Anomalous Sex Chromosome Systems

Abstract

Modern cytological investigations have firmly established the conclusion that in mammals the male is always heterogametic, or has an XX/XY sex determination system. Many earlier reports of XO males prove to be erroneous. The sizes of the X and the Y may vary considerably, but rarely are the two morphologically indistinguishable. Ohno et al. (1964) concluded that the X chromosome of the eutherian mammals constitutes approximately 5% of the genome, regardless of the diploid number. In species with numerous autosomes (such as the dog), the X may be the longest element. In species with low diploid numbers, such as many bats, the X may be among the shortest elements. Exceptional cases occur when the X chromosome carries a large amount of constitutive heterochromatin, such as the Chinese hamster and the European field vole Microtus agrestis. As far as the functional protion (euchromatin) of the X chromosome is concerned, the genetic content of the X in relation to the entire genome is remarkably constant.