1/ Alright the people have spoken and they want more cat genetics. So, I present to you all "Cat Coat Genetics 101: A Tweetorial", feat. pics of many real life cats (for science, of course...this baby is Caterpillar). 
2/ Note I will be writing this on the level of someone who has a basic background understanding of genetics (vocab: dominant vs recessive, autosomal vs sex-linked, genotype, phenotype, allele). If something doesn't make sense, please don't hesitate to ask for clarification!
3/ Also, a disclaimer: I am not an expert in cat genetics. I am however a bona fide Cat Person and a 6th-year grad student in cancer bio with a strong background in cell bio and genetics. If any of you ARE experts in cat genetics, I would LOVE to hear from you! Now, cats:
4/ Let's start with fur length. This is Mike & Sulley, two past foster kitten brothers. Mike has short fur and Sulley has long fluffy fur. Hair length is regulated by the gene FGF5 (fibroblast growth factor 5).
5/ The dominant FGF5 allele L = short hair, recessive allele l = long hair. I know Mike & Sulley's dad had long hair (ll) and mom had short hair (Ll). Therefore, Sulley is ll genotype (homozygous recessive; long hair) and Mike is Ll genotype (heterozygous; short hair).
6/ Next, let's talk about basic fur colors. Ignoring white (it's different), the two basic colors in cats are black and orange; pretty much every other coat color is a variation of black or orange. Black fur comes from the pigment eumelanin, orange = the pigment pheomelanin
7/ This is Freddie Mercury, my cousin's cat. He has solid black fur because he has a dominant allele of a gene called TYRP1. TYRP1 is part of the metabolism pathway creating eumelanin. The wild-type dominant B allele = black fur (like Freddie here)
8/ This "black fur" gene has multiple recessive alleles in cats, including b (brown) or b1 (cinnamon, aka lighter brown) that create pigments of different structures and therefore different colors. Order of dominance: B > b > b1
9/ Next, this is my kitten Bennie. He is an orange tabby. In cats, orange fur is sex-linked, meaning the gene is on the X chromosome (unlike black fur, which is an autosomal trait). This means XX females get two copies of the "orange" gene and XY males (i.e. Bennie) get one.
10/ Now here's the cool part. Nobody knows (yet) what the "orange" gene actually is! Pheomelanin (aka orange pigment) is encoded elsewhere, but the "orange" gene on the X chromosome encodes some protein that BLOCKS the production of black pigment.
11/ The dominant O allele = blocks black pigment -> orange cat; recessive allele o = not orange (black pigment still intact). It doesn't matter what variant of black a cat has, the O orange allele will always override it. This is called epistasis, aka one gene overrides another
12/ A human example: baldness. If you have the baldness gene, it doesn't matter what color your hair is, you're still gonna lose your hair. Aka baldness is epistatic to hair color. Anyway, back to cats 🐈
13/ This is Aydin, a chonky boy I catsit on occasion. Aydin is a cream/buff color, which looks like diluted orange (spoiler: it's literally just that).
14/ Dilute colors like this come from variants of melanophilin, a protein that transports pigments into hair. Less pigment added to hair = paler colors. Dominant D allele = dark colors, recessive d allele = diluted colors. Aydin = XO Y (orange) dd (dilute) -> cream fur
15/ Here's another example, this is Orson, my labmate's cat. His coat is grey, but the technical name is "blue", which is dilute black (it's weird, I know, I didn't pick the name). So, BB DD or BB Dd genotype = black fur, BB dd = blue fur (like Orson).
16/ Chocolate dilutes to lilac (aka a slightly lighter grey), so bb dd genotype. Cinnamon dilutes to fawn (yet another shade of grey...only 47 to go), so b1b1 dd genotype.
17/ This is my cat Lucy. Just like Caterpillar up top, she's a calico, or tricolor cat, aka black, orange, and white. But if orange fur overrides black fur, how does she have both?? It's all due to X-chromosome inactivation!
18/ In female mammals with XX, one X in every cell is "turned off" so that they have the same amount of "X" genes active as XY males (aka dosage compensation).
19/ Lucy is BB or Bb genotype for black (autosomal) and heterozygous for orange (XOXo, sex-linked); one X has Orange and one does not. During development, X inactivation is random.
20/ So some random fur cells have XO turned off --> black fur, and some have Xo turned off --> orange fur. This means that a cat must have two X chromosomes to have both black and orange fur.
21/ Most of the time, this is XX females, but also XXY (or other XX+ variations) intersex cats. XXY "male" cats will have male genitalia but are sterile
22/ There's also very rare cases of XY male calico cats. The best hypotheses for now are partial X duplication, mosaicism from a mitotic error in early development, chimeras (aka two embryos fused in one), or some unknown epigenetic regulation!
23/ Now, Lucy and Bennie both have some white fur, which is just a lack of pigment. Turns out white fur in cats comes from variants of the KIT gene (yes, this is the same c-KIT proto-oncogene in humans!). White fur comes from the spotting allele S, color fur is the s allele.
