Now that I'm not dealing with my tablet, I can give a more detailed explanation.
As you probably realize, simple recessive traits are shown when offspring receive the genetic material for that trait from both parents. It is easiest to think of this in terms of gene pairs...and I'll use albinism as a convenient example. Each parent provides half of the genetic information, (think: half of a gene pair), and a visual albino has the full pair (a,a). It doesn't matter which half it gives to a given offspring, it is giving the ability to produce an albino (because both halves are albino, which is what makes it visual)
Pairing a visual albino with a normal (x,x) - no offspring will get both halves of the albino gene pair, because the one parent doesnt have it to give....but all of the babies will get one half (from the visual parent); so they are all hets. So a visual bred to a normal produces no albinos, but all of the babies will be heterozygous for albinism. These can be simply called het albino, but some choose to use the terminology "100% het".
Using the letter "coding" shown above, a het would be (a,x) - the a being the half that carries the albino gene, and the x being the half that doesn't.
Now, if a visual (a,a) is paired with a het, the het will provide either the "a" or the "x" to each offspring. Since we have established that the visual will always give an "a", that means that the offspring will either end up as (a,a) or (a,x)....so visual or het. Pretty simple at this point, right?
So, were do the percentages come from?
If you pair two hets (a,x) and (a,x), each parent will give half of their pair to each offspring. So, in this case, each parent will provide either an "a" or an "x". That means each offspring has the chance of being (a,a), (a,x), (x,a), or (x,x). For our purposes, (a,x) is the same as (x,a)...both have half of the gene pair needed for albinism - in other words, they're hets. Looking at those combinations, we have 1 visual, 2 hets, and 1 normal. We can easily identify the albino, so we don't have to consider that one. Of the other 3 possibilities, 2 are hets. So each nonvisual baby has a 2 out of 3 chance of being a het. 2 out of 3 is 2/3 is 66%. When breeding het to het, the non-visual offspring are called 66% het. (Remember, though, in reality the baby is either het, or it isn't...it's just that we don't know by looking at it; so the number is just a way of expressing the likelihood). With a 66% het, the odds are pretty good (2 out of 3) that the offspring carries the trait.
Breeding a het to a normal though, would give (a,x), (a,x), (x,x), or (x,x). There are no visuals, and a baby has a 2 in 4 (50%) chance of receiving the albino trait. Those are the 50% hets (again, each baby is or isn't...we just don't known which until "proven").
If you breed a 66% or 50% het baby to a visual and get visual babies, you can drop the % designation because they have been proven het for the trait.
