How to Know if it's Pure

[Maxwell Grantly]

You can always be 100% certain a trait is pure if you “hit” the most recessive trait in that category. For example, if you were to breed a cat with small pupils than you can be 100% certain that the gene is pure (small-small) for if it were not, then the shown pupil size of the cat would be large instead of small. The same is true for other traits such as fur, for example. You could only ever know that the trait is pure if you managed to breed a cat with the most recessive fur type on the grid (if you knew what that was.)

However, in some other cases, you cannot be totally 100% sure. Your confidence may rise if, after breeding generation after generation, you always obtained the same offspring. However, this is no indication that the trait is pure. It could be that one out of the parents is homogenous (AA) and the other is heterogeneous (Aa). There would be no way for you to tell which is which. All their offspring would show a common trait, however, half would be hetreogenous (Aa) and the half would be homogenous (AA). Again, you would never know which is which. You might pick a pair of the offspring to breed for a further generation and mistakenly believe that you had indeed gained a pure strain (simply because you had selected a AA and Aa in your choice by error.) Again, half of these offspring would be “impure” (Aa) and each time you constructed another breeding pair, you could very easily be using one pure (AA) and one impure (Aa) parent.

This explains why, sometimes, a breeder may get a 100% success rate with their breeding programme for many generations and then, after selecting a new pair of cats for a further generation, a more recessive fur is given. It was simply that each time they were using a pair of cats for breeding, one was pure (AA) and one was not (Aa). That meant that every offspring had a 50% chance of being impure. In the end, they had simply picked two of these impure offsprings (Aa and Aa by a fluke) as a new breeding generation and gained a recessive-recessive gene (aa) instead.