Yes, I knew what the cross is to make ISA Brown. But unfortunately, they can still have fast or slow feathering.
Purebred chickens are supposed to meet a certain standard for appearance: color, pattern, size, body shape, comb type, etc. But the standards are telling about the adult chicken, and do not say what genes must be used to get that result. For example, there are at least three combinations of genes that can make a chicken have all-white feathers, and a white chicken can have any of them while still meeting the standard for its breed.
ISA Browns are a commercial hybrid. They come from a particular cross that makes the chicks sexable at hatch by color: females are red, males are white. That kind of cross is commonly called a Sexlink. But the parents are specially bred for certain production traits, as well as the color genes that make the sexlinking work right. They may have originally been developed from Rhode Island Reds and Rhode Island Whites, but they now they do not meet the standard for either pure breed.
Yes, the Rhode Island White is part of why your chick is white. But your chick most likely has a different combination of genes than the Rhode Island White grandmother.
A Rhode Island Red is mostly red, with some amount of black (especially in the tail.)
A Rhode Island White (or at least the version used to produce sexlink chicks) is genetically silver (turns red to white) and has the gene Dominant White (turns black to white.) So it is a silver chicken with a white tail, which looks white. The red father x silver mother is what makes sexlink ISA Brown chicks.
Your chick most likely has Dominant White (turns black to white, inherited from the ISA Brown mother), but has a base color of black (from the father, with or without the blue gene.) So instead of having the genes for large amounts of gold or silver, it has the genes for large amounts of black-turned-white.
With chickens, there are some genes that affect where the color goes on the chicken (solid black vs. red with black tail vs. gold with black lacing vs. other patterns of gold and black) and some other genes that change the specific colors (black to blue, black to white, gold to dark red, gold to silver/white, etc.) There are also genes that add white in particular places (white barring, white mottling, or one called recessive white that makes a chicken white all over.) Your chick most likely has a different set of genes for where the colors go, as compared with what an ISA Brown has, but uses the same gene for changing black to white (Dominat White).
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. Your knowledge about the biology of chickens is stunning not gonna lie. You must be a teacher or something?
