Question

When Gregor Mendel crossed true-breeding tall plants with true-breeding short plants, why was it impossible to observe segregation?
A. Alleles for height do not segregate in the F2.

B. Alleles segregate only in the F2 generation.

C. Alleles segregate best when two tall plants are crossed.

D. Alleles in the F1 must be Tt to have height variety in the F2.

Answer 1

Okay.

I've been reading this question, over and over.
It's all about breeding, and crossing genotypes with one another. And I know how THAT works.

But what I may not completely understand is the idea of "observing segregation."
What does it mean to "observe segregation"?

I THINK that what it means, to "observe segregation," is to see a difference in the appearance of offspring.
I THINK it's the idea that if two parents produce many offspring,
then it is possible for those offspring to have traits that are different, from one another.

I mean, what do you think? Is that how you would also understand "observing segregation"?

For the sake of explanation, I'll work with that idea.

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Now, let's figure out this question.

The first thing we have are two parents:

One parent has the genotype T T (true-breeding tall).
The other parent has the genotype t t (true-breeding short).

To figure out the F1 generation,
we have to cross those parents.
Let's use a Punnett Square, to show that.



You see here,
if we cross (T T) with (t t)....
then all of the offspring are (T t).

In other words, all of the offspring of the F1 generation will be tall.
Because the capitalized letter matters more than the lower-case letter.

T stands for "Tall."
And since all of the offspring of F1 are (T t),
then that means that they will all be Tall.

You see?

And we can't observe segregation here, in F1,
because all of the offspring are the same.
None of them is different from the others. None of them is short.

~~~~~~~~~~~~

Now, let's make a Punnett Square for the F2 generation.

In order to do that,
we have to cross two offspring from the F1 generation.

All of the offspring from the F1 generation were (T t).

So that means we have to cross (T t) with another (T t).

Here's the Punnet Square:



This is the Punnet Square for the F2 generation.

And in THIS Punnett Square, we CAN see segregation.

Take a look:
Three of the offspring are going to be tall.

But ONE of the offspring has the genotype (t t).
That offspring is going to be short. Not tall.
Because lowercase t stands for "short."

~~~~~~~~~~~~~~~

So, I think that the answer to this question is D.
"Alleles in the F1 must be Tt to have height variety in the F2."

What that means is that in order for us to "observe segregation,"
in order for us to see differences in the offspring in F2,
the offspring in F1, that we cross, have to be (T t).

You saw what happens when we cross (T T) with (t t).
All of the offspring are tall. None are short.

But then you saw what happens when we cross (T t) with (T t).
Most of the offspring are tall,
but ONE is short.
Here, we can "observe segregation."
We can observe a difference.