Viewing page 78 of 133

This transcription has been completed. Contact us with corrections.

32         The Journal of Heredity

described previously as snowy belly by Schwarz and Schwarz.

This mouse was mated to light-bellied (Aw), gray-bellied (A), and black (aa) females. Data in Table I include the F1,F2 and backcross progeny from the original male and the progeny of later generations.

Results suggest that snowy belly is dominant to Aw and to lower allels in the agouti series. Tests have not been made with Ay, but it probably belongs between Ay and Aw in the series. The column designated light includes those which were not pure white, but generally were lighter than Aw and had a slight grayish underfur. There is the possibility that some of these were Aw. The original male must have been heterozygous, since he produced both Aw and A, as well as snowy-bellied young. Since this is true, the F1 progeny should consist of snowy and Aw in the ratio of 1:1 from homozygous Aw females, and 2 snowy: 1Aw: 1A from heterozygous Aw females. That these ratios were not obtained is due, undoubtedly, to the small numbers involved and to inability to classify the progeny correctly. This also holds true for the F2 and backcross progenies.

Selection of breeding stock has been always for the whitest bellies. This has resulted in gradual purification of the strain, so that in the later generations grayish underfur seldom appears. Also crosses of these later generations with lower alleles of the agouti series give a larger percentage of pure white bellies than were found in the earlier crosses.

The term snowy belly is now proposed as a genetic designation for this allele, with As as its symbol.

Author's Note: Since this report was submitted for publication, black mice with snowy bellies have appeared in the F2 progeny from crosses between As and at. They are not old enough to have been tested to determine their genetic constitution. Probably the simplest explanation is to assume that S is a gene on the same chromosome with agouti, but at a different locus. The snowy-bellied agouti would be AS and other agouti A,with the proper exponent,and s. The snowy-bellied black would represent a cross-over class. It is possible also that a more complex situation may be responsible for the snowy-bellied black.

Literature Cited

1. KEELER, CLYDE E. The Laboratory Mouse. Its Origin. Heredity and Culture. Harvard University Press, Cambridge, Mass. 1931.

2. SCHWARZ, E. and H.K. SCHWARZ. Jour. Mammal. 24: (11): 59-72. 1943.