F3 generation

[MedPR]

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Confused about what EK is trying to say here.

F2 is PP, Pp, Pp, pp. P = dominant = purple. p = recessive = white.

When the F2 generation was self-pollinated, 33% of the dominants produced only dominants, and the rest of the dominants produced the Mendelian ratio (3:1). The white flowered plants produced only white flowered plants. Thus, half of the F2 generation expressed the dominant trait with the recessive trait latent

It's easy to see that F2 is PP, Pp, Pp, pp from the fact that F1 is all heterozygotes, but how does the F3 generation (what the quote is talking about) show/prove that F2 was half heterozygotes, and half homozygotes?

Also, the bold is talking only about the hybrids, right? Because the PP's express the dominant trait, but since they don't have a p, the recessive is not latent, but not there at all.

 

[dsoz]

You need to look at the subsets in the F2 to F3 breeding. You are correct in your PP, Pp, Pp, pp distribution. Now look at ONLY those that have at least one dominant allele...

1/3 of the F2 (the ones that are PP) can (and will) produce only dominant phenotype (Pp or PP genotype) F3 offspring. The other 2/3 of the dominant expressing group (the F2 that are Pp) will produce a 1:2:1 Mendelian ratio of 1:2:1 genotype (3:1 phenotype, since you can't tell the difference between PP and Pp just by looking at the flowers) This "proves" that in the F2 generation there is a 1:2 ratio between PP and Pp.

The last little bit only reinforces that the white flower is recessive with the genotype pp.

You had most of the answer already. Just the "why does it prove it" part.

Trust what you know from genetics...

dsoz

 

[MedPR]

You need to look at the subsets in the F2 to F3 breeding. You are correct in your PP, Pp, Pp, pp distribution. Now look at ONLY those that have at least one dominant allele...

1/3 of the F2 (the ones that are PP) can (and will) produce only dominant phenotype (Pp or PP genotype) F3 offspring. The other 2/3 of the dominant expressing group (the F2 that are Pp) will produce a 1:2:1 Mendelian ratio of 1:2:1 genotype (3:1 phenotype, since you can't tell the difference between PP and Pp just by looking at the flowers) This "proves" that in the F2 generation there is a 1:2 ratio between PP and Pp.

The last little bit only reinforces that the white flower is recessive with the genotype pp.

You had most of the answer already. Just the "why does it prove it" part.

Trust what you know from genetics...

dsoz

Ah I see. Thank you 🙂 I didn't really learn anything in genetics other than how to memorize answers to homework questions. I'm going through the EK genetics chapter very slowly to make sure I don't miss anything. I know genetics is a good bet to be on the MCAT.