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Beyond Mendel

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Beyond Mendel

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  1. Beyond Mendel Not simply “dominant & recessive”

  2. Multiple allelic traits • Can occur in two ways: • New alleles arise by mutation • Genes may have more than 2 allele types

  3. New alleles by mutation • mutations: rare, stable, & inherited changes in DNA • Most common, “expected” allele is defined as the wildtype • Other alleles are defined as mutant alleles • Ex. Eye color in fruit flys: red eyes  wildtype vs. white eyes  mutant

  4. New alleles by mutation • polymorphic allele: genetic locus with a wild-type that occurs < 99% of the time

  5. Genes with multiple alleles • Occur from random mutation • More than 2 allele types exist for the gene, although each individual contains only 2 alleles for the gene (one from mom & one from dad) • Increase the number of phenotypes (not just Mendel’s two types)

  6. Genes with multiple alleles • Example: coat color in rabbits one gene with 4 alleles with a dominant heirarchy: C>cch>ch>c C (dark gray) ch (chinchilla) h (himalayan) c (albino)

  7. Genes with multiple alleles

  8. No dominant over recessive • incomplete dominance: one allele is not dominant over another • heterozygotes: show interimediates between 2 allele types • codominance: one or more alleles are dominant & • heterozygotes: both alleles get expressed in the phenotype

  9. Incomplete dominance • Ex. Snapdragons CWCW x CRCR white x red

  10. Incomplete dominance • Follows Mendel’s laws because in F2 generation, all 3 phenotypes appear (no blending) • Instead of expected 3:1 ratio in F2 generation, get 1:2:1 ratio of offspring

  11. Codominance • Two alleles at a locus produce two different phenotypes that are both expressed in the heterozygote

  12. Codominance • Ex. Blood types • Four phenotypes possible: A, B, AB, and O • 3 possible alleles: IA, IB, iO

  13. Pleiotropic alleles • single allele can affect more than one phenotype Ex. Allele responsible for coloration pattern in Siamese cats ( light body, darker extremeties) Same allele is responsible for the crossed eyes of the cats

  14. Pleiotropic alleles • Allele results in the same protein, but affects two different characteristics

  15. Genes effecting genes • epistasis: the phenotypic expression of one gene is affected by another gene Ex. Coat color in mice - wild-type (B): agouti (grayish pattern resulting from bands on the individual hairs) bb: no bands  black

  16. Epistasis in mice • A second locus on another chromosome affects early development of hair pigmentation A (normal pigmentation) aa (no pigmentation)

  17. Epistasis in mice • All mice having aa are albino, regardless of the genotype at the B locus • When considering both loci together in a cross, don’t get the expected Mendelian ratio in the F2 generation (9:3:3:1)

  18. Epistasis in mice AABB x aabb AaBb AaBb x AaBb 9 agouti: 3 black: 4 albino

  19. Effect of environment • Genotype and environment interact to determine the phenotype of an organism. • Light, temp., nutrition

  20. Effect of environment • Consider 2 parameters: • Penetrance: proportion of individuals in a group with a given genotype that show the expected phenotype • Expressivity: degree to which a genotype is espressed in an individual

  21. Environment & genotype • Result in a population with a continuous variation (instead of two extremes) • due to multiple genes, each with multiple alleles, and environmental influences

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