. The elements of genetics. Genetics. ADJUSTMENT AND BALANCE to be observed, no matter how long the inbreeding may be continued: something like the maximum depression has been reached. At the same time that vigour and fertility are decreasing, so is the variation between the plants of any line in each generation. This decrease also virtually ceases after the first five or six generations. Evidently the increasing depression is due to increasing homo-. Fa F3 F4 Fig. 57.—Characteristic plants to show the relative heights of two inbred lines (P) of maize, of the Fj^ obtained by crossing them, and
. The elements of genetics. Genetics. ADJUSTMENT AND BALANCE to be observed, no matter how long the inbreeding may be continued: something like the maximum depression has been reached. At the same time that vigour and fertility are decreasing, so is the variation between the plants of any line in each generation. This decrease also virtually ceases after the first five or six generations. Evidently the increasing depression is due to increasing homo-. Fa F3 F4 Fig. 57.—Characteristic plants to show the relative heights of two inbred lines (P) of maize, of the Fj^ obtained by crossing them, and of the subsequent generations up to Fj, obtained by continued sclfing. The maximum hybridity, following crossing, is associated with maximum height, and the reduction in hybridity by selfmg is paralleled by a reduction in height. In F7 and Fg, when homozygosis in almost complete, height is reduced once more to the minimum. The increase with crossing is termed heterosis, and the decrease is termed inbreeding depression (based on Jones, in Sinnot and Dunn, 1939). zygosity and to decreasing heterozygosity. The heritable variation will then decrease until full, or nearly full, homozygosity is reached and only non-heritable variation remains. Other things being equal, an average of only i part in 32 of the heterozygosity will remain after five generations of selfing, and only i in 64 after six generations. As we should expect, the vigour of the original parents is restored, even exceeded, by crossing inbred lines derived from different varieties, or, if from tlie same variety, separated early in the inbreeding programme (Fig. 57). Though each line is homozygous, or nearly so, different lines are homozygous for different allelo- morphs of many genes, with consequent heterozygosity in the F/s 236. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfe
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Keywords: ., bookcentury1900, bookpublishernewyorkmacmil, booksubjectgenetics
