[PDF电子书]Principles of Selection by Lasley

Selection for a recessive gene is relatively simple, if penetrance is complete and genes do not vary too much in their expression. Selection unde.
such conditions is merely a matter of keeping those individuais which show the recessive trait.A good example of such selection would be for the horned gene in cattle. To produce all horned cattle, one merely has to obtain horned breeding stock and mate them together. The only time polled individuals would be produced from such a mating is when a mutation from the hormed to the polled gene occurs. This is so infrequent that it is seldom observed in an average-size herd.

10.4.4 Selection Against a Recessive Gene Selection against a recessive gene is the same as selection for a dominant gene. In both instances the homozygous recessive individuals can be identied and discarded. Even when this is done, the recessive gene stil remains in the herd, or population, being possessed by heterozygous dominant individuals. To climinate the recessive gene entirely, the homozygous recesive and heterozygous dominant individuals both must be dliscarded, leaving only the homozygous dominant individuals.

Discarding or cling all homozygous recessive individuals reduces the frequency of the recessive gene but does not eliminate it. However. if the heterozygous dominant individuals in the population are prefered in selection, the frequency of the recessive gene may increase in spite of the fact that all homozygous individuals are eliminated.