The common spiketopped apple snail or Pomacea diffusa is know in many colour variations. This page is my effort to clear this a bit out and to help people understand the basic mechanisms that determine the colour variations in these apple snails. Remember, however, that there is not much known about colours in apple snails, and that this page is more a sort of hypothesis of how the possible mechanisms could work.

Play with the genes:
In the interactive example below, one can switch on and off the colour genes. These genes are available in at least two copies (as the genetic code is stored in pairs of chromosomes that essentially consists of nearly identical DNA sequences).
Playing with this example should help to get some insight in the fact that one active (switches on) gene is enough to allow the snail to reproduce the corresponding pigment. Only when both genes are switched off, the colour is absent in the snail (single-gene recessive).
Keep in mind that this is an example of how the inherit colour variations in these snails may work.

Select which genes should be active:	Resulting snail type
Gene for dark shell pigment on chromosome copy 1 on chromosome copy 2 Gene for yellow shell base colour on chromosome copy 1 on chromosome copy 2 Gene for dark body pigment on chromosome copy 1 on chromosome copy 2
	Real life example

More in dept information:

The pigments
The colour variations in the snails are the result of mutations in several genes responsible for the pigmentation of both the shell as well the body. In the natural (wild) type, the snail has a dark body, and a dark-brown shell with dark spiral bands.
As far known the shell has at least two main pigments: the yellow base pigmentation and the brown to brown-reddish colur of the bands. Both pigments are embbedded in the outer layer of the shell that consists of conchioline (a protein). This conchioline also contributes to the shell colour, although not much and becomes only visible in shells that lack further pigmentation (switch off all colour genes above and see what is left).
The body of the snail has at least one main, dark pigment that deterimines if the body is dark or albino (colourless). Nevertheless it should be noted that even albino snails are not completely white: the body still contains a yellowish-greenish colour, mainly condensed in small spots.
Snails that do not lack pigments will still vary in the amount of pigments they possess. Many genes influence the expression of single genes so that the intensity of expression can vary even for normal treats. For example, in humans that are not albino, the darkness of the skin is widely variable and influenced by a multitude of genes. So even normal snails may be darker or lighter than their normal siblings. The mutations that cause 'knockouts' of gene functions can wipe the entire pathway to the pigment and none at all is produced.

The genes
Well, the truth is that there is not much known about apple snail genetics, and certainly not about the genes that control the pigmentation.
The genetic code of apple snails is, like in all life forms (except retro-viruses) stored in the DNA. Each cell of the snail has 28 DNA strands* in its nucleus, and each strand is available in two, nearly identical copies. So there are 14 pairs of DNA strands*, and fom each pair, one strands is received from the father, the other from the mother.
In a normal cell situation, the DNA strands are invisible with a microscope, but when a cells is about to divide into two new cells, the DNA is duplicated and each old strand and its new duplicate is compacted into a chromosome. The chromosomes are, in constrast to the non-compacted DNA strands, visible under a microsope (that's why the chromosomes got so much attention in the past before DNA was well understood).
Now about the colour genes: somewhere in the DNA, there are sequences that encode the way the pigments are to be build (such sequence is called a 'gene'). These genes can consist of a single piece of DNA, or several pieces that depends on each other.
As the DNA is available in two nearly identical copies, each gene is at least available in duplicate, although multiple copies of one gene can occur in a single DNA strand. It's not known, unfortunately, which colour gene resides on which DNA strand or at which place in the DNA strand.
*: The exact number of chromosomes in Pomacea diffusa is not reported yet. Pila ovata, Pila virens, Pila globosa and Marisa cornuarietis are know however to have 14 pairs of chromosomes.

