. The physical basis of heredity . on the environment. For example, in thenormal vinegar fly the black bands of the abdomen showgreat regularity (Fig. 4), but in a mutant race calledabnormal abdomen (Fig. 5) the bands may be irregu-larly broken up, or even absent. In cultures with abund-ance of fresh food and moisture, all the individuals havevery irregular bands, but as the culture gets old, and the MENDELS FIRST LAW 29 food and moisture become less and less, the bands becomemore and more regular until at last the flies are indistin-guishable from normal flies. If a cross is made betweena fem
. The physical basis of heredity . on the environment. For example, in thenormal vinegar fly the black bands of the abdomen showgreat regularity (Fig. 4), but in a mutant race calledabnormal abdomen (Fig. 5) the bands may be irregu-larly broken up, or even absent. In cultures with abund-ance of fresh food and moisture, all the individuals havevery irregular bands, but as the culture gets old, and the MENDELS FIRST LAW 29 food and moisture become less and less, the bands becomemore and more regular until at last the flies are indistin-guishable from normal flies. If a cross is made betweena female with abnormal bands and a wild male, the off-spring that first hatch under favorable conditions are allvery abnormal. Here abnormal completely dominatesnormal bands. But as the culture dries up, the hybridoffspring become more and more normal, until finally theyare all normal. At this time it might be said that normaldominates abnormal. Both statements are correct, if weadd that in, one environment abnormal banding dominates,. Fig. 5.—Normal and abnormal abdomen of D. melaitogaster. in another environment normal banding dominates. Thegenetic behavior of the pairs of genes is the same hereas in all other cases of Mendelian behavior, but this isrevealed only when the environment is one in which theabnormal gene produces one effect, the normal a differentone. That the gene is not itself affected by the environ-ment can be shown very simply. If a female from theabnormal stock be picked out, at a time when the stockhas only normal bands, and crossed to a wild male, theoffspring will all be as **abnormal as when the motherherself is abnormal, provided the food and moistureconditions are of the right kind. The late hatched normalflies of abnormal stock may be bred from for several 30 PHYSICAL BASIS OF HEREDITY generations, but as soon as a generation hatches underfavorable conditions they are as abnormal as though alltheir ancestors had been of this sort. Thus it is evidentthat
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectheredit, bookyear1919
