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Genetic problem, help! A tall plant of unkown genotype is test-crossed. Of the offspring, 869 are dwarf and...

...912 are tall. What is the genotype of the unknown parent? Show the cross to prove it.

ok so a test cross is used to identify the genotype of an unknown individual.. the key is to cross this unknown to a homozygous recessive individual for trait X.

SO for your question, you need to know whether your tall plant is TT or Tt. You cross this plant with tt and notice a roughly 1:1 ratio of Tall to Short. This means that your unknown plant must've been Tt otherwise ALL the progeny will be Tall. [TT means that the only allele present in the gametes is T, so all progeny will have a T, and therefore all progeny would be tall].

always do the punnett squares though to forecast what to expect: Option 1: TT x tt. .. there is 100% probability of all progency being Tt, just as we stated above.

Option 2: Tt x tt.. half the progeny will be Tt and half will be tt. I can physically illustrate a punnett square on here, but the first parent can give either a T or t ..and the second can only give 't'.. so the two genotypes that can arise are Tt and tt, in equal ratios.

you can clearly see by illustrating the different scenarios of the test cross that your answer sits in Option 2. The genotype of the unknown parent is Tt.

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