The late replication pattern of the pigeon chromosomes was studied by means of the technique of terminal uptake of tritiated thymidine and autoradiography. The W-chromosome in the female completed replication after the other macrochromosomes. The Z-chromosome in the female and both Z-chromosomes in the male terminated DNA synthesis at the same time as the remaining macrochromosomes. The microchromosomes were later replicating than the macrochromosomes. It is inferred that late replication, at least in the case of the W-chromosome, signifies genetic inactivation in somatic cells. The absence of asynchronous replication of a Z-chromosome in the homogametic sex suggests that sex determination in the pigeon might depend on a gene dosage mechanism similar to that responsible for the ‘autosexing’ phenomenon.
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Galton et al. (1966) studied this question.