You need to understand that not all charlies have mega-colon. There are many people who breed them quite successfully. and breeding two brokens together does not always result in charlies. I've had two broken throw me some really nice broken bunnies.... MIND.... a HEAVILY marked broken to a standard broken. I would not breed two minimally marked brokens together and not expect to have a litter of mostly charlies.
Absolutely right, charlies do not always have megacolon. Two entire breeds are made up of charlies: Blanc de Hotots and Dwarf Hotots. There are many healthy rabbits in both breeds. A friend who has been breeding Dwarf Hotos for 2-3 years now has yet to encounter megacolon in her rabbits.
There is some confusion about what a "charlie" actually is, perhaps because of the two different ways the term is used. "Charlie" can refer to either genotype (what its genetic code is) or phenotype (what it looks like). A
phenotypic charlie is a very lightly-marked rabbit which may be <EnEn> or <Enen>; this term is used in showing because a judge makes determinations based on what the rabbit looks like (phenotype) and has no information about the rabbit's genetic make-up (genotype). But technically, a charlie is a rabbit with two broken genes <EnEn> which results in a rabbit with very little color. This is important to know partly because of the megacolon connection, and partly because it is useful if you want to produce only broken colored rabbits. A rabbit that is
genotypically broken <Enen>, but has so little color that it looks like a charlie, is sometimes called a "false charlie."
Anyway, breeding two brokens together will not
always produce charlies, because genetic prediction is statistical, not literal. The broken allele <En> is dominant; the allele for solid is recessive <en>. A rabbit gets two alleles of the gene at this loci, one from each parent. So a normal colored rabbit is <enen>, a broken is <Enen>, and a charlie is <EnEn>. Breeding two brokens will
statistically produce 25% charlies, 25% solids, and 50% brokens. A Punnet square "breeding" two broken-colored rabbits, each being <Enen>, shows how this is true (the sire is across the top, the dam is down the side):
| En | en |
En |
EnEn
(Charlie) |
Enen
(Broken) |
en |
Enen
(Broken) |
enen
(Solid) |
However, because genetics is a statistical thing, there is a chance you could breed two brokens and get
all solids,
all brokens or
all charlies, or any combination in between; it's purely the luck of the draw. Each litter probably won't have those exact proportions, but over time they should pan out that way.
Some rabbits in the latter two groups (broken Enen or charlie EnEn) have more or less color. You can get a fairly heavily-marked charlie; or you can get a very lightly-marked broken, but that does not make it a charlie (genetically, anyway).
As far as breeding two lightly-marked brokens, they are no more likely to produce genotypic charlies than breeding two heavily-marked brokens. Broken genes do seem to run in lightly-marked, heavily-marked, blanket-pattern or spotted-patterned lines. So, when I bred my false charlie buck to a solid doe, his babies were roughly half solids and half brokens, several of them were very lightly marked like him (interestingly, some others were quite heavily-marked).
As far as we know, whatever produces megacolon is either a part of the expression of the broken gene, or a gene/genes linked to the broken gene (which I think is more likely). It seems that the effect of the broken gene is additive. In the classic explanation, one broken gene throws a pail of whitewash over a colored rabbit. Two broken genes throw two pails of whitewash over the rabbit. In a related way, one broken gene doesn't seem to risk megacolon, but two can add up to trouble. So even if a rabbit is a very lightly-marked broken, it does not have a higher risk of megacolon than any other broken; it is the presence of two broken genes that seems to confer the risk.