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Mendelian Principles and Extensions: Pure Lines, Dominance, Segregation, and Linkage

This text explains the Mendelian principles, including pure lines, dominance, segregation, and linkage. It also discusses extensions of Mendel's rules such as sex linkage, incomplete/co-dominance, multiple alleles, epistasis, and quantitative traits.

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Mendelian Principles and Extensions: Pure Lines, Dominance, Segregation, and Linkage

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  1. “Pure” lines Reliably produce progeny like parents. Now often called inbred or isogenic lines. Mendel worked with pure lines of garden peas.

  2. Garden peas normally reproduce by self-pollination.

  3. Mendel could control parentage of crosses.

  4. P1 = parents Pure lines in this case. X F1 = 1st generation In this example, one trait dominates over the other. F2 = 2nd generation Produced by selfing. The hidden trait re-appears!

  5. Dominance means that the phenotype will reflect the presence of that allele equally, whether or not the other allele is present. Short hair is dominant to long hair in trolls. • Dominance does NOT mean: • Better (there are dominant lethal alleles) • More common (dominant alleles can be uncommon) • Also -- for many traits the heterozygote is intermediate • between the two homozygotes, reflecting incomplete or • co-dominance. More on this later…..

  6. Segregation= separation of two alleles of parent into gametes. “particulate Inheritance”

  7. Do alleles for different genes segregate together? For example, must green peas be wrinkly?

  8. Independent Assortment. Transmission for alleles of one trait is independent of transmission for (most) other traits.

  9. “Dihybrid cross” between parents heterozygous for two traits. Get famous 9:3:3:1 ratio of phenotypes.

  10. Testcross confirms predictions of independent assortment. Recessive parent used to reveal genotype of hypothesized RrYy (dominant phenotype is difficult to genotype!).

  11. Practice problems for Mendelian principles. Next, extension of Mendel’s rules: 1)sex-linked genes.

  12. Drosophila has same system.

  13. Females have 2 copies of allele; Males have only one, Inherited from mother. red eyes dominant

  14. This slide shows meiosis to produce sperm. Meiosis to produce eggs would begin with two X-chroms, which can cross over (recombine).

  15. Do sex-linkage problems from www.biology.arizona.edu

  16. About 10% of human males cannot distinguish long & medium wavelengths (red & green). The opsin gene for short wavelengths is on an autosome. Which chromosome carries the genes for long & medium wavelengths?

  17. Pedigree analysis can reveal sex-linkage in humans. These two had 9 kids. Work backwards to identify carriers.

  18. Extension of Mendel’s rules: 1) Sex linkage 2) Incomplete/co-dominance

  19. Enzymes generally show co-dominance, with the heterozygote producing both forms of the enzyme This slide actually shows a pcr, so reveals genotype rather than phenotype. But, gels of enzymes look very much like this, with the heterozygote producing two types of enzymes that migrate differently, and may have different activities in particular environments. ADH is an example.

  20. Extension of Mendel’s rules: 1) Sex linkage 2) Incomplete/co-dominance 3) Multiple alleles at a locus

  21. Distribution of ADH* alleles in human populations. There are generally >2 common alleles in each population. *Alcohol dehydrogenase-4 Class 1

  22. Red blood cell membranes have stuff sticking out….proteins, carbohydrate, etc. The ABO molecules are carbohydrates. cell membrane From http://www.blood.co.uk/hospitals/training/IntBGS/IntBGS02.htm

  23. There are three common alleles at the ABO locus. What are the dominance relationships among the alleles? (In some cases, complete and in some co-dominance.)

  24. Extension of Mendel’s rules: 1) Sex linkage 2) Incomplete/co-dominance 3) Multiple alleles at a locus 4) Epistasis -- interactions among alleles of DIFFERENT loci affect phenotype.

  25. Genes C and B both affect hair color in mice CC or Cc Color BB or Bb Black Agouti cc bb Albino Brown Agouti

  26. What will be the F2 ratios of a cross between pure lines of black and albino mice?

  27. LINKAGE Image from http://www.jurassiccoast.com

  28. Alleles on the same chromosome don’t assort independently.

  29. Quantitative traits

  30. Complex diseases influenced by multiple genes in interaction with the environment: Diabetes Cardiovascular disease Cancer Alzheimer’s disease Schizophrenia Epilepsy…. Go to http://www.ncbi.nlm.nih.gov/mapview/map_search.cgi Type in Alzheimer; you will see the locations 29 genes currently associated with the disease. You can then click on a chromosome to see information about each gene.

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