mark kennedy said:
I'm not sure where you are getting the 1:1 ratio of alleles or what your point is. Further I don't have a clue what kind of an implication you see in explicitly demonstrated hybrid peas on possible genotypes. This isnt about implicite possibilities but explicitly demonstrated hybrids reduced to mathmatical ratios. Now H-W equilibrium is when there is no selected presure, natural selection,migration and only two alleles. Why would Mendel want to get to H-W equilibrium while crossing hybrids.
OK, Mark, now you have firmly and completely established that you have no clue about simple genetics or the implications of Mendels work. That is why you are making basic errors and incorrect conclusions.
I am getting the 1:1 ratio of alleles directly from the simplest of Mendels experiments. Look at my questions 2 & 2a.
2. Do you agree Mendels' work was based primarily on crossing two varieties which bred pure for the trait(s) he was testing, e.g. smooth or wrinked skin, yellow or green seed colour, etc.
2a. What does that imply for the genotype of P1 generation?
Although he didnt know it at the time, by assuring that his parental plants were breeding true, Mendel was assuring that they were homozygous for the trait he was interested in. So, if he was interested in seed texture (smoothly round or wrinkled), he was making sure that round peas always yielded round peas and wrinkled peas always yielded wrinkled peas.
One of the things he discovered is that in the F1 generation of the hybrids, all the peas were round, but that in the F2 generation 25% were wrinkled. This is what led to the formulation of the 3:1 ratio for the dominant trait in the phenotype.
But what about the genotype? You are already aware that the dominant allele masks the expression of the recessive allele. That is why the F1 generation shows only the dominant trait.
But the genotype of the F1 generation is necessarily heterozygous. Since Mendel had unknowingly assured that his starter plants were both homozygous, the round-seeded peas could only pass on an allele for roundness (R) and the wrinkled-seeded peas could only pass on an allele for wrinkled seeds (r). Consequently, all F1 plants have the heterozygous genotype Rr. That is a 1:1 ratio of the alleles in spite of the complete dominance of the round phenotype.
Now to go to the F2 generation, we begin with plants that are
all heterozygous. In meiosis each gamete receives either an R or an r allele, and receives them in a 1:1 ratio. So we now have four possible genetic outcomes in the zygote.
An R allele is received from both parents=RR genotype= round phenotype
An paternal R allele mates with a maternal r allele=Rr genotype=round phenotype.
A maternal R allele mates with a paternal r allele=Rr genotype=round phenotype.
An r allele is received from both parents=rr genotype=wrinkled phenotype.
The 3:1 ratio of phenotypes is obvious. But count up all the alleles.
1 paternal R allele in each RR zygote and in ½ of the Rr zygotes
1 maternal R allele in each RR zygote and in ½ of the Rr zygotes
1 paternal r allele in each rr zygote and in ½ of the Rr zygotes
1 maternal r allele in each rr zygote and in ½ of the Rr zygotes
The allelic ratio is 1:1 Is it clear now how I got that?
See also
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/Mendel.html
Why would Mendel want to get to H-W equilibrium while crossing hybrids.
He wasnt trying to get to an H-W equilibrium. Mendel did not know that such a thing existed. He
discovered it!!!
And it didnt get a name until the formula was worked out in the 20th century.
But Mendel made the initial discovery and also discovered something elsewhich should have been your answer to questions 3 & 3a
3. How does dominance and recessiveness produce the H-W equilibrium in the F1, F2 and subsequent generations?
3a. What does dominance and recessiveness imply about the possible genotypes of organisms showing dominant and recessive traits?
Weve just seen how the 1:1 distribution of alleles combined with the dominance of one of them leads to a heterozygous F1 generation with the dominant phenotype. In the F2 generation 25% are homozygous for the dominant trait and 50% are heterozygous.
Both of these groups exhibit the dominant trait. The remaining 25% are homozygous for the recessive trait and exhibit the recessive phenotype. Hence the 1:1 allelic ratio yields a 3:1 phenotypic ratio.
It also shows that those expressing the recessive phenotype are homozygous for the recessive allele. But of those expressing the dominant phenotype, only 1/3 are homozygous for the dominant allele. The other 2/3 are heterozygous and can pass the recessive allele to their offspring even though they don't express it themselves.
What Mendel also discovered is that if you continue to allow the plants to self seed,
these genotypic and phenotypic ratios will both be maintained.
In short, you will
not get a reversion to the homozygous condition of either of the grandparents: all RR or all rr. In each generation 25% will have each of the two homozygous conditions, but 50% will continue to maintain the heterozygous genotype.
Now I am sure we will both agree the scenario above is very simple and ideal. You will only get it normally in the controlled situation Mendel created. In nature you seldom have a 1:1 allelic ratio. You can have more than two alleles at one gene locus. You get a spectrum of dominance---it is not always just one or the other. You have factors other than dominance also influencing the expression of genotypes.
And, you have selection pressure favoring one phenotype over the other.
But lets stay with our controlled situation for now. See what you can do now with question 4, assuming that Mendel decided to apply a 10% selection pressure against either the dominant or recessive phenotype.
4. What if Mendel had decided to impose a selective pressure on his test subjects? How would he exert a selective pressure against a dominant trait? What would be the expected result in the next two-three generations? How would this differ if he exerted a selective pressure against a recessive trait?
It is a little disturbing that a relativly minor point can get turned into a major issue. Mendel's laws wouldn't even address the long term changes in things like bones and brain sizes in mammals. It would seem that all a creationist has to be is a creationist to be wrong on here.
Well, its not as minor as you think. And Mendelian genetics do address the long-term changesbut that is something to explore once you have answered question 4. No it's not a matter of being wrong because of being a creationist. It's a matter of being wrong because of not knowing genetics well enough and making mistakes because of it.