quick looks chromosome editing guide
this is a quick guide to gene editing, for the looks chromosome specifically. once you know what you're doing, it's not too difficult!
after opening a pet in a hex editor, set the bytes per line to 15. then, scroll down past the header and LNZ and such until you find something that looks kind of like this, a little past the YALP section and other veterinary info. this is the pet's first looks chromosome, the part that specifies what kind of appearance the pet can pass down!
here, i've isolated the important columns and labeled which parts are which.
for all the parts labeled in green (the ones that specify body parts or otherwise point to a breed), you want to change the bytes indicated on that line to the breed ID for the breed you want that part to be taken from. i have a quick reference for genetics values here that lists all of them for the base game breeds.
each part that specifies a breed id also specifies a seed, which is important for breeds that have variations, since it's what determines which variation in that breed the part is taken from (for example, for alley cat tails, which way the tail will zig-zag). most of the time, you can just leave this as-is, but if you're going for a very specific part like that, you can adopt an AC pet with the variation you want and use the seed from that pet.
the parts labeled in gold (the extensions, scales, etc.) all use regular numbers, converted to hexadecimal, so if you want a markings factor of 50, you'd just put in 32. for the extensions, four bytes are highlighted, because extensions can be negative, and for negative extensions, you change the three bytes after the number to FF, so if you wanted the leg extensions to be -5, you'd put 05, then three FF bytes after that.
the parts highlighted in cyan, which determine the colors, have their own ranges of values that correspond to different colors, which again i have listed in the quick reference list. as for which color slots correspond to which parts of the coat, there's a guide to coat color genetics for each breed on Unique Petz here!
each pet has two looks chromosomes, so you'll also want to scroll down a bit and change the second one, but after that, you can run the pet through PetzByte and they should pass on the new appearance when breeding!