Summary
1. The determination of the relationship of crossing-over to chiasmata and of chiasmata to segregation has made it possible to define the conditions of variation of pairing chromosomes in complex-heterozygotes and sex-heterozygotes and to analyse their behaviour in genetic terms. The object of the present studies is, first, to test the predictions made with regard to sex-chromosomes deducing their genetic structure from cytological observations and, secondly, to note those respects in which the behaviour of the sex-chromosomes throws light on the problems of chromosome mechanics.
2. The sex-chromosomes of the Norway rat each consist of a pairing segment and a differential non-pairing segment. The pairing segment includes the spindle attachment and chiasmata may be formed on one or both sides of it so that the first division is either reductional or equational for the differential segments (Text-fig. 17).
3. The differential segments will have complete sex-linkage, the pairing segments will have partial sex-linkage diminishing in proportion to the crossing-over distance from the differential segments.
4. The shape, movements, and staining capacity of the sex-chromosomes in the rat and elsewhere agree in suggesting that they have a lower surface charge than the autosomes, and this is held to be responsible for the special mechanism ofX-chromosome segregation in organisms lacking theY.
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Koller, P.C., Darlington, C.D. The genetical and mechanical properties of the sex-chromosomes. Journ. of Genetics 29, 159–173 (1934). https://doi.org/10.1007/BF02982193
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DOI: https://doi.org/10.1007/BF02982193