Heather - This topic is kinda controversial.
Some people totally disregard the "marker" theory, others (myself included) think that there has been more than just coincidence involved? I think it fasinating either way.
Also, it seems that when people breed 50 or 66%ers, they keep back the ones that have the markers by the snakes vent.
Randy Remington has been following this (and anything else genetic in nature)for the last few years and always gives the best break-down of this topic, ImHO.
Also, with the cost of het-pieds coming down in price, the marker theory seems to have become less important when you can get a 100% male for $125.