genetics question

[justletmein]

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hey everyone,

was studying and ran across this problem:

Red is dominant over white in a certain flower. To test
whether a red offspring is homozygous or heterozygous
in this flower
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. Two of the above will work
E. None of the above will work

Kaplan says "D", meaning both A and C will work. Now, typically the answer should be C since it's a test cross. "A" could work, but probably doesnt determine actual outcomes; since the genotype is Rr, it doesn't mean you're going to get a 'red' flower every time, know what i'm sayin? So anyway, my question is ... is the real answer C? thanks!

 

[dshnay]

I think D is correct

lets assume R stands for red and r for white
You know this rule, to test if its homo or hetero always test it with a testcross (homozygous recessive, rr), so that's going to be C

Now in order to prove that A will also work, here's my reasoning:
A red plant which came from a cross of red and white so here it goes
R- * rr = 1/2 Rr, 1/2 rr
Now the red plant that we want to test if its hetero or homo
we take our Rr * R- (the one that we want to know if its hetero or homo)
if it was Rr, then you would get the 3:1 ration 3 red 1 white (RR, Rr Rr rr)
If it was RR then all offspring will turn out to be red

i hope i didn't confuse you even more

 

[justletmein]

probabilty doesnt determine outcome. There is a 1/4 chance it will be white, but that doesnt mean that if you have 16 offspring, 4 will be white. they could all be RED. with the test cross, 100% will be RED. So although "A" could be an answer, what are you suppose to choose here? C or D??

 

[busdriver]

the real answer is D...

i think you're thinking too much into it...the testers aren't trying to trick you.

 

[justletmein]

alright, thanks busdriver.