When you breed merle to merle, there is a 25% chance PER PUPPY of producing a double merle and those puppies are quite often blind and deaf. It’s not a 25% chance per litter. One in four of the puppies you produce is expected to be homoygous merle.
So I’ve produced a little table to look at this issue in a different light. The more puppies you have, the more likely you are to produce one of these puppies, but it is possible that you won’t produce one. So I’ve calculated the odds of producing no doubles and the odds of producing at least one double, the more puppies you produce.
|Litter Size||Odds of No Doubles||Odds of at Least One Double|
So you can see, even if you have a breed that has small litters, say 4 on average, two-thirds of those litters will include a double-merle. This, of course, assumes that all double merles survive to birth, which is not the case. Some people might find this comforting, as if intentionally producing puppies that will be aborted is a saving grace to this breeding ethic, others might not want to burden their bitches or their conscience with these unnecessary complications. It has not been well documented what percent of MM fetuses fail, so consider these percents to be tracking potential fetuses and an average upper limit on the number of puppies that survive.
You can see that when we correctly apply these odds to puppies instead of litters, in a litter of 5 puppies, we have only a 25% chance of NOT producing a double. I’ve read people claim that 25% sounds low, so it’s not something to worry about. But that’s a horrible misunderstanding of what the odds are really saying. In a normal litter of 5-6, we’re looking at an 80% chance of one of these puppies. That’s not a small number by any means.
* * *
If this post made you think and you'd like to read more like it, consider a donation to my 4 Border Collies' Treat and Toy Fund. They'll be glad you did. You can subscribe to the feed or enter your e-mail in the field on the right to receive notice of new content. You can also like BorderWars on Facebook for more frequent musings and curiosities.
* * *