After poking around the site a bit, you'll run into a mechanic that is coveted by a large group: genetics. The letters and numbers making up a cat's genetic string may look confusing and be overwhelming to decipher at first, but with a bit of practice, you'll find them less daunting.

Darkfang8901 has a genetics guide here. It contains a lot of information all at once, so if you're a complete beginner to genetics, I would start here and go to their guide later. The official Physical Traits encyclopedia article is very similar to this guide if you'd prefer to start there!

I am someone who isn't deep into genetics at the time of writing this guide writing for people who are similar to me. Keep that in mind as you read this guide — it's basic and as condensed as possible!

A cat's genome (also called a genetic string) is made up of different alleles to determine what they look like. These alleles work together with a select few in order to determine certain physical traits.

Here is an example of a genome:

Let's break this down and give it some color:
C SN LL OBFF2 YYMS YY1C AB

But what does this mean, exactly?

C
Every single string on the site at the moment will start with this C. This is the species indicator. 'C' means Not-Cat.

SN
The next set of alleles determines a cat's wind.

LL
This set determines the length of a cat's fur.

OBFF2
This set is where things get a little bit tricky! This set is what determines a cat's color. The first two letters determine which color group the cat is in and the last two letters determine whether or not the color is diluted. The number is the density of the color in the group.

YYMS
This set determines a cat's pattern. The first two alleles determine whether or not the pattern shows and the last two determine which pattern is showing.

YY1C
This set determines a cat's white spotting. The first two letters determine whether or not the spotting shows, the number determines which white spotting it is, and the last letter is what group of white spotting to pull from.

AB
This set determines the growth rate of a cat's stats.

Now that we know what makes up a cat's genetic string, it's time to learn more about the alleles!

Cat Species
Every cat on site at the moment is a Not-Cat, so every genome starts with C.

Wind
Possible alleles making up a cat's Wind:

1. N - North
   
2. S - South
   
3. O - Null

The combinations for wind outcomes are below:

1. North: NN, NO, ON
   
2. South: SS, SO, OS
   
3. Trade: NS, SN
   
4. Null: OO

The order of the alleles does not matter for wind!

Null (O) is a recessive gene, meaning you need two alleles to express it. Trade wind cats can never carry an O allele as they need both N and S to be a trade, so only North and South cats can carry null (making them known as a "null carrier").

Fur Length
Possible alleles making up a cat's fur length:

1. S - Shorthair
   
2. L - Longhair

The combinations for fur length outcomes are below:

1. Shorthair: SS, SL, LS
   
2. Longhair: LL

The order of the alleles does not matter for fur length!

Longhair (L) is a recessive gene.

Colors
Possible alleles making up a cat's color:

1. O - Orange
   
2. B - Black
   
3. F - Full
   
4. D - Dilute

Because color is more complex than other sections, I won't be going over it here, but will dive into it in the next post!

Pattern
Possible alleles making up a cat's pattern:

1. Y - Shows pattern
   
2. N - Doesn't show pattern

The combinations for pattern outcomes are below:

1. Shows pattern: YY, YN, NY
   
2. Doesn't show pattern: NN

Pattern is also a bit complex and will be looked at in a few posts down.

White Spotting
Possible alleles making up a cat's white spotting:

1. Y - Shows white spotting
   
2. N - Doesn't show white spotting

The combinations for showing white spotting are below:

1. Shows white spotting: YY, YN, NY
   
2. Doesn't show white spotting: NN

The order of the alleles does not matter for showing white spotting!

Not showing white spotting is a recessive gene.

The number determines which white spotting is being expressed. The chart using the Classic whitespotting group, for reference:

0. None
1. Locket
2. Locket & Toes
3. Bib & Boots
4. Bib, Boots, & Belly
5. Classic Bicolor
6. Piebald
7. Spotted Piebald
8. Freckled Piebald
9. Van
10. Albino

0 is always 'None' while 10 is always 'Albino.' 1-9 are based on the different whitespotting group.

The last letter determines the group of white spotting to pull from. Currently, there are five groups:

1. C - Classic
   
2. P - Piebald
   
3. R - Right
   
4. L - Left
   
5. I - Inverse
   

When breeding two cats with different whitespotting groups, the beans can be of either group.

Growth Rate
A, B, and C are used to showcase a cat's stats' growth rate. The order of these does matter.

A + A = Very Early
A + B = Early
A + C = Decreasing

B + A = Arch
B + B = Steady
B + C = Dip

C + A = Very Late
C + B = Late
C + C = Increasing

These are used for a cat's physical stats, determining how much they gain at each lifestage. As it's only possible to use math to figure these out and it's only relevant to non-adult cats, I won't be going into this.

As stated earlier, color is more complex than other portions of a cat's genome. Most importantly, while many of the alleles can be in any order and have the same effect, the positions do matter when it comes to color.

To reiterate, here is the color portion of our example genome:


If your cat is North Wind, the color group it expresses is taken from the first allele.

If your cat is South Wind, the color group it expresses is taken from the second allele.

If your cat is Trade Wind, you have two outcomes. Tortoiseshell cats express coloration from the first allele, with the second being their other color. Watercolor cats require both alleles to be the same and pull their color based on whether it's O or B.

