Abstract
1. In Datura stramonium a recessive gene (bd) which is located in the 21·22 chromosome, is responsible for a complete lack of pairing of homologous chromosomes during the first meiotic division in both microsporocytes (pollen-mother-cells) and megasporocytes. The 24 univalents may be scattered along the axis of the bipolar spindle. This results in irregular distribution of chromosomes or even failure to form more than one group (non-reduction).
2. In shape, size and behavior, these univalent chromosomes are similar to the univalents of haploids.
3. The second meiotic division is normal and equational.
4. The tetrad stage is characterized by a varying number of microspores and microcytes.
5. Dyads are the result of non-reduction which varies from zero to eight percent.
6. PMC's showed a limited number of kinds of irregular chromosome distribution; a few occurred more frequently than others.
7. During anaphase I there was a tendency for the chromosomes to separate into two groups, along with a lesser tendency toward non-reduction. There was considerable lagging of chromosomes.
8. Chromosome groups which contain less than a genom produce microcytes only. A formula is given for the probability of a genom in groups of 12 or more chromosomes.
9. Pollen abortion varies from 92 to 98 per cent. Good pollen consists of varying amounts of large grains which have the size characteristic of the pollen of tetraploids and of smaller grains which presumably contain a genom with none to several extra chromosomes.
10. The amount of good pollen found is in fair agreement with the amount expected from application of the formula for probability of a genom to chromosome counts at metaphase II.
11. Trisomic (2n+21·22) plants which are triplex for this gene also show lack of metaphase pairing, 2n+21·22 plants which are duplex for it show normal pairing.
12. Two tetraploids were obtained when bd2 pollen was used on a 4n female.
