Mismark Case Study: Boston Terrier

Boston terriers come in three acceptable colors: black, brindle, and seal, all with white.

This dog would have too little white

The Boston terrier is a breed that came from crossing a number of different breeds together. In its ancestry are such breeds as the French bulldog and bull and terriers types, crosses between bulldogs and terriers that are the ancestors of such modern breeds as the bull terrier and pit bull. The reason why modern Bostons are so small is that they have since been bred down in size from the original stock. This ancestry also explains all of the colors that pop up on occasion in the breed. What are these mismarks?

• Too little white
• Lacking the required white chest, muzzle and blaze between its eyes
• Too much white
• Excessive facial white or "splash white"
• Any base color other than black, brindle, or seal 
• Liver, blue, lilac, fawn, gold, or tan point
• Not enough skin pigment
• The nose must be completely black
• Blue eyes
• Even a touch of blue disqualifies

Too much white and a blue eye

To begin, let's look at variations in the amount of white on the dog. Facial white is forever a conundrum in the canine world. In every breed that had facial white, there is a great amount of variation. This variation is controlled by as-of-yet unknown modifiers and is likely polygenic and inherited mostly independent of the Spotting locus that controls body white. This is the issue with having a rather strict requirement for facial white: there is no real guarantee that your breed will always have the same exact amount. This is why in breeds like the Boston terrier, too much white on the face happens so often. Too little white also happens, but in the Boston this appears to be a more unusual occurrence.

A splash rescue

The "splash" white is completely different. Piebald and extreme white can both be sources of the splash pattern, and both are recessive alleles on the Spotting locus. Most Bostons express the Irish white phenotype, which can vary from the minimal markings required by the breed standard up to what's often called collared Irish, which has white extending all the way around the neck and prominent white on the legs. Irish white is dominant to both piebald and extreme white, and as such it's very possible for dogs who conform to the standard to carry these alleles. Though not always the case, it's possible that some of the heavily marked collared Irish dogs have as much white as they do thanks to the recessives they carry. Extreme white especially can cause a dog to have more white than it would otherwise have. Minimal piebald can also mimic collared Irish markings, and so it's possible the occasional minimal piebald may slip into the show ring.

As for the dogs with off-standard base colors, there are really quite a number of possibilities. Here's the full list of possible colors, what they're called in the breed and the genes that cause them. Wherever I say black, this is basically interchangeable with brindle and seal. All three colors are caused by the K-black locus, with brindle (kbr) being recessive to black (K) or seal (K, bad black, possibly recessive to regular black).

Masked fawn puppy

• Sable/fawn 
• Recessive to black on the K-black locus, allows the Agouti locus to show through: Ay- kk results in sable/fawn
• Dog may or may not have a black mask and may be liver, blue, or lilac instead of black
• Tricolor/tan points
• Also on the Agouti locus: atat kk results in tan point; may be liver, blue, or lilac instead of black

Liver pigment

• Non-black pigment that would otherwise be black
• Liver/brown, recessive to black on the Brown locus: bb results in liver pigment
• Blue/gray, recessive to black on the Dilution locus: dd results in blue pigment
• Lilac/champagne, combination of the blue and liver genes: bb dd results in lilac

Recessive red

• Gold/honey/cream/blonde
• Caused by recessive red on the Extension locus: ee results in recessive red/clear red
• This gene hides all black body pigment, so any of the above, including blue/liver/fawn, but skin and eyes can still be effected by the liver and blue genes

Liver pigment and too much white

The major thing to note about all of the above is that they're all recessive. Due to this fact, you won't know your dog is a carrier until it throws a mismark puppy. Even if you remove those mismark puppies from your breeding program, its siblings have a two-in-three chance of being carriers. Why is that? It comes down to basic genetics: mismark puppies can only show up in litters bred from carrier parents or those expressing the gene. If breeding from two carriers, the expected ratio in the puppies is 1:2:1. That is, about one-forth will be non-carriers, half of the litter will be carriers, and the last forth will express the recessive mismark. If you remove the mismarks, you're left with a ratio of 1:2, or one-third non-carriers, two-thirds carriers. This means there's a rather high chance of the gene persisting, particularly if a breeder then chooses to linebreed or do other forms of inbreeding. Inbreeding only increases the likelihood that the recessive mismark will appear. This is why so many breeds have persistent mismarks.

Two dogs, three blue eyes

The two mismarks that I haven't yet mentioned are blue eyes and noses lacking in pigment ("Dudley nose"). Both are disqualifications according to the standard, and both are natural consequences of breeding for dogs with lots of white. White around the nose makes it quite possible that a dog will end up with a pink spot on its nose, or even a completely pink nose. So, expecting all dogs to have a fully pigmented nose when you are breeding for prominent white markings is a bit silly. It's basically the same with blue eyes. Though they can be independently inherited, the more white a dog has, the more likely it is to have one or more blue eyes. If the white touches a corner of the eyes, this also increases the chances. This is why so many dogs with a lot of white on their face have one or more blue eyes. Since the Boston standard desires very prominent white, blue eyes shouldn't be a surprise.

Liver pigment

As with every dog, color should be the last thing a breeder should worry about. There are so many more important things that a breeder should be striving to produce, like dogs with even temperaments and ones that are healthy. By disallowing certain colors, especially ones that were perfectly acceptable at one point in the breed's history (like splash in Bostons), there's a very high likelihood that perfectly good, healthy dogs are being dropped from the breeding population for extremely superficial reasons. This does nothing but narrow the gene pool and damage the breed as a whole. However, this doesn't mean that one should purposefully breed for mismarks. On the contrary, breeding for recessive is always bad because it usually means inbreeding, and potentially very heavy inbreeding at that. Inbreeding in any form and by any name (such as linebreeding) is very bad as it narrows gene pools and makes it more likely that deleterious recessives pop up. This is why so many purebred dogs have inherited health problems. Color should not matter. Purebred dogs have enough problems.

Apparently, there are some people out there selling merle Bostons. This color has been introduced through mixing with another breed. Since merle has so many negatives, there is absolutely no good reason to out-cross to purposefully introduce merle into a breed. 

Images are from Wikimedia Commons or Flickr.com under Creative Commons licenses: one, two, three, five, six, seven, eight, nine, ten. Image four is copyright to Rescue Furdaddy on Flickr.com.