Developing one’s own line of any particular variety, to me is one of the central elements of showing and breeding fancy mice. Apart from applying careful inbreeding and rigorous selection, one of the most important qualities of a line of show mice is line purity. If you have a line of blacks, you don’t want regular blues or chocs to crop up. There are some varieties that lend themselves to running two colours like black/choc tans or agoutis and cinnamons. In brokens, you may have even more colours running in a line, however, you certainly don’t want satin to crop up.
I am facing this challenge now with my pearls, where I am trying to develop a line of typy blues to use as an outcross. If you read my earlier article on pearls, you may remember that the likelihood of a pearl cropping up in a second-generation brother to sister mating is 6.25% if outcrossed to blue, whereas an outcross to black will reduce this to 1.56%.
To get fast results, I used creams and PEW (which, in a lot of cases are PE creams) to improve type and because most creams seem to carry blue. This introduced just about any recessive trait I could NOT wish for, meaning all of the self blues I have so far managed to breed, albeit being typy, carry various recessive traits, such as satin, PE dilution, chocolate, and possibly stone. (Although I am not sure I understand the stone gene fully. There is very little literature on it). Not to mention piebald, which will cause white tail tips and toes.
So, the next step will be to keep all the blues, select for type, and build up numbers. After this, some test breeding will be required to eradicate the recessives. This brings us to a central part in line purity. Test breeding is the only way to reliably get rid of recessive traits.
So, how does one go about it and why is it reliable beyond reasonable doubt?
To test breed, you need one mouse that displays one or all of the recessive traits you want to select against, so that you can breed it to a mouse you want to test for that trait. The method is simple: If the trait is present, an average of 50% of the individuals in the litter will display the trait. So, if you have a litter of 8, and none of those show the recessive trait, you can be pretty sure that the tested parent is not a carrier of that trait. To express it in terms of probabilities, the likelihood of a litter of 8 NOT showing the trait if it is present in the tested individual is 0.39%. This probability will decrease with decreasing litter size, becoming 0.78% for a litter of 5, 1.56% for a litter of 4, and so on. So, even with a litter of only four, you have a pretty slim chance of missing the recessive trait.
If it turns out that the tested mouse is very unlikely to carry the trait, you can breed it to another mouse that you know is pure. Under no circumstances should you breed with any of the offspring, as they will ALL be carriers of the trait.
Unfortunately, with my blues, I am fighting against more than one trait, so a pink-eyed lilac satin would be what I need for test breeding but it is not strictly required. Test breeding can be done for a single trait and once that has gone, the next recessive can be tested against.
All in all, this is a lengthy process, which will throw me back months, if not years, but it is well worth it, as I will be left with a pure line. The advantage cannot be overstated, as I will have many more individuals to pick from instead of constantly discarding unwanted colours, satins and white tail tips.
I did this successfully with my blacks, which were throwing blues when I first got them, my brokens, which had satin running in them and I am in the process of doing it with my agoutis, because I don’t want cinnamons.
In conclusion, I think it would be good practice for any fancier to keep lines pure and not outcross without making sure no unwanted recessives are introduced into a line. Test breeding is the only way to achieve line purity.