Evolutionary genetics / edited by D.L. Jameson.

Date:
[1977], ©1977
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Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Evolutionary genetics / edited by D.L. Jameson. Source: Wellcome Collection.

    169/360 (page 149)
    No. 518] VARIATION 77 increased gametic variability results. Johannsen^'^ has shown that there is no such increase in fluctuation when close-pollinated plants are crossed. I have crossed several distinct varieties of maize where the modal num¬ ber of rows of each parent was twelve, and in every instance the progeny had the same mode and about the same variability. Finally, a possibility of gametic coupling should be considered. Our common races of flint maize all have a low number of rows, usually eight but sometimes twelve ; dent races have various modes running from twelve to twenty-four rows. AVhen crosses between the two sub¬ species are made, the tendency is to separate in the same manner. Attention is not called to these obscuring factors with the idea that they are universally applicable in the study of supposed continuous variation. But there are similar conditions always present that make analysis of these variations difficult, and the facts given here should serve to prevent premature decision that they do not show segregation in their inheritance. Table VI shows the results from several crosses be¬ tween maize races with different modal values for number of rows. Several interesting points are noticeable. The modal number is always divisible by four. This is also the case with some twentj'^-five other races that I have examined but which are not shown in the table. I suspect that through the presence of pure units zygotes having a multiple of four rows are formed, while heterozygous units cause the dropping of two rows. The eight-rowed races are pure for that character, the twelve-rowed races vary but little, but the races having a higher number of rows are exceedingly variable. '\¥lien twelve-rowed races are crossed with those having eight rows, the resulting generation always—or nearly Johannsen, W., Does Hybridization Increase Fluctuating Variabil¬ ity? liept. Third Inter. Con. on Gendics, 98-113, Lonclon, Spottiswoode, 1907. 149