mark kennedy said:
gluadys said:
Mutations are an additional wrinkle. Mendel didn't deal with mutations. So let's keep it simple till we know we are on the same page.
Of course he did, there was the wild type phenotype which was the most common expression of a particular gene in a population. A mutant phenotype is a variant of a gene's expression that arises when the gene undergoes a change, aka mutation.
Not knowingly, he didn't. You are committing a retrospective anachronism. You know and I know that the alternate phenotypes are produced by alternate alleles, which in turn are a consequence of mutation. But Mendel did not know this. Nobody knew that genes and genetic alleles even existed until Mendel did his experiments. Nobody connected them with mutations until more than 50 years after he published.
So Mendel made no provision in his experiments for studying mutations. In fact, he practically (though unwittingly) excluded them. Remember that he did not invent or breed the alternate phenotypes he chose to work with. Peas and his other test subjects had come in these variations since long before he was born. All Mendel did was assure that his initial stock bred true for the characteristic he was studying---so that he could be sure of the character of his parental generation.
Your math is out. In the F1 generation we get 4 heterozygotes. In the F2 generation we get 2 homozygotes and 2 heterozygotes.
Ok, If you say so dear, I'm not going to make an issue of it.
Don't patronize me, honey. It has nothing to do with "if I say so". It's the math of the Punnett square. Your numbers refer to the phenotype, not the genotype.
P1 cross: RR x rr. We can designate the two genes as (p)aternal and (m)aternal, thus:
R(p)R(m) x r(p)r(m)
Which provides the four possible genotypes of the F1 generation:
R(p)r(p); R(p)r(m); R(m)r(p); R(m)r(m)
As you can plainly see all four are heterozygotes and produce one phenotype (dominant).
This means the F1 cross will be Rr x Rr (note the contrast with the P1 cross)
And this produces the four F2 genotypes of
R(p)R(m); R(p)r(m); r(p)R(m); r(p)r(m)
Note that the two genotypes in the centre are heterozygous and the two on the end are homozygous (for the dominant and recessive character respectively). So 2 homozygotes and 2 heterozygotes.
But phenotypically the heterozygotes are identical to the dominant homozygote, so the phenotype ratio is 3 dominant:1recessive.
Mark, if you can't keep the count straight at this level of simplicity, how do you expect to keep track of more complex genetic scenarios?
At this level it doesn't. So far we are dealing strictly with variation in a single species. No speciation. But that will come.
I just wonder how it would look in the Punnett square.
We can introduce mutation to a Punnett square (once you have the square itself down ok), but speciation might be a different matter. There are additional factors to consider.
I'll save you the trouble. They're not. And if by genetic changes you mean mutations, that is not what Mendel dealt with at all. The point here is to see the result of selection on the H-W equilibrium in the absence of mutations.
Mendel deliberatly changed the gene frequencies and if its over two alleles then its a mutation. Equilibrium was just something I suggested using for a baseline. You may be confusing a mutation with speciation, creatures speciate all the time I was just wondering what that would look like in Mendelian terms.
No, you are the one who is confusing things because you are rushing ahead to questions of mutation and speciation (and I am not confusing those terms) when all Mendel was dealing with was variation, without mutation.
This doesn't mean that the alleles which gave green or yellow colour to seeds, or a round or wrinkled shape, or pink or white flowers or tall or short plants have nothing to do with mutations. Yes, the alleles orginated
at some time by mutation. But that time could have been a century, a millenium, many millennia before Mendel began his experiments.
Mendel at no time had to rely on a new mutation surfacing in order to begin or continue his experimentation. He studied alleles which were already part of the gene pool of his test subjects.
So for the moment assume the existence of the two alleles and assume no new mutation, and take them through say 5 generations with a selective component. (I don't know that Mendel ever did this). See what happens to the frequency distribution of your alleles.