From gene to pigment
When the colour pigments are to be made, the DNA sequence is read (translated into RNA) and translated into the colour pigments (-proteins).
At this point, several things can go wrong:
The gene code of the colour pigment can be deleted completely or partial.
Or the gene can be inactive because the regulating sequences are absent or misfunctional.
Or the gene can be partial altered (changes in DNA sequence), resulting in a partial or even no translation into protein pigments.
In all such cases, the colour gene is not functional and the pigment cannot be made by the snail, at least not from that particular DNA copy. If the other, matching DNA strand still contains a functional copy, the pigment can still be made. In such case the inactive, non-functional gene acts like a reccesive gene, while the functional gene acts dominant. In practise this means that the one working colour gene copy determines if the colour is made or not, so that the inactive gene simply doesn't matter. Only of both gene copies are not functional, the colour is lacking from the snail.
If a snail has two working copies of a colour gene, the snail is called homozygote wildtype for the colour gene.
If a snail has two non-working copies of a colour gene, the snail is called homozygote knockout for the colour gene.
If a snail has one working gene copy and one non functional gene copy, the snail is called to be heterozygote for that particular colour gene.
Once more it should be emphasized that this explanation is based on a hypothesis, in which each colour treat is depending on a single, recessive gene.

Genes and reproduction
When the egg-cells are made, the DNA is compacted into 'meiotic' chromosomes and during what's called the 'meiotic' division, the egg-cells are created.
Special in this 'meiotic' process is that the resulting egg-cells, only contain one copy of each DNA strand (most likely 14 in total for Pomacea diffusa)*. So an egg cell does not have two nearly identical copies of each strand that the normal cell have.
One cannot predict which copy of the initial paired DNA strands the egg-cells receive, as this happens at random for each egg-cell. On top of that, the initial paired DNA strands (nearly identical copies) exchange parts of their DNA at random (crosslinking). This exchange of DNA parts (crosslinks), make that each DNA strand in each egg-cell is a unique combination of the paired DNA strands of the parent.
The same reduction and at random passing of DNA strands (in the form of chromosomes) happens in the production of the male sperm cells, so that the both the egg cells as well the sperm cells have a random selection of 14 DNA strands* in total.
When the sperm cell and the egg cell fuse (fertilisation), the resulting cell has gets 14 DNA strands of the father and the 14 from the mother, so that in the end (or better beginning) the fertilised egg-cell has 14 pairs of nearly indentical (one of each parent) DNA pairs (in total 28 strands).
*: The exact number of chromosomes in Pomacea diffusa is not reported yet. More about the chromosomes here

Some examples
If both of the parents have at least one non-functional copy of a colour gene, part of the offspring can receive both the non-functional gene from the father as well a non-functional copy from the mother. In such case the resulting snails will lack the particular colour (homozygote knockout). If the offspring receives, however, at least one functional copy, the resulting snail won't lack the colour as it can still be made from the functional copy.
If at least one of the parents has two functional copies of a colour gene, all the offspring will have that colour, even if the other parent has two non-functional copies of that colour gene. This is because each snail gets one copy from each parent and when one of the parents has two functional copies, the offspring will always get one functional gene, which is enough to make the colour.

Final notes
This model for pigmentation genes in apple snails is only a hypothesis (certainly the interactive example). And it's also a simplfied model/explaination. For example, a gene can reside in several copies on a DNA strand. Also it's not known if the number of copies are also related to the intensity of the pigmentation. If so, one should be able to see a direct relation in the number of copies and the amounnts of pigmentation. At the other hand it can also be that the amount of pigmentation is regulated with a feedback mechanism, so that the cells can regulate the amount of production and can increase of decrease the production. In such cases, the number of copies do not matter that much.
Also the absence or not of a particular pigment can also be indirectly due to defects in the regulating mechanisms etc.
But despite all these uncertainties, this model can somehow explain the colour variations in Pomacea diffusa, and can give some insight in the mechanims of colour, genes and breeding results.

Recommended websites:
Donya's Apple Snail Color page: http://www.angelfire.com/va/myevolution/snails/main.html

Thanks go out to Michael Marcotrigiano for the helpful comments and suggestions to improve this text.

'Chromosomes' section, 'Reproduction' section, 'Pomacea diffusa'' page and 'Shell' section.

http://www.applesnail.net