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Genetics Biology 12

Genetics Biology 12. Mendel’s laws are operating alleles are segregating independent assortment fertilization is random. but…Mendel didn’t look at how complex things can get!. Post-Mendelian studies have revealed that relationships between genotype and phenotype can be complex.

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Genetics Biology 12

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  1. Genetics Biology 12

  2. Mendel’s laws are operating • alleles are segregating • independent assortment • fertilization is random but…Mendel didn’t look at how complex things can get!

  3. Post-Mendelian studies have revealed that relationships between genotype and phenotype can be complex • degrees of dominance • multiple alleles • epistasis • polygenic inheritance • environmental effects

  4. Example of incomplete dominance

  5. Snapdragons www.florifacts.umn.edu/Trials/Annual%20Beddin...

  6. Multiple alleles: Individual - 2 alleles for any autosomal gene - one on each homolog Population - gene can exist in more than two forms

  7. A medical problem - some blood transfusions produce lethal clumping of cells. Don’t worry about details yet...

  8. The Human ABO Blood Group System multiple alleles codominance

  9. Table 14.2

  10. Codominance in the Human ABO Blood Group System Dominance Dominance Codominance

  11. Fig. 17.4

  12. Possible Dominance Relationships between Two Alleles Dominance RelationshipHeterozygote Phenotype Complete (Mendelian)same as homozygous dominant Incomplete intermediate between the two homozygotes Codominance phenotypes of the two homozygotes are expressed

  13. Epistasis: one gene alters or blocks the expression of another independently inherited gene. Disclaimer: Made-up example A mutant gene that caused complete baldness in humans would be epistatic to a gene that determines hair color.

  14. A d D a b b e e

  15. Epistasis Normal dihybrid ratio is altered from 9:3:3:1 to 9:3:4:0 C/c gene is epistatic to the B/b gene

  16. Epistasis example: Coat color - Labrador retrievers Gene 1 - production of pigment BB - Black Bb - black bb – chocolate Gene 2 - pigment deposition EE or Ee - pigment deposited ee - no pigment deposited

  17. BBEE BBEe BbEE BbEe BBee Bbee bbee bbEE bbEe

  18. BbEe X BbEe BE Be bE be BE BBEE BBEe BbEE BbEe Be BBEe BBee BbEe Bbee BbEE BbEe bbEE bbEe bE BbEe Bbee bbEe bbee be

  19. environment often influences phenotype • the norm of reaction = phenotypic range due to environmental effects • norms of reactions are often broadest for polygenic characters. Blue require low pH

  20. Polygenic inheritance: additive effects (essentially, incomplete dominance) of multiple genes on a single trait. More than one gene controls the outcome of the phenotype. AA = dark Aa = less dark aa - light And similarly for the other two genes - in all cases dominance is incomplete for each gene. Think of each “capital” allele (A, B, C) as adding a dose of brown paint to white paint.

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