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Homework Help: Genetics problem

  1. Nov 8, 2004 #1
    You have genetically engineered a tomato plant to be resistant to an insect pest by introducing a gene into tomato seeds that codes for a protein that is poison to the insect, but harmless to humans. After several generations you have obtained a true-breeding line of insect resistant tomatoes.

    Is the insect-resistance trait likely to be dominant or recessive?

    I would think recessive only because it needs to go several generations before it becomes true-breeding. Can someone please give me a good explanation?
  2. jcsd
  3. Nov 8, 2004 #2
    Resistance has generally been found to be a recessive or partially recessive trait, anyone else?
  4. Nov 8, 2004 #3


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    Gold Member

    Think about it...we will represent the allele with r, and the regular non-resistance with R. The first generation would be RR, since they don't have the allele. You introduce the allele into 2 plants and have them reproduce:

    r R
    r rr Rr
    R Rr RR

    You breed two of these together, seeing from experimentation that one is resistant but most are not:

    r r
    r rr rr
    R Rr Rr

    Now you find that two are resistant (the two homozygous recessive) and breed these. Now, you notice that all of the offspring are resistant. If the allele was dominant, it would be nearly impossible to develop a line of true breeding plants that produce only resistant plants. It is impossible to tell whether a plant that represents the trait is homozygous or heterozygous. Impossible? Of course not...but probability points to it being recessive.
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