>essentially, the reason your argument is flawed is because you
>are talking about a very large number of genes.

Nuclear genes are part of the DNA that makes up chromosomes. Reticulated pythons have a diploid number of 36 chromosomes. There is a fair liklihood that Angolan and ball pythons also have that number. Assuming that is so, an F1 Angolan-ball hybrid will have 18 chromosomes from the Angolan parent and a matching 18 chromosomes from the Ball parent. Since the hybrid is fertile, meiosis is probably normal with the chromsomes of each species matching up pair by pair and gene by gene. It is likely also that crossing over can occur so that some of the cromosomes resulting from meiosis will contain genes from both species.

An egg or sperm from a hybrid will contain one set of chromosomes, here assumed to be 18 chromosomes, and the set of genes contained on those chromosomes. Assuming crossing over, i.e, genetic exchanges between pairs of chromosomes, some of those chromosomes will carry Angolan genes, some will carry ball genes and some will carry a mix of genes from the two species. The exact combination will vary widely from gamete to gamete. Even if a gamete receives exactly 9 Angolan chromosomes and 9 Ball chromosomes without crossing over, however, the gamete will not likely carry exactly half Angolan genes and half ball genes simply because individual chromosomes vary in size and hence in number of genes. Even if the gametes carried exactly half the genes from each species, the half from the Angolan parent would all be different from the half from the ball parent and no two gametes would carry exactly the same combination of Angolan-ball genes.

>to provide an anology, if you flip a coin, you get 100% heads
>or 100% tails. no inbetween. this is the case for your one
>gene example, of eye color. however, if you flip 30,000 coins,
>you get damn near 50% heads, 50% tails.

But since genes are on chromosmes, and there are probably only 18 pairs. There is going to be much wider gamete to gamete variation in numbers of genes inherited from one species or the other in a Angolan-ball hybrid than if one assumes each gene of a pair is inherited independantly of every other gene.

>lets start with the original cross. we are in agreement, no?,
>that this F1 animal received one allele each from its two
>parents. so it is 50% ball/50% angolan.

True

>(incidentally, within genetics circles, F1 refers to the
>generation since the orginal cross, within a closed breeding
>system. so if we adhere to that definition, you are correct
>the backcrosses of the hybrid to a parental species are not
F2s.

True.

>however, within animal breeding circles, and especially among
>snake breeders, F1,F2,etc is used to denote the generation
>from the orginal breeding of interest regardless of whether
>subsequent crosses are retained in an inbred system or not)

They need to stop doing that. It is a corruption of the terminology and leads to confusion.

>now, when that hybrid animal is crossed to a ball, the hybrid
>again passes along one allele and the ball one allele FOR EACH
>GENE. because there are thousands of discrete genes, the
>chances are extremely high that the F2 animal (or second
>generation animal if you prefer) will receive from its hybrid
>parent nearly exactly 50% ball alleles and 50% angolan alleles
>(so combined with the pure ball parents alleles 75% ball
>alleles and 25% angolan alleles).

Given that only 18 pairs of chromosomes are involved it is unlikely any given backcross will be nearly exactly 75% ball, 25% Angolan. That ratio is a generalization and applicable to any given snake only as an approximation. For any given pair of genes in a hybrid x ball backcross, there are 2 possibilities, one Angolan allele plus one ball allele or both ball alleles. The variation from the ball phenotype will depend on the specific mix of Angolan genes inherited that are dominant to their respective ball alleles and what epistasis is involved.

>so, you would be correct to say an F2 animal (or 2nd gen.

Actually a 1st generation backcross.

>animal) MIGHT have 100% ball python alleles for eye color.
>however, to say that it has anything but 25% angolan alleles
>(ie. genetic material) overall is incorrect.

25% can only be an approximation. Individual backcross snakes would vary widely in the percentage of genes inherited from the Angolan parent.

>incidentally, there are likely only a small number of genes
>(just guessing, perhaps 5-30 or so of the thousands the animal
>has) that affect color and pattern. so the animal could, quite
>by chance, receive say 8 ball alleles of 10 color/pattern >genes from its hybrid parent (plus 10/10 ball alleles from the
>pure parent) and thus have 2/20 angoloan alleles in the
>color/pattern genes. and so, look very much like a ball
>python. but it is still 25% angolan.

As an approximation, not as an exact or even nearly exact percentage.

>as far as the misrepresentation thing. i suppose as long as
>you present the whole thing as just your opinion, and also
>disclose that the animals are crosses between a hybrid and one
>parental species, it isn't misrepresentation, you are just
>giving an (incorrect) opinion to go with the facts.

I fail to see the incorrectness of his opinion. Looks to me like the two of you are looking at the same thing from different angles. He's emphasizing the potential backcross individual snake to snake variability. You are emphasizing that among a large number of backcross progeny, the gene ratio among them will be close to 75% ball, 25% Angolan.

Bigfoot

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