Mendel 2

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Mendel 2

  1. 1. More of Mendel<br />
  2. 2. What we know…<br />Inheritance of biological characteristics is determined by genes<br />Principle of Dominance<br />When there are two or more alleles for a gene, some are dominant while others are recessive<br />Law of Segregation<br />In sexually reproducing organisms, adult cells have two copies of each gene—one from each parent; these genes segregate when GAMETES are formed<br />
  3. 3. Does the segregation of 1 pair of alleles affect the segregation of another pair of alleles?<br />
  4. 4. Let’s look at Pea shape and pea color<br />Round (R) vs. wrinkled (r)<br />Yellow (Y) vs. green (y)<br />Remember, there are 4 possible gametes<br />If a parent is homozygous dominant (true-breeding dominant) for both shape and color, what will all possible gametes be?<br />If a parent is homozygous recessive (true-breeding recessive) for both shape and color, what will all possible gametes be?<br />
  5. 5. Make a test cross of:<br /><ul><li>True breeding Round Yellow Peas
  6. 6. Genotype (RRYY)
  7. 7. True breeding Wrinkled Green peas
  8. 8. Genotype (rryy)</li></ul>ry<br />ry<br />ry<br />ry<br />RY<br />RY<br />RY<br />RY<br />
  9. 9. What is the phenotype of the F1 offspring?<br />What is the genotype of the F1 offspring?<br />
  10. 10. What this first cross told us…<br />All F1 offspring were heterozygous for seed shape (round) and seed color (yellow) RrYy<br />The F1 plant was made from fusing a gamete carrying RY and a gamete carrying ry<br />Will the dominant alleles stay together or separate when making the F2 offspring?<br />
  11. 11. Now cross these hybrid (RrYy) plants on a new 4X4 Punnett Square<br />
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  13. 13. What does the F2 cross tell us?<br />Are there combinations of alleles that we did not see in either of the parents?<br />This means that the alleles for seed color separated independently than the alleles for seed shape<br />Genes that segregate separately do NOT influence each other’s inheritance<br />
  14. 14. Principle of Independent Assortment<br />Genes for different traits can segregate independently during the formation of gametes. This principle helps account for many genetic variations in plants, animals and other organisms.<br />
  15. 15. Mendel’s 4 Principle’s<br />Inheritance of biological characteristics is determined by genes<br />Principle of Dominance<br />Law of Segregation<br />Principle of Independent Assortment<br />
  16. 16. Exceptions to Mendel<br />Genetics more complicated<br />Some alleles are neither dominant nor recessive<br />Many traits are controlled by multiple alleles or multiple genes<br />
  17. 17. Incomplete Dominance<br />When red flowered (CRCR) plants were crossed with white flowered (CWCW) plants they made…pink flowers (CRCW)<br />Which allele is dominant?<br />neither<br />Incomplete dominance:<br />Case in which one allele is not dominant over another<br />The heterozygous phenotype is somewhere between the two homozygous phenotypes<br />
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  19. 19. Codominance<br />Both alleles contribute to the phenotype<br />Chickens<br />Allele for black feathers is codominant with allele for white feathers<br />Chicken looks speckled with black and white feathers<br />Not like the blending of dominant phenotypes…<br />BOTH dominant phenotypes show up<br />In humans<br />Gene for protein that controls cholesterol levels in the blood<br />People with heterozygous form make both types of protien<br />
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  21. 21. Multiple Alleles<br />When a gene has more than two alleles<br />NOT more than 2 alleles for a person but MORE than 2 alleles for the trait exist<br />Coat color in rabbits<br />A single gene for coat color<br />At least 4 different alleles<br />Simple dominance and make 4 possible coat colors<br />Genes for human blood type<br />3 different alleles: IA, IB, I<br />You can get different genotypes:<br />IAIA<br />IAi<br />IAIB<br />IBIB<br />Ibi<br />ii<br />You can get different Phenotypes:<br />Type A (dom)<br />Type B (dom)<br />Type AB (dom)<br />Type O (recessive)<br />
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  26. 26. Polygenic Traits<br />“Poly” many<br />“-genic” genes<br />Traits controlled by two or more genes<br />Several genes interact to produce a trait<br />Wide range of phenotypes<br />Skin color<br />Four different genes<br />Fruit Fly eye color<br />Three genes make the reddish brown pigment <br />
