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1. Multiple alleles ( more than 2 alleles for gene in population)

Modified Mendelian Ratios Ch. 13. 1. Multiple alleles ( more than 2 alleles for gene in population). Example: Blood Groups Karl Landsteiner 1900’s. Chromosome 9 I gene. ABO blood system = polymorphic I gene Blood type genotype A I A I A or I A i B ? AB I A I B O ii

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1. Multiple alleles ( more than 2 alleles for gene in population)

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  1. Modified Mendelian Ratios Ch. 13 1. Multiple alleles (more than 2 alleles for gene in population) • Example: Blood Groups Karl Landsteiner 1900’s Chromosome 9 I gene

  2. ABO blood system = polymorphic I gene Blood type genotype A IAIA or IAi B ? AB IAIB O ii What is the mechanism of inheritance of A, B, AB, O? Autosomal or sex chromosome? Which two alleles are codominant? Which allele is recessive?

  3. Example • A child has type O blood. The mother of the child has Type B blood. What could the blood types of the f IBi • ii

  4. A,B,O carbohydrate antigens

  5. Bombay phenotype • A man IAIA (Type A) and a woman of type IAIB have a child with type O blood. How??

  6. HH IAI A Hh IAIi • I gene encodes glycotransferases • Glycotransferases add sugar group to glycolipid • H gene encodes the glycolipid antigen

  7. 2. Incomplete dominance Example: snapdragons Allelic symbols do not specify dominance CR = red color Cw = white color CR CR X Cw Cw

  8. 3. Codominance Each allele encodes separate gene product distinct in heterozygote L gene for human blood cell surface protein LM = M antigen LMLN x LMLN LN = N antigen 1M

  9. A man with the M bloodtype has a child with a woman of the MN bloodtype • Expected ratio of offspring?

  10. 4. Lethal alleles MM = normal spine MM’ = manx cat (no tail) M’M’ = lethal Cross two manx, what is ratio of phenotypes in offspring? How do breeders obtain manx cats?

  11. Lethal alleles in humans Tay Sach disease tt is fatal Huntington disease HH is fetal lethal Hh causes death ~ age 50.

  12. Polydactyly, dominant 5. Penetrance • % individuals that exhibit phenotype corresponding to genotype Pp pp 5,5 6, 5 6, 6

  13. 6. Expressivity=the extent to which a trait is exhibited Piebald spotting

  14. Both sisters have same genotype Human piebald trait autosomal dominant defect in melanocyte development mutation in the KITgene, which encodes cell surface receptor tyrosine kinase http://dermatlas.med.jhmi.edu/derm/indexDisplay.cfm?ImageID=386927278

  15. Incomplete penetrance AND expressivity • NF-1 = Neurofibromatosis1 • (1/4000, 17q11.2) • (350 kb gene, 60 exons) • Autosomal dominant trait NN and Nn with 50- 80% penetrance • Variable expressivity • Mild form tumors on nerve CT coverings, skin, eyes, organs, face • speech, blood pressure, spine curvature, headaches

  16. Gene expression also affected by: • Sex (baldness) • Temperature (melanin in Siamese cats) • Chemicals (PKU) • Diet (height, cancer)

  17. 7. Epistasis- gene product interactions • A product of one gene influences, or masks, the expression of another gene(s) • Modification of dihybrid cross ratio AaBb X AaBb 9:3:3:1

  18. Epistasis in labrador retrievers • B and E color genes (labs) B black b brown E color e no color (yellow) ee is epistatic Cross two double heterozygotes Phenotypes of parents? Phenotypes of offspring? ratio?

  19. The genotype rrppgives roosters a single comb. R-P- gives a walnut comb. rrP- gives a pea comb, R-pp gives a rose comb. Determine the ratio of phenotypes in the F2 generation of the following cross P ? F1 ? F2 ? ROSE PEA SINGLE WALNUT

  20. Epistasis in Cats • W = white w = not white • B = black b = brown Mate 2 heterozygous cats What is the expected ratio?

  21. 8. Polygenic traits • Vary continuously • Weight, height, IQ

  22. Example: CAD is a multifactorial trait affected by • Obesity, diabetes, high blood pressure, high levels of LDL-cholesterol, and gum disease • Although CAD runs in families, does not show Mendelian inheritance patterns • CAD occurs more often in men • Risk higher among US African Americans than Caucasians or Asians • >200 genes identified • LDLR and familial hypercholesterolemia (dominant)

  23. Influenced by many genes and environmental factors • Predisposition genes + environment • Cleft palate and identical twins, colon cancer may run in families

  24. 4. Dominance series – C series/ rabbits c+ = full color cch = chinchilla (hypomorphic) ch = himalayan (hypomorphic) c = albino (apomorphic allele = nonfunctional) Himalayan Albino Chinchilla

  25. Genotypephenotype? cch cch cch ch ch c c+ cch c+ = full color cch = chinchilla (hypomorphic) ch = himalayan (hypomorphic) c = albino (apomorphic allele = nonfunctional)

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