24/ However, S is codominant to s, meaning that you see BOTH color patches and white patches on one cat (see also: spotted cows and ABO blood types), AND S has variable expression, meaning % white fur can be different amounts in cats with the same genotype.
25/ Typically an Ss cat will have <50% white and an SS cat will have >50% white, but it's complicated and there's probably other factors at play that affect where white patches occur and how much white fur there is.
26/ This is Spot, another one of my catsitting cats. He had...rather unfortunate white spotting patterns.
27/ Remember Mike and Sulley from earlier? They are solid white, which comes from a fully dominant KIT allele W. So if a cat has a W allele, it overrides all other colors (epistasis again) and will be solid white. They also have heterochromia, aka one blue and one amber eye!
28/ Another common coat pattern: tabby cats. This is Fozzie (RIP), he was a black tabby, meaning that the dark stripes were black, with brownish stripes in between.
29/ Fozzie was also polydactyl, aka had two thumbs on each front paw! Polydactyly comes from an autosomal dominant mutation in the sonic hedgehog (Shh) pathway and is seen most frequently in cats on the east coast of the US and Canada.
30/ Tabby patterns depend on at least 2 known genes, agouti and tabby. Agouti has dominant "A" and recessive "a" alleles. "A" changes melanin production, so that individual hairs have stripes of black and pale colors; "a" non-agouti = solid color hairs and no stripes.
31/ If a cat has AA or Aa agouti genotype (aka striped hairs), the patterns of paler striped hairs vs darker non-striped hairs are controlled by the tabby gene. The tabby gene has many different alleles that lead to all sorts of cool stripe patterns (will skip details for now).
32/ Another cool fact: ALL orange cats are tabby, there are no solid orange cats! Why? The orange gene is epistatic to the agouti gene. This means that no matter what agouti alleles a cat has, the orange gene automatically produces tabby stripes (even if they're super faint).
33/ Now on to my favorite coat pattern of all: colorpoints! This is Oliver (aka Ollie), labmate's cat and brother to Orson. Ollie is a chocolate point, meaning he only has brown on the tips of his body, i.e. ears, face, feet, tail...and scrotum.
34/ But how does the color show up only on extremities? Color point cats have a mutation in a protein that turns tyrosine into melanin...but it's a TEMPERATURE SENSITIVE mutation!
35/ This protein unfolds when it gets too hot, preventing melanin from forming properly, but at cooler temps works just fine. Body temp is warmer towards the core and cooler towards the extremities (ears, tail, feet, etc), so pigment forms only where the body is cooler.
36/ The wild-type dominant C allele = full-body color. Recessive c allele = colorpoint. There's actually a couple different variants of c that lead to more or less difference in the color of the core vs extremities (look up Siamese vs Burmese).
37/ A lot of people incorrectly call all colorpoint cats Siamese, but Siamese cats are actually a very specific breed. Your average street cat can be a colorpoint but not a Siamese.
38/ You can also get color points in ANY pigment color, like orange or grey! Here is Bububu, a majestic three-legged cat I catsit who is a blue tabby colorpoint. [B- dd A- T-] = Black dilute Agouti Tabby)
39/ Also, as cats age, their body temperature drops. So, as colorpoint cats age, their whole body tends to get a bit darker over time!
40/ You can also see colorpoint differences depending on geography and season due to differences in local temperature. Colorpoint cats are basically furry thermometers.
41/ Anyway, I hope you learned something new and exciting about cat coat color today! These are merely the basics, because genetics are delightfully weird and complicated.
42/ For more info that's not too technical, I recommend the Wikipedia page on cat coat genetics (and cited links): en.wikipedia.org/wiki/Cat_coat_….
43/ For a wonderfully detailed deep-dive on cat genetics of all sorts of traits, I highly recommend the site Messybeast: messybeast.com/catarchive.htm
44/ Also, shoutout to a cat genetics blog/Twitter recommendation I got from my previous post! @sparrowsgarden sparrows-garden.com/genetics
45/ Adopt, don't shop, spay/neuter your cats, take them to the vet regularly, vaccinate them, keep them indoors only, and for goodness sakes do NOT feed them vegan food. They are adorable little carnivores who need their meat.
46/ Thank you all for indulging my deep nerdiness for both cats and science. If you would like to learn more about specific cat genetics or have any questions, I will do my best to google my way to an answer! And please post your cat photos below as well, the more the merrier 🙂
43.5/ This is the page on Messybeast specifically on coat color and patterns. Great info in particular on tabby variations and white patterns (i.e. tuxedo) and how development plays a role in patterning! messybeast.com/colour-charts.…
47/ A couple educators asked me if they can use this thread/examples for teaching, so to make it public: YES PLEASE! I would LOVE for anyone to spread the love of cats & genetics to students of all ages, so feel free to use these examples! You have my blessing (and my cat pics)
