Imagine it like this, you've got 100 blue marbles in a bucket, these represent the genes of a 100% pure greyhound. Now 100 red marbles in another bucket, these are collie, 50 marbles out of each bucket into a third bucket = 1 first cross pup. You now have a bucket containing 50 red and 50 blue marbles, mix them thoroughly, this is your first cross collie greyhound bitch. Make up another bucket the same to represent a first cross collie greyhound dog. From each of these mixed buckets make up another bucket, 50 marbles from each, to represent a pup ,but because the genes are passed at random you