Bridgesii Color Genetics

        As seen on the Color Examples page, an apple snail's color can be broken down into 3 primary "traits" that have different forms: stripe color, shell base color, and body color. Obviously the color of those will be determined by the inheritance of dominant and recessive coppies of the traits from the snail's mother and father.

Gene Basics
        The three traits listed above (stripe color, shell base color, and body color) are, in the simplest form, controled by three pairs of genes (in reality it is more complex, but for the moment a simple model will suffice for explaination). The first is a pair of genes for the dark shell pigment, and second a pair genes for yellow shell pigment, and the third a pair genes for dark body pigment. Each of these genes has two forms: dominant, and recessive. The genes in each trait's pair are independant of each other, meaning that the trait's pair can contian both a dominant and a recessive gene for the trait. The presence of a dominant gene will always override the presence of a recessive gene in the pair. So, for the recessive trait to be expressed (the expression of a set of traits is called a PHENOTYPE), the gene pair must contain two recessive genes. Otherwise, the resulting expression of the trait will be that of the dominant trait.

Color Classification
        Using these three color traits and their various combinations, a snail's overall color can be "classified" according to its traits. Similar to the steps for identifying a snail's color, the snails can be broken into 2 major categories: albinistic and dark. All snails in the "albinistic" category will not be truely albinos, but rather have a white foot with various pigmented spots; all dark snails will have a blue-ish black foot with the same pigmented spots. Each of those two categories has 3 subset categories: yellow, green, and purple. When looking at the large number of colors found in P. bridgesii, it becomes obvious that snails' shells tend to either have primarily yellow and brown pigments, primarily purple pigments, or primarily green-ish brown pigments. So, the resulting categorization of colors, arranged with similar trait "patterns" in each category on the same level, looks like this:

Albinistic			Dark
Yellow	Green	Purple	Yellow	Green	Purple
Striped Yellow	Striped Olive	Striped Purple	Wild Type	Dark Striped Olive	Dark Striped Purple
Golden	Plain Olive	Plain Purple	Jade	Dark Plain Olive	Dark Plain Purple
Red-White		Light Striped Purple	Red-Brown		Dark Striped Purple (csb)*
Ivory			Blue

*csb=clear shell base

        Why the shell colors seem to fall into 3 primary categories (yellow, green, & purple) is not understood yet, though I am currently working on genetic models to explain it. When a more detailed look is taken at just how many traits control the overall shell color of P. bridgesii, the above table is not 100% possible to use because, as it turns out, some purple snails can express yellow traits that are masked, and likewise some yellow and olive snails can express some purple traits that are masked. However, despite those complications, the resulting overall color that you see in a given snail can still be fitted into the table above with little problem.

The Shell
        When an apple snail's shell structure is taken into account, it quickly becomes evident that the simple three-trait model is not sufficient to fully explain the inheritance and expression of color genes. Apple snails have two layers of pigment-carrying shell: an inner layer, and an outer layer.

A schematic example of shell layers in a Wild Type P. bridgesii [left], and a Dark Striped Purple with a yellow inner shell layer that is masked by the outer purple-carrying layer [right].

A schematic example of shell layers in a Light Striped Purple [left], and a Transparent Golden snail [right]; the inner layer is always semi-transparent, so a clear outer layer results in overall transparency.

Examples of different hues produced by different inner shell layers in 2 Striped Purple bridgesiis and one Dark Striped Purple (clear shell base). From left to right: yellow inner layer/purple outer layer/purple stripes, pink inner layer/purple outer layer/purple stripes, clear inner&outer layers/dark purple stripes/dark body (shines through as blueish-purple).

        The inner layer can contain a uniform pigment (a "base color"), but no stripes. The outerlayer caries both a "base color" and the stripe color. So far, it appears that the shell alone can be considered to have 3 traits: one for the inner layer's color, one for the outer layer's base color, and one for the outer layer's stripe color.
        To further complicate things, the genes controling the color of each of the 3 shell traits are not as simple as an on/off switch. While at a glance, it would seem that most snails either have a pigment or they don't (on/off switch), the actual case is more like an on/on/on/off switch, in which a snail can have one of several colors for a trait, or lack the pigment alltogether. This would suggest to me that there may be multiple genes controling each trait, but the inheritance is relatively straight-forward so (for the moment anyway) I feel I can still treat the factors/genes controling each trait as a unit. With that approach, a snail's color for a given trait can be considered a result of relative dominance between two gene "units", one inherited from each parent.

