A (hard) lesson in colour genetics

So…last weekend I’m out with the dogs and Dazzle’s coat looks an absolute disgrace, all this rain we’ve been having isn’t great for a dog with a huge coat that acts like a sponge! So I decided a good bath and groom was in order when we got home. She’s always like a little barrel, like her mother before her she is prone to weight gain, despite me cutting her food back, which is not helped by her great thick coat either. Well, I got her in the bath and soaked her and…what’s this?? OMG…Dazzle is in whelp...panic!! I looked back at when she was in season…around the time my grandad died, so the week she must have been mated was the week of the funeral when we had a whole load of family staying in the house and everything was a bit of a blur!! I just had no idea who the sire was, since I had no inkling that she’d been mated at all it really could have been anyone and no matter which way I looked at it, none of the options were particularly favourable but Spark was probably ‘worst case scenario’ being her son and a merle too :o(

Now normally when I have a bitch due to whelp it’s been a long awaited event that results in lots of excited anticipation. This last week I have to say has been one of nervous dread, waiting to see what arrived. So…she whelped Monday night and when the first pup was born…a chocolate merle…I briefly thought they might be cocker crosses (sired by Travis) but they were the size and shape of normal collie pups with neat little triangle ears, another choc merle and then the third one…an almost all white choc merle…a double merle…and I knew for sure then that the puppies were Mac’s, he is the only dog I have here that carries chocolate and he is obviously also a merle; probably second worst case scenario :o(

We finished up with 3 ‘normal’ (heterozygous Mm) choc merle boys and a ‘normal’ blue merle boy as well as a choc/white boy along with the ‘double’ (homozygous MM) choc merle. I spoke with my vet and we agreed that the kindest option was to have the white pup humanely put to sleep. The risk of deafness, blindness, lack of eyes etc was just too great to justify rearing this poor baby and there was no hesitation from my vet that this was the most sensible course of action, as sad as it seems now I’m sure we made the right decision but that doesn't actually make it any the less tough.

the unexpected arrivals...


I am totally unsure how I feel about this litter now…they certainly weren’t planned or expected and their arrival is tinged by sadness, guilt and regret but the five remaining pups are beautiful so I can’t help but smile when I look at them and I’m also quietly excited at the prospect of Mac babies; something I had resigned myself would never happen again so this is an unexpected surprise, it’s all so very bitter/sweet and I feel guilty again about being a little bit excited. The irony is that when I took all my dogs for gonioscopy testing last year, I had planned to mate Dottie to Mac so I could keep a puppy. When Mac failed the gonio test my hopes were dashed and I was desperately disappointed. I asked Stuart (opthamologist) whether it would be possible to use him again at all and he pulled his face but suggested that if I really MUST use him again it should be to a bitch with completely perfect eye structure and drainage angles, such as Dazzle (who he claimed has the best drainage angles he’s ever seen) which I obviously pointed out was never going to happen since they were both merles! I did contemplate having Mac castrated at the time but decided against it because I didn’t want to upset the hierarchy in my pack; all my boys get along so well with Mac being the ‘top dog’ amongst them and because he’s not really one for bothering when we have girls in season I’ve never found him being entire a problem (huh!).

So, here I am, well read and experienced in breeding and colour genetics and a staunch advocate against merle to merle matings, in the situation I never imagined I would find myself in, it just goes to show…you never quite know what’s around the corner in this game!

So I’m just going to share a few facts about merle to merle matings and double merles as I'm sure some people reading this will have questions.

Merle is a dominant gene and as such a dog only needs one copy of the gene to exhibit the coat pattern. So the vast majority of merles are heterozygous for merle Mm (that means they have two different genes on that allele…one for merle M and one not for merle m). When you mate a merle to a solid coloured dog the merle should (statictically) pass on the merle gene to 50% of the offspring and the non merle gene to the other 50%, so half the litter will also be heterozygous merle (Mm) and the other half will be homozygous (two copies of the same gene) for non merle (mm).

Problems arise when two merles are mated together because you now have the probability that 25% of the pups will inherit the merle gene from both parents so statistically 25% of the litter will be homozygous non merle (mm), 50% will be heterozygous ‘normal’ merle (Mm) and 25% will be homozygous ‘double’ merle (MM). The effect of the double merle gene is to drastically dilute the colour pigmentation so you end up with a puppy that is almost entirely white and this is where the problems lie. The colour pigmentation cells are involved in the development of optical and auditory systems (sight and hearing) so the lack of pigmentation can result in a whole host of deafness and eyesight issues.

What this means is that the fact that these double merle puppies have issues is secondary to the fact they are mostly white (if that makes sense)…the genes themselves don’t cause the problems, only the fact that the genes result in a lack of pigment so, while the double merle pups themselves will likely have health issues, the rest of the pups in the litter are perfectly normal healthy puppies and should a double merle be retained and bred from (whether it be deaf and/or blind itself) it would not pass any of these issues onto it’s offspring. It was (and probably still is in some places) common practice for ‘colour breeders’ (those breeding solely for colour and nothing else) to deliberately do a merle to merle mating and retain a double merle pup as a stud dog. Being homozygous MM means that a double merle, when mated to a solid coloured bitch, can ONLY pass on the merle gene and therefore can only produce merle puppies. It is also much more commonplace in breeds where merle is more prevelant, such as Australian Shepherds and shelties.

Here are a few interesting links regarding double merles but let me just clarify that I WOULD NOT EVER recommend a deliberate merle to merle mating; intentionally producing defective puppies is not my idea of good breeding practice and having to make the heart breaking decision to have a newborn puppy pts is not something I would wish on any other caring and conscientious breeder.

LethalWhites.com

Double Merle Genetics

Deafness in White Aussies