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Genetics in Humans and Mammals: PART1

1. 2. 3. 4. 5. 6. Genetics in Humans and Mammals: PART1. Pleiotropy. Some genes have multiple phenotypic affects For example: Genes that cause diseases in humans that are responsible for multiple symptoms. Epistasis:.

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Genetics in Humans and Mammals: PART1

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  1. 1 2 3 4 5 6 Genetics in Humans and Mammals: PART1

  2. Pleiotropy • Some genes have multiple phenotypic affects • For example: • Genes that cause diseases in humans that are responsible for multiple symptoms

  3. Epistasis: • When a gene at one locus affects the expression of a gene at another locus. • For example, in mice fur color can be black (B) or brown (b). There is another gene C or c determines if pigment will be deposited for hair. Depositing pigment is dominant over not disposing pigment. Individuals that do not deposit pigment are albino (white) even if they have the gene for brown or black fur

  4. A practice question • Mate two mice that are hetrozygous for fur color and pigment deposition. What is the expected phenotypic ratio?

  5. Epistasis in Labrador retrievers • 2 genes: (E,e) & (B,b) • pigment (E) or no pigment (e) • pigment concentration: black (B) to brown(b) eebb eeB– E–bb E–B–

  6. Polygenic Inheritance • Traits that are controlled by 2 or more genes that have an additive affect • For example: skin pigmentation is controlled by three separately inherited genes Lets consider three genes A, B, C, where the upper case letter stands for dark pigment and the lower case letter stands for light or no pigment

  7. Order from darkest to lightest • AABBCC • aabbcc • AaBBCC • aabbCC • aaBBCC

  8. Craziness • AaBBCC x AaBBcc How many different shades of skin color are possible for this cross?

  9. Polygenic inheritance • phenotypes on a continuum • human traits • skin color • height • weight • intelligence • behaviors

  10. Recessive diseases • The diseases are recessive because the allele codes for either a malfunctioning protein or no protein at all • Heterozygotes (Aa) • carriers • have a normal phenotype because one “normal” allele produces enough of the required protein

  11. male / sperm A a AA Aa A A a A female / eggs Aa Aa a Aa aa a Heterozygote crosses • Heterozygotes as carriers of recessive alleles Aa x Aa AA Aa carrier Aa aa carrier disease

  12. Cystic fibrosis (recessive) • Primarily whites of European descent • strikes 1 in 2500 births • 1 in 25 whites is a carrier (Aa) • normal allele codes for a membrane protein that transports Cl- across cell membrane • defective or absent channels limit transport of Cl- & H2O across cell membrane • thicker & stickier mucus coats around cells • mucus build-up in the pancreas, lungs, digestive tract & causes bacterial infections • without treatment children die before 5; with treatment can live past their late 20s normal lung tissue

  13. Tay-Sachs (recessive) • Primarily Jews of eastern European (Ashkenazi) descent & Cajuns (Louisiana) • strikes 1 in 3600births • 100 times greater than incidence among non-Jews • non-functional enzyme fails to breakdown lipids in brain cells • fats collect in cells destroying their function • symptoms begin few months after birth • seizures, blindness & degeneration of muscle & mental performance • child usually dies before 5yo

  14. Sickle cell anemia (recessive) • Primarily Africans • strikes 1 out of 400 African Americans • high frequency • caused by substitution of a single amino acid in hemoglobin • when oxygen levels are low, sickle-cell hemoglobin crystallizes into long rods • deforms red blood cells into sickle shape • sickling creates pleiotropiceffects = cascade of other symptoms

  15. Sickle cell anemia • Substitution of one amino acid in polypeptide chain hydrophilicamino acid hydrophobic amino acid

  16. Sickle cell phenotype • 2 alleles are codominant • both normal & mutant hemoglobins are synthesized in heterozygote (Aa) • 50% cells sickle; 50% cells normal • carriers usually healthy • sickle-cell disease triggered under blood oxygen stress • exercise

  17. Heterozygote advantage • Malaria • single-celled eukaryote parasite spends part of its life cycle in red blood cells • In tropical Africa, where malaria is common: • homozygous dominant individuals die of malaria • homozygous recessive individuals die of sickle cell anemia • heterozygote carriers are relatively free of both • reproductive advantage • High frequency of sickle cell allele in African Americans is vestige of African roots

  18. Prevalence of Malaria Prevalence of SickleCell Anemia

  19. 1872 Huntington’s chorea (dominant) • Dominant inheritance • repeated mutation on end of chromosome 4 • mutation = CAG repeats • glutamine amino acid repeats in protein • one of 1st genes to be identified • build up of “huntingtin” protein in brain causing cell death • memory loss • muscle tremors, jerky movements • “chorea” • starts at age 30-50 • early death • 10-20 years after start Testing…Would youwant toknow?

  20. Genetic testingAminiocentesisCVS sequence individual genes

  21. Genetic counseling • Thousands of genetic disorders are inherited as simple recessive traits • from benign conditions to deadly diseases • albinism • cystic fibrosis • Tay sachs • sickle cell anemia • PKU

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