In Search of Nonrandom X Inactivation: Studies of the Placenta from Newborns Heterozygous for Glucose-6-Phosphate Dehydrogenase
The version of the X-inactivation hypothesis presented by Lyon (1961, 1972) proposed that inactivation was a random event with respect to the parental origin of the X chromosome. Although there have been some suggestions that inactivation may not always occur randomly, the evidence has not been compelling. On the basis of enzyme phenotype in small numbers of blood samples from interspecies hybrids, it has been suggested that the paternally derived X chromosome of marsupials is inactivated preferentially; on the other hand, studies of other tissues have revealed that paternal alleles are expressed (Cooper 1971; Cooper, Johnston, Murtagh, Sharman, VandeBerg, and Poole 1974; Cooper, Johnston, Murtagh and VandeBerg 1975). Observations compatible with preferential expression of the maternal allele in the mule, the interspecies hybrid between female horse and male donkey (Hamerton, Richardson, Gee, Allen, and Short 1971), proved to be the result of a relative selective advantage for cells with an active horse X chromosome (Hook and Brustman 1971). Similarly, the nonrandom pattern of X-inactivation observed for structurally abnormal X chromosomes is now known to result from selection rather than X inactivation (Leisti, Kaback, and Rimoin 1975; Russell and Cacheiro 1978).
KeywordsCord Blood Chorionic Villus Maternal Allele Maternal Cell Interspecies Hybrid
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