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Mendel 2 revised


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  • 1. More of Mendel
  • 2. What we know…
    Inheritance of biological characteristics is determined by genes
    Principle of Dominance
    When there are two or more alleles for a gene, some are dominant while others are recessive
    Law of Segregation
    In sexually reproducing organisms, adult cells have two copies of each gene—one from each parent; these genes segregate when GAMETES are formed
  • 3. Does the segregation of 1 pair of alleles affect the segregation of another pair of alleles?
  • 4. Let’s look at Pea shape and pea color
    Round (R) vs. wrinkled (r)
    Yellow (Y) vs. green (y)
    Remember, there are 4 possible gametes
    If a parent is homozygous dominant (true-breeding dominant) for both shape and color, what will all possible gametes be?
    If a parent is homozygous recessive (true-breeding recessive) for both shape and color, what will all possible gametes be?
  • 5. Make a test cross of:
    • True breeding Round Yellow Peas
    • 6. Genotype (RRYY)
    • 7. True breeding Wrinkled Green peas
    • 8. Genotype (rryy)
  • 9. What is the phenotype of the F1 offspring?
    What is the genotype of the F1 offspring?
  • 10. What this first cross told us…
    All F1 offspring were heterozygous for seed shape (round) and seed color (yellow) RrYy
    The F1 plant was made from fusing a gamete carrying RY and a gamete carrying ry
    Will the dominant alleles stay together or separate when making the F2 offspring?
  • 11. Now cross these hybrid (RrYy) plants on a new 4X4 Punnett Square
  • 12.
  • 13. What does the F2 cross tell us?
    Are there combinations of alleles that we did not see in either of the parents?
    This means that the alleles for seed color separated independently than the alleles for seed shape
    Genes that segregate separately do NOT influence each other’s inheritance
  • 14. Principle of Independent Assortment
    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.
  • 15. Mendel’s 4 Principle’s
    Inheritance of biological characteristics is determined by genes
    Principle of Dominance
    Law of Segregation
    Principle of Independent Assortment
  • 16. Exceptions to Mendel
    Genetics more complicated
    Some alleles are neither dominant nor recessive
    Many traits are controlled by multiple alleles or multiple genes
  • 17. Incomplete Dominance
    When red flowered (CRCR) plants were crossed with white flowered (CWCW) plants they made…pink flowers (CRCW)
    Which allele is dominant?
    Incomplete dominance:
    Case in which one allele is not dominant over another
    The heterozygous phenotype is somewhere between the two homozygous phenotypes
  • 18.
  • 19. Codominance
    Both alleles contribute to the phenotype
    Allele for black feathers is codominant with allele for white feathers
    Chicken looks speckled with black and white feathers
    Not like the blending of dominant phenotypes…
    BOTH dominant phenotypes show up
    In humans
    Gene for protein that controls cholesterol levels in the blood
    People with heterozygous form make both types of protien
  • 20.
  • 21. Multiple Alleles
    When a gene has more than two alleles
    NOT more than 2 alleles for a person but MORE than 2 alleles for the trait exist
    Coat color in rabbits
    A single gene for coat color
    At least 4 different alleles
    Simple dominance and make 4 possible coat colors
    Genes for human blood type
    3 different alleles: IA, IB, I
    You can get different genotypes:
    You can get different Phenotypes:
    Type A (dom)
    Type B (dom)
    Type AB (dom)
    Type O (recessive)
  • 22.
  • 23.
  • 24.
  • 25.
  • 26. Polygenic Traits
    “Poly” many
    “-genic” genes
    Traits controlled by two or more genes
    Several genes interact to produce a trait
    Wide range of phenotypes
    Skin color
    Four different genes
    Fruit Fly eye color
    Three genes make the reddish brown pigment
  • 27. Epistasis
    When the expression of one gene effects the expression of another gene
    Ex. Fur color in mice…controlled by 2 separate genes
    Gene 1
    Brown fur pigment (BB or Bb) is dominant over gray fur pigment (bb)
    Gene 2
    Coat Pigment depositing gene
    Dominant gene (CC or Cc) means fur will get pigment and this is determined by gene one
    Recessive gene (cc) means that no pigment will be deposited on fur…whether the they have the gene for black or brown fur
    The gene for Pigment Deposition is the EPISTATIC gene because it alters the Phenotypic ration
  • 28. Homework
    Complete a Dihybrid cross for 2 mice that are heterozygous for both Brown fur (Bb) and Pigment deposition (Pp)
    Predict your phenotypic ratio…
    What are you final phenotypic ratios?
