Basic Genetics Question

[Gingi]

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Hi there people.

I know that following question is very basic, but for some reason I miss the point. Please be kind enough to help me with it.
(it's from the mcatquestion.com)


A researcher crosses a heterozygous yellow (dominant) plant with a homozygous recessive red plant. In the next generation, 653 plants were yellow. Which of the following answers is an approximate prediction for the number of red plants in the same generation as the 653 yellow plants?

(a) 0
(b) 238
(c) 653
(d) 978

Here is how I was thinking:

heterozygous yellow (dominant) = Yy
homozygous recessive red = rr

cross:
Yy*rr = Yr, yr

Yr = Yellow yr=mixed phenotype?

My first reaction was that since the genotype is either Yr or yr, no red plants will be present in this generation, therefore, (a) is the answer. Apparently the right answer is (c) - and I don't understand why.

Since we're talking here of mixing two traits, I got a little confused.

Thanks in advance to all the good people out there.

Gingi

 

[TropicalKitty]

I'll try...

You've mixed some extra stuff in here...

Heterozygous - two different alleles (not different genes, in this case)
Homozygous - two same alleles.

They have not indicated in the question that there are two genes at play here so stick with one gene with two alleles (Y - yellow, dominant. y - red, recessive) It's a simple Mendelian cross.

Your cross is as such: Yy x yy

Punnet square it:

___Y ___y
y | Yy | yy
y | Yy | yy


So, your progeny will be 50% Yy and 50% yy...

They gave you the numbers 653 are yellow (Yy). Thus, by the punnet square you'll see that the number of red (yy) should be the same. The answer is C.

Remember the square and keep your genetics lingo straight. 🙂

 

[Gingi]

Firstly, thank you !

I think I understand my misconception:

Allel - a version of a gene -- in this case: the same gene might be phenotypically either red or yellow.

Since it's one gene, bot Yy and yy will resemble the genotype in the question.

Correct me if I'm wrong.

 

[TropicalKitty]

Yes, an allele is a version of a gene. And yes, the phenotype of the allele will either be yellow or red.

(Extra detail note: They gave nothing about co-dominance, which could give say, an orange flower, since all the flowers with the Y allele were yellow. Also, they said the recessive was red, so any yy would have to be red based on the way recessive should work.)

In this question you have a gene for color with two alleles: a yellow allele, or red allele. The yellow allele is dominant, so if the plant has a yellow allele, it will be yellow, regardless of it having a red allele. Think of eye color with people. Brown eyes are dominant, but a brown eyed person can have a blue eyed child with a blue eyed other parent. This is because the brown eyed person is heterozygous for eye color: they had a brown allele and a blue allele. Same for the yellow plant in this question.

And lastly, yes, the Yy and yy represent the genotypes of the plants in the question.

Sometimes it's just getting from the description of the individual to the notation. You can do it!

 

[Gingi]

Thanks Tropical !!!