4. Genes producing heterosis.
The recessive mutations of a degenerative nature such as dwarf plants, crooked stalk, pale chlorophyll, narrow leaf and others which show heterosis when crossed back to the normal inbred lines in which they originated, have now been extracted from these crosses and compared with the original lines. Both the normal homozygous dominants and the degenerate homozygous recessives come out of these crosses larger than they went in, as shown by preliminary tests. A statistical comparison of these extracted deviations is now being made. The indication is clear that other genes are involved. Whether these originated at the same time the visible alterations appeared, or whether they were previously in these lines and carried along by enforced heterozygosity has not been determined.
Additional evidence for genes having a small but significant effect on heterosis are being obtained from backcrosses using dominant gene markers that have no noticeable effect on yield, such as the normal alleles of glossy seedlings, white cob, white endosperm and colorless silk. After seven generations of backcrossing without selection, the resulting plants both with homozygous recessive and heterozygous dominant gene markers are slightly larger in growth and earlier in flowering than the original lines. This indicates that there are many genes closely linked with these marker genes that have a small effect on heterosis.
Donald F. Jones