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Phenotypic ratios help

  1. Oct 12, 2009 #1
    Phenotypic ratios help....

    Hello all,

    I am trying to do some revision on basic genetics for a biology exam coming up; there is one practice question I cannot understand, I was hoping if someone could verify how the answer came to be ^^. The question is below:

    Color of lentils is determines by a gene with three alleles. The phenotypes are marbled, spotted and clear. Four crosses were repeated many times. The crosses and the outcomes of these crosses are show in the table below.

    Cross Parents Offspring
    1 Marbled x Marbled 3 Marbled : 1 Clear
    2 Spotted x Clear 1 Spotted : 1 Clear
    3 Marbled x Marbled 3 Marbled : 1 Spotted
    4 Clear x Clear All Clear

    The corresponding question is:

    From the data, it is possible to conclude that: Two thirds of the marbled offspring in cross 3 are heterozygous.

    How did they come to that conclusion?

    Cheers, thanks for reading,
    Adrian ^^
  2. jcsd
  3. Oct 12, 2009 #2


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    Re: Phenotypic ratios help....

    Well, if 1/4 of the offspring of a Marbled x Marbled cross are spotted, what does this tell you about which traits are dominant and which traits are recessive? For this data can you infer the genotype of the two marbled parents? Once you've done this, try drawing out a Punnet square.
  4. Oct 13, 2009 #3


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    Re: Phenotypic ratios help....

    Ygggdrasil is right, the crosses are there to give you clues about the mode of inheritance of the alleles (dominant/recessive). Based on that you can infer the genotype of the parents of cross three and with a Punnet square write out the genotypes of the offspring.
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