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    Determining the mode of inheritance of genotypes

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    In pings, coat color may be sandy, red, or white. A geneticist spent several years mating true-breeding pigs of all different color combinations, even going so far as to obtain true-breeding lines from different parts of the country. For crosses 1 and 4 below, she encountered a major problem: her computer crashed and she lost the F2 data. She nevertheless persevered, and using the limited data shown here, she was able to predict the mode of inheritance, the number of genes involved, and assign genotypes to each coat color. Based on the available data generated from the crosses shown, attempt to duplicate her analysis.

    Cross P1 F1 F2
    1 sandy X sandy all red data lost
    2 red X sandy all red 3/4 red: 1/4 sandy
    3 sandy X white all sandy 3/4 sandy: 1/4 white
    4 white X red all red data lost

    Once you have formulated a hypothesis to explain the mode of inheritance and assigned genotype to the respective coat colors, predict the outcomes of the F2 generations where the data were lost.

    After considering the above problem, concentrate on the following one.

    Labrador retrievers may be black, brown, or golden in color. While each color may breed true, many different outcomes occur if numerous litters are examined from a variety of matings, where the parents are not necessarily true-breeding. Shown here are just some of the many possibilities. Propose a mode of inheritance that is consistent with these data, and indicated the corresponding genotypes of the parents in each mating. Indicated as well the genotypes of dongs that breed true for each color.

    A. black X brown ---> all black

    B. black X brown ---> 1/2 black; 1/2 brown

    C. Black X Brown ---> 3/4 black; 1/4 golden

    D. Black X Golden ---> all black

    E. Black X golden ---> 4/8 golden; 3/8 black; 1/8 brown

    F. Black X golden ---> 2/4 golden ; 1/4 black; 1/4 brown

    G Brown X Brown ---> 3/4 brown; 1/4 golden

    H. Black X Black ---> 9/16 black; 4/16 golden; 3/16 brown

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    Solution Preview

    Question 1
    To start we must first establish the number of alleles and genes.
    - 3 traits could mean one gene with 2 co-dominant alleles or 2 genes in a more complex relationships.

    Co-dominance cannot explain the test crosses available so we will make a model that has two genes.

    gene 1 (R,r) encodes for red such that
    RR or Rr= red
    rr= no color

    gene 2 (S,s) encodes for sandy such that
    Ss or Ss= sandy
    ss= no color

    Now we must look at the relationship between these two genes that contributes coat color.
    -Since sandyx sandy= red and redx sandy or white = red ...