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Question by 2012: Genetics question about tomatoes please help!?
In tomatoes, purple stem (T) is controlled by a dominant gene in contrast to the recessive allele that produces a green stem (t). In addition, “cut” leaf shape is controlled by a dominant gene (R) with the recessive allele producing “potato” leaf (r). Below, you are give the data from several crosses of tomatoes. Work backwards from the data to determine the most likely genotypes of the parents
a) Purple cut X green cut = 322 purple cut, 101 purple potato, 310 green cut, 107 green potato
b) purple cut X green cut = 722 purple cut, 231 purple potato, 0 green cut, 0 green potato
c) purple cut X green potato = 404 purple cut, 0 purple potato, 387 green cut, 0 green potato
d) purple potato X green cut = 70 purple cut, 91 purple potato, 86 green cut, 77 green potato
thank you for any help you give me
Answer by Lena
a) Your offspring includes tomatoes with recessive characters (green cut, purple potato and green potato). That means that the parent with the dominant characters (purple cut) can’t have only the dominant alleles (TTRR).. So it has to be TtRr. About the green cut parent: It’s genotype would be either ttRR or ttRr. To check which one is correct we look at the offspring again. If it’s ttRR there would be no “potato” plants, so it has to have the recessive allele “r”. So, the green cut plant is ttRr.
b) You have no green stems at your offspring, although one of the parents has (the green cut, thus it’s tt). So, the purple cut parent should be homozygous to the dominant (TT). About the leaves: There is offspring with the “potato” character, so both parents should have the recessive allele. They express, though, the dominant phenotype, so they are heterozygous (Rr). Summarizing, the purple cut parent is TTRr and the green cut ttRr.
c) We know that the green potato parent is ttrr, because it expresses the recessive characters. About the purple cut parent: The offspring shows that there are plants with green stems (tt), so this parent can’t be homozygous to the dominant, because that way all offspring would have purple stems. You can conclude he is Tt. On the other hand, we have no plants with “potato” leaves in the offspring, so the purple cut parent should have only the dominant alleles (RR). Summarizing, the purple cut parent is TtRR.
d) The purple potato parent would be either Ttrr or TTrr and the green cut parent would be either ttRr or ttRR. But, you have all types of offspring, so the parents can’t be homozygous to their dominant characters. So they will be Ttrr and ttRr.
In these exercises, you may also want to do the crossings, so you can check if you were right. The ratios will approximately match, too.
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