Who's good at solving genetic problems? I need help with this problem!

Discussion in 'Secondary Education' started by Peachyness, Jun 24, 2010.

1. PeachynessVirtuoso

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Jun 24, 2010

So, I'm studying for the science CSET and I'm working on some practice genetics problems. I'm stuck on the reasoning behind their answer.

Here is the problem:
Before I give you their answer, maybe you can figure it out and tell me why.

Thanks!!

3. TeacherGroupieModerator

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Jun 24, 2010

I would expect the answer to be c, and the reasoning is most likely this:

a. We're starting with green plants: they are necessarily either homozygous dominant (GG) or heterozygous (Gg).
b. Yellow pea plants are strictly homozygous recessive: gg.
c. Heterozygous plants (Gg) bred together have a 1:4 chance of producing yellow offspring.
d. Breeding a known GG to a plant of unknown genotype will yield green, period.
e. Breeding a known gg to a plant of unknown genotype will yield all and only green peas if and only if the plant is GG; it will yield 1/2 yellow peas if and only if the plant is Gg; I'm assuming we already know that the plant is not gg, but the result of a gg-gg cross would be yellow only.

It's entirely possible that I'm wrong, but then I'm no geneticist.

4. jwteacherCohort

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Jun 24, 2010

Nope, you're right TeacherGroupie. The question states to find out if it is GG or Gg. Crossbreeding a GG or Gg plant with a gg plant is the only possible way to figure out its genotype. If it's GG, you'll still get a green plant 100% of the time. If it's Gg, you'll get a green plant only 50% of the time.

EDIT: You could also crossbreed with a Gg plant as you'll still get a green plant 100% with a GG plant, and 75% of the time with a Gg plant.

So I guess the answer is A and C?

5. mmswmModerator

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Jun 24, 2010

You ALWAYS perform test cross experiements with homozygous recessive specimens. If you cross with anything else, your level of certainty goes down significantly. The reasoning is essentially what TG said. The answer is C.

6. czaczaMultitudinous

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Jun 24, 2010

nevermind. Now TG's answer makes perfect sense to me...(had to think that one through more)

7. kcjo13Phenom

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Jun 24, 2010

A bit late, but agree with TG (as usual).

8. PeachynessVirtuoso

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Jun 24, 2010

That's what I was thinking at first too! That's why I was so confused. They said the answer was C, but no explanation.

Right after I posted this question, I redrew all of my punnet squares and realized that crossing with a gg would be the best possible answer, but wasn't 100% certain if the answer could also possible be A too.

Thanks to everyone who helped!!! I'm using UC Irvine's CSET test prep site, and I caught so many mistakes, that I needed to be sure.

9. kickchick2000Rookie

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Jun 25, 2010

As a former associate researcher in genetics the answer is C. As stated you always perform the test cross with the homozygous recessive.

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