The "Unit" Approach
        I realize that my description of the gene "units" is hard to understand and give a decent description of, so I feel I need to give a more detailed definition of it before launching into further genetics explaination. Assume for a moment that there is indeed one gene for each of the 3 traits involved in the shell color. This would mean that a snail would have two coppies of the gene, one inherited from each parent, and that if each gene was different, the dominant one would be expressed (please not, I am not accounting for co-dominance in this). Now, assume that rather than a gene being a simple either/or, that it can take one of a variety of forms. So, one copy of a stripes gene could be for purple stripes, brown stripes, black stripes, or no stripes. In this case, each of those gene "forms" (purple, brown, black, or no stripes) would have a specific relative dominance, meaning that you could stack their heirarchy. For instance, black might be dominant over brown, brown over purple, and purple over no stripes. This eliminates the possibility of inheriting conflicting information from two parents. If a snail gets a stripe gene that is for no stripes, and one gene for black stripes, that snail will end up with black stripes.

The Body
        Now that the gene "units" have been explained using the stripes as a model, the body and body spot characteristics can also be taken into account. Like the various shell traits, body color traits do not follow a specific on/off for pigment production. In reality, there are actually 3, possibly 4, body colors. Those colors are: white, blue-ish black, yellow, and possibly brown. Again, each of those forms would have a certain dominance or recessiveness relative to the other forms. So far, all we know is that blue-black is dominant over white. Dominance of the others are not known.
        Body Spots also follow a similar pattern. There are two primary colors of body spots: orange, and yellow. Orange is dominant over yellow. While these are the most common, and probably account for a huge percentage of bridgesiis in the world, there are other spot traits that appear. One is a very strange form of orange spots, in which the orange pigmentation is so dense that it makes red-ish blotching all over the foot. This form of spot has only been observed in animals with a blue-black foot, and it is in association with vertical black stripes on the back half of the foot. Another spot form which I have only seen once is a no-spot version, in which no body spots appear to be present at all. I saw this in exactly one light-bodied individual who, unfortunately, died shortly after I discovered and purchased it.

Summarry of Gene Forms
        So far, numerous color forms have been documented in both shell layers, the stripes, the body, and the spots of P. bridgesii. The following table is a summary of all recorded color forms. (Forms marked with a "?" are suspected but so far there is no conclusive evidence, and "null" means an abscence of pigment for that trait).

Inner Layer	Outer Layer	Stripes	Body	Body Spots
null	null	null	blue-black	orange
yellow	yellow	brown	white	yellow
brown	brown	black	yellow	red-blotch
green?	green	green	brown?	null?
pink	purple	purple
purple

        Unfortunately, most of the inner and outer shell layer color forms have had to be documented opertunistically, taking note of what colors show through durring accidental shell breaks or sudden growth of one layer or the other. I have never and will never deliberately injur or otherwise harm a snail to gather data. Though this limits my ability to gather complete data, with the help of other people, I have been able to solidify the presence of most of the color forms for the traits in the table above. Hopefully I will be able to confirm or rule out uncertain forms in the near future.

Variations/Variables Within Gene "Units"
        Each gene unit most likely containes several individual genes, which can account for certain inconsistencies within a given color type of snail. For instance, if two Golden snails are chosen at random, it is probable that there will be a very small difference in shell thickness, golden hue, and several other things. While this difference is often imperceptbale in the Golden color, there is a much wider and noticable variation in other colors. The variation in stripes on Light Striped Purples is a good example of this. Some Most Light Striped purples (purple stipes, clear shell base, white body) have wide, somewhat "messy" stripes. However, there is a version of the color which has very neat, regular stripes that are very thin; this is the absolute hardest kind of snail to come by short of a true red-white. Stripes seem to come in two general forms in most colors, not just the purples: wide/irregular ("messy"), and narrow/neat. Irregular and messy stripes appear to be dominant over the narrow ones. The darkness of the stripes also seems to be variable: some snails have thin stripes that are very dark, others are very light, and the same applies to messy stripes. This suggests to me that a gene "unit" for a given snail's stripes would contain 3 sub-units controling the following traits: stripe color (pigment type), stripe width (messy vs. narrow), and color intensity (amount of pigment).
        So far I have not been able to identify any "sub-units" in any of the other traits, though I suspect both shell layers may have sub-units controling shell thickness. Thin/thick shells seem to be hereditary, so that would tend to support the idea.

Color Combinations
        With the assumption that all of the traits (NOT counting any sub-units, just the whole gene units for both shell layers, stripes, body color, and spots) in the table above can assort independantly, and that no traits are dependant on any others, the resulting possible number of color combinations would be astronomical. If any trait can appear in the presence of any other trait, it would make the total number of possible combinations 6*5*5*4*4=2,400. Even if the uncertain traits are ruled out, the possible combinations would still be 5*5*5*3*3=1125. I seriously doubt that there are that many possible colors out there, so that suggests that some of the traits are linked and/or dependant on the presence of sertain trait forms. Also, the fact that snails still tend to fall into the categories of yellow, green, and purple, suggests links between traits, otherwise snails with pink shells and dark green stripes would show up...and so far I'm not aware of any of those.

