Chromosome preparations from 102 ICR albino mice were examined using a modified trypsin Giemsa staining technique. The mice were from four lines selected for maximum rate of
post-weaning gain (28 generations), one line selected for
minimum rate of post-weaning gain (25 generations), and two
unselected control lines. Mitotic metaphase chromosome preparations
were obtained from bone marrow cells of adult male
and female mice. Two similar treatments were utilized in
obtaining the chromosome preparations. The first treatment
consisted of: 50 minute colchicine pretreatment, 30 minute
hypotionic pretreatment, four-glacial acietic acid-methnanol fixation
periods, 15 second trypsin period and 15 minute Giemsa
staining period. The second treatment varied from the first,
basically, in the length of hypotonic pretreatment, and length
and number of fixation periods.
A mean number of 40 telocentric chromosomes with very similar banding patterns was observed in all lines. Relative
chromosome lengths for each chromosome were obtained. The lengths
for the various lines, control, high and low, were compared by
means of a pooled "t" test. Non-significant α2 levels were
obtained for the pairwise comparisons of the lines. Significant
α2 levels were obtained for the effects of the two treatments,
trypsin and no trypsin. Karyotypes for each line were
made with no chromosomal abnormalities detected in any of the
lines. The selection regime followed has produced significant
genetic change in several characteristics of these mice. However,
these changes have apparently not been accompanied by
observable alterations at the chromosomal level.
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