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Question

I know about crosses Inheritance and this question is about Test Cross. I want to know if we can determine the genotype of hybrid plant by crossing with a pure recessive parent from Filial generation 1 or if we require more generations to know the genotype?

My thoughts

if the plant is homozygous then no need if it is dwarf as we know that dwarf genes are recessive but if the phenotype is tall then we have to find its genotype whether it is Tt or TT. I have come across a MCQ : the genotype of hybrid plant obtained from a cross can be determined by crossing the plant with - . To me the ans should be Pure recessive parent at F1 generation. But ans given is Pure recessive parent at F2 generation

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  • $\begingroup$ If you both parents are homozygous (which is the consequence of being a pure line), then you don't need to even do any cross to infer the genotype of the F1. Maybe you should give an example as to clarify your question and try to solve it yourself so that we see where is your misunderstanding $\endgroup$
    – Remi.b
    Commented Aug 13, 2017 at 16:26
  • $\begingroup$ @Remi.b Yes, if the plant is homozygous then no need if it is dwarf as we know that dwarf genes are recessive but if the phenotype is tall then we have to find its genotype whether it is Tt or TT. I have come across a MCQ : the genotype of hybrid plant obtained from a cross can be determined by crossing the plant with - . To me the ans should be Pure recessive parent at F1 generation. But ans given is Pure recessive parent at F2 generation. $\endgroup$
    – Abdul Khan
    Commented Aug 14, 2017 at 8:02
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    $\begingroup$ I added your comment into your post and created two sections. Feel free to roll back if you don't like my edit. $\endgroup$
    – Remi.b
    Commented Aug 14, 2017 at 8:21
  • $\begingroup$ @Remi.b Thank you for the edit, now the post looks more appropriate. Now, If we look at the question from statistical view the ans looks simple that more data sampling means more reliable effects i.e F3, F4.... generations, as the same done by Mendel. So, as we know in MCQ there is one correct ans either we gain or loose mark. Little caution, if you have to choose from two above options, which you will choose? $\endgroup$
    – Abdul Khan
    Commented Aug 14, 2017 at 11:21

1 Answer 1

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Real genetics is complicated

Biology is more complicated that you seem to expect. The answer very much depends upon the underlying genetics of the trait (and the underlying non-genetic factor affecting the trait; see here) as well as the genotype you are crossing it with.

Simplified model

But let's assume an extremely simple model of pure dominance / recessivity at a single bi-allelic locus. Let a be the recessive allele and A be the dominant allele. Let's assume the pure line you are using for the crossing is homozygote for the a allele (and let assume that we know it).

  • If the parent is AA, then all offspring will be Aa and all will have the dominant phenotype.

  • If the parent is Aa, then 50% of the offspring will be Aa and have the dominant phenotype and 50% of offspring will be aa and have the recessive phenotype.

  • If the parent is aa, then all offspring will be aa and have the recessive phenotype.

So, under this simple model, if you can produce a high enough number of offsprings to make good statistics, then yes, looking at the F1 generation is enough to determine the genotype of the parent.

Under a more complex model, it is not necessarily possible to only look at the F1 generation.

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  • $\begingroup$ The concept of a test cross is still relevant. It was on my US medical licensing exam. Though that was quite a few years ago, I hope it is still on it. It is still important for evaluating risk based on family history. There are a number of diseases that demonstrate Mendelian inheritance. You can't just run genetic tests on everyone. $\endgroup$
    – De Novo
    Commented Jul 10, 2018 at 22:53
  • $\begingroup$ I'd add that a test cross is very specific: it's a parent that doesn't show a recessive trait crossed with a parent that does (just the first two bullets in your answer). $\endgroup$
    – De Novo
    Commented Jul 10, 2018 at 23:02

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