test cross question

[Farcus]

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Red is dominant over white in a certain flower. To test whether a red offspring is homozyguous or heterozygous in this flower, one would

A)Cross it with a red plant that had a white parent
B)cross it with a red plant that had two red parents
C) Cross it with a white plant
D) B and C
E) None of them work

Why is the answer only C? it doesn't make any sense because if something is RED it can be XX or Xx and if we test cross with A) red plant that had a white parent that would mean crossing Xx with Xx, or XX with Xx which you have 25% chance of getting a recessive phenotype for the former and 100% chance of getting dominant phenotype for the latter. I think the book just gave a wrong answer or something.

 

[TheBoondocks]

Red is dominant over white in a certain flower. To test whether a red offspring is homozyguous or heterozygous in this flower, one would

A)Cross it with a red plant that had a white parent
B)cross it with a red plant that had two red parents
C) Cross it with a white plant
D) B and C
E) None of them work

Why is the answer only C? it doesn't make any sense because if something is RED it can be XX or Xx and if we test cross with A) red plant that had a white parent that would mean crossing Xx with Xx, or XX with Xx which you have 25% chance of getting a recessive phenotype for the former and 100% chance of getting dominant phenotype for the latter. I think the book just gave a wrong answer or something.

A test cross is defined as one with a dominant genotype along with a recessive phenotype. You always do a test cross with a recessive homozygous genotype The only answer is C. You wouldn't do it with a red plant that had two red parents because they could be either Ww or WW. HOWEVER, in this case you're correct. Since a red plant with a white parent has to be Ww. One could distinguish between WW which would be all red and Ww which would have 25% white. However, this works but is generally not done which is why they said C. In theory you're correct. However, from all the literature I've read, when one needs to determine what the genotype of a dominant phenotype is, they always use a homozygous recessive to determine it.

 

[pretender]

Exactly. If you cross WW and ww, you will get all red, meaning you have a homozygote. If you cross Ww and ww, you will get 1/2 red and 1/2 white, meaning you have a heterozygote.

 

[Quantum Chem]

You always use a recessive parent when doing a cross to determine the genotype of one that is showing a dominant phenotype. Think about it like this: If you test an unknown dominant with a known genotype of XX then all of the offspring would show dominance no matter what the genotype of the unknown is. If you test an unknown dominant with a known genotype of Xx then a quarter of the offspring would show recessive. But if you test an unknown dominant with a recessive xx then either all of the offspring will be dominant (XX is the unknown) OR half will show dominance and the other half will be recessive (Xx is the unknown).

While yes it is possible to test an unknown with an Xx testing with an xx just makes things easier since you would definitely know the genotype of the xx. Of the choices, the only ones that would give you 100% confidence in determining the unknown's genotype would be choice A and C.
A: "Cross it with a red plant that had a white parent" You know that the test will be genotype Xx because one of its parent's was white.
B: "cross it with a red plant that had two red parents" You don't know the genotypes of the parents, therefore you couldn't tell the genotype of the offspring that is going to used to test the unknown with.
C: "Cross it with a white plant" See previous paragraph for explanation.
D: "B and C" B would not give definite answer, so this cannot be the answer.
E: "None of them work" You know that C works, so this isn't it.

So out of the choices, choice C gives the best, and correct answer. Just remember always use a recessive when doing a cross test to determine the genotype of an unknown.

Hope this helps! 🙂

 

[Farcus]

but a red flower from 1 white parent is Ww, and the thing we're testing is pheontypically dominant. So dominant could be WW or Ww, so there are two possible choices. WW with Ww, or Ww with Ww, if you do one and the other, you only get recessive for the latter. I don't see it...

 

[Farcus]

You always use a recessive parent when doing a cross to determine the genotype of one that is showing a dominant phenotype. Think about it like this: If you test an unknown dominant with a known genotype of XX then all of the offspring would show dominance no matter what the genotype of the unknown is. If you test an unknown dominant with a known genotype of Xx then a quarter of the offspring would show recessive. But if you test an unknown dominant with a recessive xx then either all of the offspring will be dominant (XX is the unknown) OR half will show dominance and the other half will be recessive (Xx is the unknown).

While yes it is possible to test an unknown with an Xx testing with an xx just makes things easier since you would definitely know the genotype of the xx. Of the choices, the only ones that would give you 100% confidence in determining the unknown's genotype would be choice A and C.
A: "Cross it with a red plant that had a white parent" You know that the test will be genotype Xx because one of its parent's was white.
B: "cross it with a red plant that had two red parents" You don't know the genotypes of the parents, therefore you couldn't tell the genotype of the offspring that is going to used to test the unknown with.
C: "Cross it with a white plant" See previous paragraph for explanation.
D: "B and C" B would not give definite answer, so this cannot be the answer.
E: "None of them work" You know that C works, so this isn't it.

So out of the choices, choice C gives the best, and correct answer. Just remember always use a recessive when doing a cross test to determine the genotype of an unknown.

Hope this helps! 🙂

I know you are suppose to cross with a recessive but if you do it with heterozyous you also get a quarter of recessive if the unknown if heterozyous, which you said it yourself which is my point. Granted the method i'm advocating doesn't have higher % yield. since if you do rr with unknown dominant you can get 50% recessive instead of my 25%. I know what you guys are saying how you go with recessive and I have read that many a times in my books and kaplan but this doesn't make any sense its like stubborn to think one way when the other way also works.

 

[TheBoondocks]

I know you are suppose to cross with a recessive but if you do it with heterozyous you also get a quarter of recessive if the unknown if heterozyous, which you said it yourself which is my point.

Check out my edited post. I agreed with you on a second look. However, the main reason is ease. If you use ww, then you will have 100% red if WW or 50% red if Ww. In theory you're correct, but no one ever uses a heterozygous individual to determine the genotype. So in theory you're right, but in practice C is the best answer.