    Write a paragraph explaining your results.
  • 29.
  • 30. Genes and the Environment
    Genes provide the plan for development
    How the plan unfolds depends on the environment
    Sunflower has genes for height and color of flowers
    But these traits are also influenced by climate, soil conditions and water availability
  • 31.
  • 32. Genes
  • 33. Applying Mendel’s Principles…
    Apply Mendel’s Principles to many organisms, including humans
    Thomas Hunt Morgan (1900’s)
    American geneticist
    Common fruit fly
    Produced offspring very quickly
    Single pair of flies=100 offspring
    Mendel’s principle’s were tested with Drosophilia and many other organisms and they applied to all of them as well
  • 34. Would genes on the same chromosome be inherited together?
    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
    This went against Mendel’s Principle of Independent Assortment…(review!)
  • 35. Gene Linkage
    Thomas Hunt Morgan gave us the answer
    50 Drosophilia genes
    Seemed to contradict Principle of Independent Assortment b/c certain genes were always inherited together
    He grouped the fly’s genes into linkage groups
  • 36. Linkage groups are made up of genes that seem to be inherited together
    Linkage groups assort independently but all genes in one group are inherited together
    4 linkage groups
    4 chromosomes
    What can be concluded?
  • 37.
  • 38. Conclusions
    Each chromosome is actually a group of linked genes
    Mendel’s Principle of Independent Assortment holds true but an adjustment needs to be made…
  • 39.
  • 40.
  • 41. How Mendel missed it…
    What 7 genes did he study?
    6 of the 7 genes were on different chromosomes
    Two genes were on the same chromosome but they were so far apart on the same chromosome that they assorted independently
  • 42. If two genes are on the same chromosome, does that mean they are linked for ever?
    No, chromosomes cross over during meiosis so they can separate
    Crossing over produces new combinations of alleles
    Important for genetic diversity
  • 43. Lucky Student
    Alfred Sturtevant 1911
    Columbia University
    Worked in Morgan’s Lab
    If two genes are farther apart on a chromosome, the they are more likely to be separated during meiosis
    Measured the rate at which linked genes were separated and recombined to make a “map” of distances between genes
    Recombination rates could be used to make gene maps
    Gene maps showed the location of a gene on a chromosome
  • 44. Gene Linkage and Crossing Over
    The farther apart 2 genes are on a chromosome….
    The more likely they are to “cross-over”
    The closer two genes are on the same chromosome…
    The less likely they are to be separated
  • 45.
  • 46.
  • 47.
  • 48.
  • 49. Steps for Dihybrid cross
    Make a Key
    Trait 1: height
    Dominant phenotype: Tall TT or Tt (ways to get it)
    Recessive phenotype: Short tt
    Trait2: color
    Dominant phenotype: Purple PP or Pp
    Recessive phenotype: White pp
    Write out genotypes for each parent
    _ _ _ _ x _ _ _ _
    Write Out Gametes for each Parent (use arrows)
    4 gametes for each parent (_ _)
    Make Punnett Square (16) boxes
    Label parent one and write gametes along top
    Label Parent 2 and write their gametes on side
    Fill in each box (should have 4 letters)
    Tally genotypes
    Write out 4 possible phenotypic combinations
    Dominant trait 1 and dominant for trait 2:______
    Dominant trait 1 and recessive trait 2:_______
    Recessive trait 1 and Dominant trait 2:_______
    Recessive trait 1 and recessive trait 2:________
    Tally Phenotypes (should =16)
    Write phenotypic ratio
    Dominant- capital letter D
    Recessive- lower case d
    2 of the same size letter
    If its two little letters recessive dd
    If its 2 big letters it is dominant DD
    2 different size letters (capital and lowercase) Dd
    ALWAYS dominant