Sorry if this doesn’t make sense or is dumb
So to my understanding, your dna contains all of the code for how your phenotypes are expressed, including in your skin of course. Like where moles/birthmarks go, and for animals, how different fur colors manifest. Pigment and stuff is programmed to go to specific areas and things like that
But how much data is there? Because if someone gains a lot of weight, they also gain a lot of new skin. Does your dna just have an endless pattern for what features your skin will have? Or when an animal is really overweight, how does the dna just keep on generating a fur pattern indefinitely? I can understand when it comes to skin how maybe moles and things are a separate manifestation, but to my knowledge fur color is very tied to pigment sources placed in a pattern. It’s not random, because the colors are never just scattered without reason. But how does it decide in what manner to continue the pattern? Of course DNA is a finite thing, so does it eventually just loop back around? A repeating pattern, reusing data?
Sorry if I’m totally misunderstanding how this works lol
In: Biology
You’ve asked a really good question that does have an example in the calico cat!
1. When a zygote/fetus is first formed, that one cell has two copies of each chromosome, but, each chromosome copy may have a different version of a gene (these versions are called “alleles”) such that one copy codes for one hair color, and another copy codes for a different color. **Cat coat genetics are actually a little more complicated than this but I’ll keep it simple**. If the cat has *black* on both chromosomes, it is a black cat. If it has *orange* on both, it is a ginger cat. If it has *black* on one, *orange* on the other, it has a brown coat.
2. As cells of the zygote proliferate (grow in number) and differentiate (turn into specific body parts), these alleles become important only if they affect that cell. Like, a cat’s liver cell doesn’t give a crap about coat color, but the fur-growing cell needs to know – what color am I? So yes, areas of the body that grow and “fill in” just copy themselves from what is already there. A tooth doesn’t randomly grow out of the skin that is growing, because tooth genes aren’t “on” in the skin. (And when this happens, which does rarely, it’s a sign of a serious disorder in those cells. They’re reading the wrong chapter in the manual).
3. There is a fascinating thing that happens in female zygotes: [Barr-body inactivation](https://en.wikipedia.org/wiki/Barr_body). Females have two X-chromosomes, and under certain circumstances, early on in fetal development, one of those two chromosomes in a cell will just shrivel up and close down shop. For these cells, and all the cells that proliferate from it, the remaining X-chromosome becomes the sole genetic source for those genes down the line. So imagine you have a female cat zygote, and it has *black* fur gene on one of its X-chromosome, and *orange* on the other; after the first cell division, both of the daughter cells ‘turn-off’ one X-chromosome, such that one is now just *black* and the other cell is *orange*. As those cells develop into tissues, the cat will present as black on one half, right on the other. This happens a lot with cats because their coat color genes are basically “white or color?” plus “if color, then black or ginger” and then… kind of a volume slider bar for intensity of color. So with a cat that has both alleles for those first two genes, and color intensity set to full, you can end up with patches of different colors (black, orange, and white). Each patch is derived from that moment when its first source cell turned one of those X-chromosomes off. We call this coat pattern “calico”. Such Barr-body-related events in development have a lot of complex and interesting outcomes.
4. So as a calico kitten grows and gains weight, their patches stay mostly the same and just fill in from the inside. Similarly, a mole on human skin is a skin cell that randomly decided to produce a lot of melanin, and then all the cells proliferated from it followed those instructions (I don’t think moles are Barr-bodied-related events, though).
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