Sister plants of a twin set, with the same chromosome numbers, are in general morphologically identical. In the F
2 of a hybrid between Sears' Nulli-VII (20
II), and Matsumura's a-gigas (1
IV+19
II) a pair of twin plants was obtained. Both had 41 chromosomes, but one of them was awnless and the other tip-awned. In the selfed F
3- and F
4-progenies of both plants appeared many normal plants with 21
II and 20
II+1
I, besides a few dwarfs with the sterile chromosome combination of 20
II. The offspring of the awnless twin were all awnless. The tip-awned twin showed a relatively lower fertility and a lower percentage of heading. Its progeny segregated with regard to awned according to the monohybrid ratio, 1 awnless: 2 tip-awned: 1 awned, independently of chromosome numbers. This shows that the tip-awned twin must have been heterozygous.
The origin of these twins can be explained in three ways:
1) One fertilized egg (awnless homozygote) was divided into two embryos in an early stage and a minute deficiency including the gene for awnless occurred simultaneously.
2) The egg cell and one synergid were both fertilized, one by a sperm nucleus, the other by the vegetative nucleus. One of these female and male nuclei had a minute deficiency including the gene for awnless which occurred at mitosis in the embryosac or the pollen grain, while the others had this gene.
3) The egg cell and one synergid were fertilized by 2 pollen grains, one with the gene for awnless and the other with a minute deficiency involving this gene.
The first hypothesis seems to be more probable than the second and the third.
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