbEe BbEE yellow be Bbee bbEe bbee BbEe BBEE X bbee ee genotype prevents pigment deposition in hairs, regardless of what pigment genotype is (BB, Bb, bb) F1puppies are all BbEe

  43. Environment and genetics • Environment can affect gene expression

  44. Environment and genetics • Environment can affect gene expression • Ex, Himalayan rabbits • Enzyme making black pigment works if skin temp. <33 degrees C

  45. Environment and genetics • Similar story for siamese cats!

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