The intensity of the color varies depending on what the third and fourth alleles say. Both color groups have two pools: the "full" set and the "dilute" set. Here are the possible combinations:

1. Full: FF, FD, DF
   
2. Dilute: DD

Dilute is recessive, so it needs two alleles to be expressed.

The number represents how intense the color is. 1 is the lightest, 4 is the boldest.

Here are the color groups:

1. Black Full: Black [4], Chocolate [3], Brown [2], Tan [1]
   
2. Black Dilute (Grey): Charcoal [4], Grey [3], Smoke [2], Silver [1]
   
3. Orange Full: Red [4], Ginger [3], Orange [2], Apricot [1]
   
4. Orange Dilute (Cream): Buff [4], Cream [3], Almond [2], Beige [1]

Looking at our example string and what we know, we can deduce what our cat looks like!

If the cat is North Wind, the expressed color would be Orange.

If the cat is South Wind, the expressed color would be Brown.

If the cat is Trade Wind, the expressed color would be Orange. It would also be a tortoiseshell, meaning the second shown color would be Brown.

The cat in question is actually a Trade Wind so is an Orange Tortoiseshell! Here is our example cat:

While more simple than color, pattern has a couple of different alleles that go into it. For clarify, let's pull our example genome:


We can break the sequence up a little bit. As stated in a previous post, the 'YY' means that the pattern shows. A pattern will show even if the genome is YN or NY.

This leaves us with MS, which states which pattern is expressed. Here are the alleles possible:

1. T - Stripe
   
2. M - Marble
   
3. S - Spot
   
4. P - Point

The order of these alleles does not matter as the two added together in general is what creates the pattern. Our example genome has a "spotted marble" pattern, so-to-speak, which is Rosette. Here are all of the possible pattern combinations:

1. Mackerel Tabby: TT
   
2. Classic Tabby: TM, MT
   
3. Broken Tabby: TS, ST
   
4. Lynx Point: TP, PT
   
5. Clouded Tabby: MM
   
6. Rosette: MS, SM
   
7. Cloud Point: MP, PM
   
8. Spotted Tabby: SS
   
9. Mink: SP, PS
   
10. Color Point: PP

Not expressing a pattern is a recessive gene, and cats with NN will have a Solid pattern.

Albinism (Albino) is to express the most white possible, having a value of 10. Albino cats come with a unique eye color depending on their whitespotting group, while other cats have dark brown. This unique whitespotting makes Albino cats quite popular, but they are rather tricky to get!

Whitespotting value, when breeding two cats, is determined based on a scale rather than combination and wind. What this means is that when breeding two cats together, their kittens can have a whitespotting value between and including the parents'. Kittens can be either whitespotting group the parents have. This may sound confusing, so here are some examples:

1. Breeding a cat with Locket to a cat with Bibs, Boots, & Belly means their kittens can have Locket, Locket & Toes, Bibs & Boots, or Bibs, Boots, & Belly whitespotting.
   
2. Breeding a cat with Spotted Piebald to a cat with Classic Bicolor means their kittens can have Classic Bicolor, Piebald, or Spotted Piebald whitespotting.
   
3. Breeding two cats with Bib & Boots together means their kittens can only have Bib & Boots.
   
4. Breeding a cat with Locket (Classic) to a cat with Tail Tip & Toes (Right) means ther kittens can have Locket (Classic), Locket & Toes (Classic), Tail Tip (Right), or Tail Tip & Toes (Right).

Because of this, when it comes to breeding an Albino cat, at least one of the parents needs to have or carry Albino in order to have Albino kittens.

How do we figure out which cats can do that?

1. Cats that express a whitespotting pattern that isn't Albino cannot, themselves, produce Albino kittens. Their partner needs to be able to.
   
2. Cats that have no visible whitespotting may carry Albino secretly. Due to a bug, if your Migrated from Earth cat has an ID lower than #11358, they have NN0 by default. Cats born to a set of parents are unaffected by this bug.
   
3. Because not expressing whitespotting is a recessive trait, at least one parent needs to express whitespotting. Otherwise, the parents will only produce kittens that don't express whitespotting.
   

The easiest way to check for hidden Albino (NN10) is to use a cat with Van whitespotting. If the Bean Sandbox gives any outcome predictions with whitespotting lower than Van, that means the partner cat doesn't carry Albino. If the predictions only show Van cats, then the partner cat is NN9.

You can use the ID 2468 when testing for NN10. This cat is Trade Wind and the ID is easy to remember, meaning testing is relatively easy. Do note that the cat in question is YN9, meaning some predictions will end up with no visible whitespotting.

reserving this for when new species get added

6/25/23 - updated this to finally account for the four new whitespotting groups

can i ask why the patterns state mackerel tabby, classic tabby etc. what does the tabby mean? I have a broken pattern tortoiseshell cat, would their pattern code still be TS or ST?

@niran
Apologies for the delay on this response, I'm not subscribed to this thread!

The patterns in the Sandbox used to say Tabby, but it seems those have been removed. So it doesn't mean anything in particular. Broken would still be TS or ST for pattern!