Abstract
The chiasma conditions have been extensively analysed in two contrasting rye genotypes, one showing a normal and regular pattern of chiasma distribution and the other showing a highly abnormal and asymmetrical pattern of chiasma distribution. The analyses show that the pattern of chiasma distribution in the abnormal genotype conforms to the statistical expectations of randomness, and this is interpreted as being due to a breakdown of the processes which govern the distribution of chiasmata in normal rye genotypes. On the basis of these findings it is proposed that two independent and fundamentally different control systems are involved in the maintenance of efficient chiasma conditions in rye. One of these controls simply gives competence for chiasma formation, the other control system is evidently concerned with regulating the distribution of chiasmata. Analysis of two trisomic genotypes reveals that the conclusions relating to large samples of bivalents are also applicable to particular identifiable chromosomes. The results of brceding tests involving the two contrasting genotypes show that the control of chiasma distribution in rye has a complex genetic basis.
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