  27. 27. Epistasis<br />When the expression of one gene effects the expression of another gene<br />Ex. Fur color in mice…controlled by 2 separate genes<br />Gene 1<br />Brown fur pigment (BB or Bb) is dominant over gray fur pigment (bb)<br />Gene 2<br />Coat Pigment depositing gene <br />Dominant gene (CC or Cc) means fur will get pigment and this is determined by gene one<br />Recessive gene (cc) means that no pigment will be deposited on fur…whether the they have the gene for black or brown fur<br />The gene for Pigment Deposition is the EPISTATIC gene because it alters the Phenotypic ration<br />Sd<br />
  28. 28. Homework<br />Complete a Dihybrid cross for 2 mice that are heterozygous for both Brown fur (Bb) and Pigment deposition (Pp)<br />Predict your phenotypic ratio…<br />What are you final phenotypic ratios?<br />Write a paragraph explaining your results.<br />
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  30. 30. Genes and the Environment<br />Genes provide the plan for development<br />How the plan unfolds depends on the environment<br />Example:<br />Sunflower has genes for height and color of flowers<br />But these traits are also influenced by climate, soil conditions and water availability<br />
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  32. 32. Genes<br />
  33. 33. Applying Mendel’s Principles…<br />Apply Mendel’s Principles to many organisms, including humans<br />Thomas Hunt Morgan (1900’s)<br />American geneticist<br />Common fruit fly<br />Drosophiliamelanogaster<br />Produced offspring very quickly<br />Single pair of flies=100 offspring<br />Mendel’s principle’s were tested with Drosophilia and many other organisms and they applied to all of them as well<br />
  34. 34. Would genes on the same chromosome be inherited together?<br />Thomas noticed that almost every single time he crossed two flies that each had red eyes and mini wings, the offspring almost always inherited BOTH red eyes and mini wings<br />This went against Mendel’s Principle of Independent Assortment…(review!)<br />
  35. 35. Gene Linkage<br />Thomas Hunt Morgan gave us the answer<br />1910 PRINCIPLE of LINKAGE<br />50 Drosophilia genes<br />Seemed to contradict Principle of Independent Assortment b/c certain genes were always inherited together<br />He grouped the fly’s genes into linkage groups<br />
  36. 36. Linkage groups are made up of genes that seem to be inherited together<br />Linkage groups assort independently but all genes in one group are inherited together<br />Drosophilia<br />4 linkage groups<br />4 chromosomes<br />What can be concluded?<br />
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  38. 38. Conclusions<br />Each chromosome is actually a group of linked genes<br />Mendel’s Principle of Independent Assortment holds true but an adjustment needs to be made…<br />IT IS THE CHROMOSOMES THAT ASSORT INDEPENDENTLY, NOT THE INDIVIDUAL GENES<br />
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  41. 41. How Mendel missed it…<br />What 7 genes did he study?<br />6 of the 7 genes were on different chromosomes<br />Two genes were on the same chromosome but they were so far apart on the same chromosome that they assorted independently<br />
  42. 42. If two genes are on the same chromosome, does that mean they are linked for ever?<br />No, chromosomes cross over during meiosis so they can separate<br />Crossing over produces new combinations of alleles<br />Important for genetic diversity<br />
  43. 43. Lucky Student<br />Alfred Sturtevant 1911<br />Columbia University<br />Worked in Morgan’s Lab<br />Hypothesis<br />If two genes are farther apart on a chromosome, the they are more likely to be separated during meiosis<br />Experiment<br />Measured the rate at which linked genes were separated and recombined to make a “map” of distances between genes<br />Conclusion<br />Recombination rates could be used to make gene maps<br />Gene maps showed the location of a gene on a chromosome<br />
  44. 44. Gene Linkage and Crossing Over<br />The farther apart 2 genes are on a chromosome….<br />The more likely they are to “cross-over”<br />The closer two genes are on the same chromosome…<br />The less likely they are to be separated<br />

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