The answer is fairly simple, though the explanation gets a little complicated, so bear with me.
There are a bunch of different genes that influence a rabbit's color; it is the interaction of all of them that determines what color the rabbit is. Some genes only have two possible forms (black vs. chocolate, dilute vs. non-dilute), while others have several possibilities; we call them gene "series."
The A series has 3 possibilities: Agouti(A), Tan(a[SUP]t[/SUP]), and Self(a). Self is the most recessive gene in the A series. for a rabbit to be a self, it must have 2 copies of the self gene. Tan (including Marten and Otter) is the next step up in the series; tans have the white "trim" like the agoutis, but the body color of a self. A tan may have two copies of the tan gene, or one tan and one self. Agouti is the most dominant form in the A series. As long as a rabbit has 1 copy of the agouti gene, it will be agouti patterned, and you can't tell just by looking whether the rabbit has 2 agouti genes, an agouti and a tan, or an agouti and a self. When you have one known, more dominant member of a gene pair, and one unknown member, the pair is written like this: A_. That means you know this animal is an agouti (because it is expressing the agouti pattern), but you don't know what other gene it is carrying but not expressing.
Tort may not look it, but it is a self-based color. Torts have two copies of the self pattern gene (aa). The other gene series that is critical to producing a tort is the E series. In that series, there are several possibilities, but the one that produces self is the non-extension gene (e). Non-extension is the most recessive in the series, so for it to be expressed, the rabbit has to have 2 copies of it (ee). Because it is the most recessive, it can hide behind the other genes in the series, ,most notably the full-color gene (E).
To keep this post as short as possible, I won't explain what all of the other genes do; but you may have seen colors laid out by the genes that are required to produce them, like this:
A_ B_C_D_E_enen vv(Chestnut Agouti)
Here are the crosses you have mentioned, with the critical genes highlighted:
A_B_c[SUP]chd[/SUP]_D_ eevv (Frosty) x aaB_C_ddeeVv (VM Blue Tort) ----> aaB_C_D_eevv (tort) because the Frosty and the Blue Tort only have non-extension genes in the E series, all of their babies will get a non-extension gene from both of them. The Blue Tort only has self in the A series; obviously, this Frosty has a self gene there too, since this cross resulted in a Tort.
aaB_cchl_D_eeenenvv (Sable Point) x aaB_C_ddE_Enenvv (Broken Blue) ---> aaB_C_D_eeenenvv (tort) Both of these parents have only self genes (a) so that's all they can give to their offspring. Clearly, though the Broken Blue has one full-color gene in the E series, he must have a non-extension gene that he isn't expressing, since he gave it to his offspring.
aaB_c[SUP]chl[/SUP]cddeeenenvv (Pearl Point)
a[SUP]t[/SUP]_B_C_D_eeenenvv (Torted Otter) x ----> aaB_C_D_eeenenvv (Tort)
a[SUP]t[/SUP]_B_c[SUP]chl[/SUP]cD_E_enenvv (Sable Marten)
If you get self-patterned babies from Otters or Martens, clearly, those tan-patterned rabbits are carrying but not expressing a self gene. Not hard, but kind of funny when that's all you get!
Well when you put it that way, you gotta wonder what the importance of a pedigree is! Lol other than to see who's lines are in your rabbit's background and what wins they have. Well, not so much the purpose of the pedigree but why some people make a such a big deal over them.
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