I'm curious, Pete, how large a breeding population is typically maintained for such lines? Do you have any info on how frequently such mutants occur (one every how many generations)?

These excerpts explain it better than I can:

Clarence Little arrived at the Bussey Institution for Research in Applied Biology in 1908, and started his work with inbred mice. He purchased a few animals from the farm in Granby, and spent the next few years mating them brother-to-sister, for generation after generation, selecting only the most vigorous pups from each litter for another bout of incest. Eventually, the animals had become so entangled in their family ties as to be virtual clones of one another—a near-perfect line of identical twins, with identical parents and children, too.

In 1929, he decided to create his own campus devoted to the mouse, and convinced a group of wealthy automobile executives to fund a research center at the edge of Acadia National Park in Bar Harbor, Maine. Little named the new laboratory after the head of a car company, Roscoe Jackson, and began applying the Detroit model to the manufacture of research organisms. His "thoroughbred" animals would become the raw material for biological research—as pure and reliable a subject of study, he promised, as the reagents on the shelf of a chemist.

Among Little's early strains were the dark-furred C57BL mice that he'd created in 1921 using the Granby stock, and colonies of these made their way into nearly every lab at Bar Harbor. After 40 generations of brother-sister matings, a feat that might be accomplished in a decade, a given strain of mouse will stabilize, its genes matched up in homozygous pairs. That's when each set of parents, and every son and daughter, becomes a perfect clone of one another, invariant over space and time. From its start, the Black-6 was meant to be as reliable as a rubber stamp.

Little's eugenic utopia [is] realized, 400 generations later, in stacks of plastic shoeboxes—Black-6 pups born [today], but carbon copies, more or less, of their great-great-a-hundred-times-more-great-grandparents. Here in Maine, these mouse twins root around like living fossils, their genomes frozen in the roaring '20s.

Given enough matings, even the best-kept inbred strains will start to shift and split, their base pairs jostled in a natural tide. In the four decades that followed the Jackson Lab's opening, at least 40 different sublines of Black-6 emerged by chance A few years ago, Black-6 accounted for three-quarters of all the mice shipped out; now it makes up five-sixths.

excerpted from an article in about Black-6, by Daniel Engber (2011)

¶

Comment: there have millions of these babies now, but the original strain needn't have involved many mice; perhaps just a pair, whose offspring were sib-crossed, over and over. To maintain diversity, one strives for a large gene pool. That is the exact opposite of what these folks did: they wanted a completely homogenous type, pure as double distilled water. No or practically no variation.

PLB

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