Attempts to test the inactive-X theory of dosage compensation in mammals

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Abstract
The inactive-X theory of dosage compensation postulates that in all somatic cells of adult female mammals one or other of the two X chromosomes is genetically inactive. This means that in a female heterozygous for two non-allelic genes acting through the same cells, and carried one on each X chromosome, one or other gene should act in all cells. Conversely, if the two genes are carried on the same X, then both genes should act in some cells and neither gene in the remainder. This point has been tested by breeding experiments with mice, using pairs of genes affecting coat colour and coat texture. In female mice carrying the colour mutant dappled,Modp, on one X and a translocation including the wild-type alleles of pink-eye,p, and albino,c, on the other, eitherModpor the translocation acted in all cells. With the genes tabby,Taand striated,Str, affecting coat texture, inStr + / + Tafemales tabby acted only in the non-Strpatches, while inStrTa/ + + it acted only in theStrones. Thus these experiments confirm that only one of the two X chromosomes is active in the somatic cells of female mammals.