Explanation Of Gene Action As Related To Physiological

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-16o <strong>As</strong> another example of what is probably plelotropy, one of you, working with a dominant gene but back-crosslng to the recessive, found that this gene had an otherwise unnoticed effect on growth rate. Because the gene was dominant and the breeding plan was to backcross continually to the recessive, the chickens could be sorted clearly into two groups; those which had the gene but were heterozygous for it and those which didn't have the dominant gene at all. Then it became clear that those which did not have the dominant gene exceeded the others by a full tenth of a pound in weight. Now this difference is big enough to be important practically but small enough that it would not have been noticed had not the other effect of the gene been so conspicuous that it was possible to separate the chickens into a group which had the gene and another group which did not. Cases like this make one wonder how many other effects, too small to be noticed, are exerted by genes which we know by and name for their major effects only. Present knowledge of genetics seems (to me) to indicate that pleiotropy is very common, --- perhaps universal. One reason for thinking that plelotropy is exceedingly common is the complex nature of the machine which is the living animal or plant. In the much simpler machines, such as watches and automobiles, the anatomy and functioning of which we know fairly well, Itls rare indeed that we can make a major change in an important part of that machine without having several different changes results in other aspects of the way that machine functions. It seems to me almost certain that this must be true ofthe vastly more complicated machines which our living animals are. Such reasoning seems to me validand plausible, although it is far less convincing than actual detailed evidence in each case would be. The practical consequences of pleiotropy are all in our favor if we like all of the effects which the gene has. Then if we select for one
-I7- r effect of the gene, we also get some improvement in the other effects as a bonus. Trouble arises when a gene has some effects which we llke and others which we do not. Then if we select for one of these effects we increase the frequency of that gene and improve the flock in that respect but at the same time we cause the flock to deteriorate in those characters which the gene affects unfavorably. In general, the favorable effects of a gene tend to be at least partiall__dominant_a_e_mthe unfavorable ones. So far as that is true and most genes have several effects, then overdomlnance__ ......for the net effects,_°fthe--g-eneis a hlghly_likely consequence. If dominance of the favorable effect were complete in all cases, the heterozygote would show all of the desired qualities of both homozygotes and none of the undesired qualities of either. It is more likely that dominance of the favored effect is not wholly complete in all cases and that one homozygote will be better than the other. Hence, overdominance might be general and yet not often extreme. That is, the heterozygote might be only a few percent better in net merit than the better homozygote, although far better than the other homozygote. Most of the variation in a flock, already improved by years of breeding, will generally be caused by genes which have some good effects and some bad ones. <strong>Gene</strong>s with wholly good effects will have already become almost homozygous all through the population. Any gene whose effects were all inferior to those of its allele will already have been made so scarce that it causes little variation and makes little trouble. In these circumstances, intensifying selection would have the effect of changing the degree of overdominance and hence the equilibrium frequencies of genes with some good effects and some bad effects. The population would change rapidly as the population approached its new equilibrium condition. Little or no,genuine fixation of the new condition would occur; if the new selec-
Page 1 and 2: "7-. J _ Gene Action as Related to
Page 3 and 4: dominance in their primary effects
Page 5 and 6: -ll- ! SUB-DIVISION OR CLASSIFICATI
Page 7 and 8: -13- A glance at this table reveals
Page 9: -15- genes have manyeffects, althou
Page 13 and 14: -19- be continued for several gener
Page 15 and 16: • -21- strains were very large. I
Page 17 and 18: "_3- ability" in general) is import
Page 19 and 20: -25- Wyatt: I would llke to ask Dr.
Page 21 and 22: -27- ! weakness in your stock you w
Page 23 and 24: -29- Hogsett: Dickerson: Dr. Dicker
Page 25 and 26: -31° We used to teach our classes
Page 27: -3_ that the pathogen, being a viru

Explanation Of Gene Action As Related To Physiological

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