Phenotypes and Genotypes
        This is unfortunately the hardest thing for most people to understand, and it's primarily for my purposes in recording my snails' traits. However, when I have any color models posted, they will always be given using my abreviations and ways of recording traits. So, to know what any color models mean that I post, understanding the phenotypes and genotypes I use will be necessary, until I find a better and more understandable means of recording data.
        Given the table above of all the possible colors for each shell layer, the stripes, the body, and spots, in order to track my own snails and create a standard way in which a snail's traits are recorded, I have created 3 systems to document a snail's genes.
        The first and simplest way, uses the simple 3-trait model, with 1 trait each for stripes, shell base, and body pigment. The letter A is given to stripes, B for shell base, and C for body pigment. For this, the traits function as strictly on/off, so either there is pigment present, or there isn't. This means that Dark Striped Olives and Wild Type bridgesii's get the same combination of letters to describe their appearance (their phenotype): ABC. Currently I am counting purples seperately. To document a purple snail, X is for stripes, Y is for shell base, and Z is for body pigment. Recessive traits are expressed with lowercase letters, so a Light Striped Purple would be Xyz, and a Golden snail would be aBc.
        To document what traits they carry, each letter is recorded twice (once for each gene inherited for a trait). These GENOTYPES are written in the form AaBbCc+XxYyZz. Pigmentation is always dominant over no pigmentation. If a snail is expressing no pigmentation for a trait, that trait gets two lowercase letters, because if they had a dominant gene (capital letter) it would override the recessive one. If dominant traits are expressed, only one copy of the gene can be recorded acurately, because the other copy could be either dominant or recessive. To record this, the placeholder _ is used. So, a Light Striped Purple would be ______+X_yyzz, and a golden snail would be aaB_cc+______. The placeholders can only be filled in by knowing if the snail's children express recessive traits that it doesn't. So, a wildtype snail that had ivory and light striped purple offspring would have the following genotype: AaBbCc+X_yyzz.
        To deal with the more complicated approach to the genetics, using the table of all possible color forms, I have created a second, more detailed way of documenting the phenotype of a given snail. (Because this uses the complicated tabel of gene forms, I cannot have a coresponding way of recording genotypes since I don't know what forms are dominant over the others). To do this, I have assigned each color a single letter:

*white and dark are for body color ONLY. Dark is equal to blue-black. For all other traits, white is considered NULL.
**Orange is for body spot color ONLY.

        Phenotypes are recorded in the following format:

                        innerLayer+outerLayer+stripes+body+bodySpots.

        So, an ivory snail with yellow body spots would have a phenotype of: nnnwy. Likewise, a Dark Striped Purple snail with one pink shell layer and one purple shell layer (with dark purple stripes and orange body spots) would have a phenotype of: pPPdo.

Color Inheritance
        So far the relative dominance of almost all color traits has not yet been established. All we know so far, using the simplistic ABC/XYZ phenotypes, is this (higher=more dominant):

RELATIVE DOMINANCE	Yellow			Green			Purple
	Stripes	Shell base	Body	Stripes	Shell base	Body	Stripes	Shell base	Body
DOMINANT	brown (A)	yellow (B)	Dark (C)	black (A)	green (B)	dark (C)	dark purple (X)	purple (Y)	dark (Z)
RECESSIVE	null (a)	null (b)	white (c)	null (a)	null (b)	white (c)	null (x)	null (y)	null (a)

        (Please note that the letters in the brackets are indicating a comparason of the simple color model to the more detailed one; they are NOT the same letters as used on the detailed phenotype table)

        It is also known that Yellow color forms are dominant over Purple ones, however the exact inheritance is not known, because some yellows can have purple stripes that appear brown, and some purples can have a yellow inner shell layer that is masked. Relative dominance of the Green (Olive) forms is not yet known, because to my knowledge there have been no selective breedings of various Olive colors to other colors, or at least there have been none with reported results that I have seen.
        Despite the apparent pattern in the dominance (i.e. that stripes are dominant over no stripes, etc), there is a loophole when looking at relative dominance of Yellow color forms and Purple ones: purple offspring with purple stripes can result from a parent cross which has no stripes to begin with. This means that an albinistic (lacking pigment) trait is in fact dominant in some way over a pigmented trait, which is generally not the norm. Color crosses observed to produce purples in ways like that (that don't make sense) are as follows:

(Golden)	+	(Blue)	=	(Light Striped Purple)
(Golden)	+	(Golden)	=	(Light Striped Purple)

        In these cases, stripes are obviously inherited from the snails which don't express them (and the presence of stripes is normally dominant). This indicates that the purple form of the stripe gene must be recessive to the null form. Inheritance of other purple traits seems to follow similar patterns, but so far those are the only documented combinations that break the null+null=null rule that all of the Yellow color forms follow.