Pearls are considered a difficult variety and, despite their striking beauty, are not as popular as other AOVs, only ranking in 12th position this year (a statistical outlier because of a BIS won by myself). Last season, they never even appeared in the Variety Popularity chart.

Part of the reason for them being a niche variety is the Pearls’ reputation of being reluctant breeders. I have heard several theories, among which are a tendency to develop haemophilia, and being generally poor mothers. I cannot confirm any of these effects and my Pearls are breeding normally, with litters of 4 to 12 young. I don’t cull until I can judge the coat by blowing into it, and even if larger litters are causing smaller sized individuals, my does are raising litters of 12 frequently.

The other reason, and this is probably more important, is the Pearls’ puzzling genetics and reluctance to return to anything that looks remotely like a Pearl, after any outcross. And it is an outcross that any breeder will think about first, when looking at a Pearl mouse because of their scrawny appearance, thin tails and small ears. Despite lacking in most typical characteristics of a show mouse, they tend to have big, bold eyes and a most delicate ticking, which is much finer than any other ticked variety, giving them a blueish grey powdered appearance.

I was given some lovely initial stock by Eric Jukes, and, naturally, to make them my own line, and perhaps improve type, I planned an outcross. Obviously, I kept the original line going unadulterated, as any responsible breeder would.

My choice of an outcross was three big typy Black does, from my line of Blacks that I keep to improve my show Blacks. I never show this line as it would not win its class, but I thought it might make a good combination.

As expected, the first litters I had from these does, sired by a Pearl buck, were all black. As my next step, I thought it might be advisable to carry on with a brother x sister mating, as that would preserve the already improved type somewhat and, statistically, would yield 25% Pearls.

Boy, was I wrong.

What I got was 75% Black (as expected) and 25% Blue. This had me puzzled, particularly, as I thought my knowledge of genetics was good and I knew what to expect. I had a hunch what might be happening, but I had to do a bit of reading to get a clearer picture.

The first false assumption was clearly, that Pearls were based on a black genotype. The result of the outcross showed that these were blue based, not black. But why did none of the 7 blues I had from the F1 outcross show any pearl? One of the 7 offspring looked like what you might call a “Silver Blue”. Quite attractive actually, but obviously not a Pearl. One looked like a lighter, somewhat diluted Blue.

The Finnish Mouse Club’s website is quite good on genetics and despite some questionable information (particularly with regard to self Cream), it helped me understand what the problem might be and a theory formed. The Finnish fancy claim that Pearls are made up of not one gene (let’s call it ‘pe’), but two, that is pearl (pe) and silver (si). The combination of pe and si makes pearls. Add to that a blue base, and you can very easily work out the statistics of an outcross to an individual whose genotype has none of these.

If a pearl mouse is

a/a B/B d/d si/si pe/pe,

and you breed it to a black genotype, which is

a/a B/B D/D Si/Si Pe/Pe, your F1 will be

a/a B/B D/d Si/si Pe/pe.

So, you need to then bring back all three recessive traits. Recombining brother and sister will give you a statistical probability of only 1.56%. And that’s the problem.

So, the second false assumptions was, that Pearl is made up of a single gene, which is very clearly not the case. You need a ‘double dose’ of both pe (‘pearl’) and si (‘silver’. This is the gene that makes Silver Greys). Plus, you need blue.

It gets even better. The footnote by Roger Hutchings under Ruth’s article “The Puzzling Pearl” mentions lilacs cropping up in outcrosses. This is consistent with the Finnish website’s statement of the lightest pearls carrying one dose of Chocolate.

So, a heterozygous (‘single dose’) recessive trait that expresses itself in the presence of other, homozygous recessive traits, like pe and si?

Yes, that seems to be the case. And it is again, somewhat surprising. A black mouse that carries chocolate is still black and you cannot see a difference between it and a pure black mouse. However, in the presence of pe/pe si/si, a single dose of chocolate seems to express itself (by making pearls lighter).

I am sure other Pearl breeders can confirm that in some litters of pearls you get brownish (like café latte) youngsters.

If the above theory is right, these seem indeed to be lilac based pearls, and again, this would be consistent with both Roger’s findings and the Finnish website (which states that the lightest individuals are a/a B/b d/d si/si pe/pe), because a heterozygous trait will eventually (i.e. if recombined with another, genetically identical individual) produce a double dose and yield 25% of a litter looking brownish.

So, to recap, if you mate two very light individuals, according to the Finns, they are likely not

a/a B/B d/d si/si pe/pe,

but

a/a B/b d/d si/si pe/pe.

B being ‘chocolate’ dilution, which is only expressed if homozygous (a ‘double dose’).

However, in Pearls, it seems to produce a lighter shade of Pearl if heterozygous (‘single dose’). Now, if you combine two very light individuals, your chances are 25%, to get:

a/a b/b d/d si/si pe/pe,

which would be lilac Pearls, and those seem to appear brownish in the nest.

So, knowledge is power, but I still need a plan for my Pearls. How do I make them more typy, whilst retaining the pearly coat?

I think a good strategy will be to first produce a line of typy Blues. They don’t need to be great colour, or showable, just as typy as possible. I will use these as an outcross to a Pearl. I will receive 100% Blues in the first litter.

Then, I can see two paths:

o Repeat a few times until I have typy Pearls.

A judge does not care about genetics and will judge what is put in front of him/her. Black based Pearls offer a unique opportunity (for a very dedicated breeder) to make some outstanding Chinchillas, with none of the problems associated with this variety (brownish tinge, dark spine). Chinchillating Pearls might be a good idea anyway, to get rid of the tendency to develop yellow vents, and when combined with Black and Tan (instead of self Blue), some very finely ticked Chinchillas might be produced. This will be a worthwhile project and I hope someone tries. If you have ever seen the coat texture of a real Chinchilla, you might agree with me that Pearls are much more like Chinchillas than our chinchillated Agoutis.

Make typy black Foxes first, then use the strategy as outlined above. Good luck!