The X inactivation ratio as measured by allelic *Pctk1* expression. The *x*-axis lists progeny tested mice with the designated allele described in . Mice were mated with 129S1 mice for progeny testing. The ratio was measured in individual female progeny that are represented by circles. Shading of the circle describes the parent of progeny tested animal: control male (light gray circles); control female (dark gray circles); RX1 and RX2 derived recombinant lines (open circles). RNA isolated from tissues of 2- to 3-week mice was analyzed. The *y*-axis provides the ratio, measured as the fraction of total RNA that is expressed from the 129S1 allele (*Xce*^{a}). To the right of each group of the ratio measurements, the corresponding mean (solid square) and standard deviation (black lines) are provided. Using a *t*-test of two samples assuming unequal variance, the ratios were compared to control animal with the CastXm or 246m X chromosome. Below each allele on the *x*-axis, ★ indicates the ratio differed from CastXm ratio and # indicates the ratio differed from the 246m ratio, according to *P*-value from two-tailed *t*-test (*P*-values are noted below). (A) *Pctk1* expression in progeny of control mice. The mean X inactivation ratio of CastXm did not differ from 199m (*P* = 0.30), 88m (*P* = 0.75), CastXf (*P* = 0.10), and 2175f (*P* = 0.09), but it did differ from the ratio of 246m (*P* < 10^{−16}) and 246f (*P* < 10^{−6}). The ratio of 246m (*P* < 10^{−13}) and 246f (*P* < 10^{−6}) both significantly differed from ratio of all other controls. The ratio of 246m significantly differed from 246f (*P* = 0.033). (B) *Pctk1* in progeny of control *vs.* RX1 mice. The ratio in CastXm progeny significantly differed from the ratio in 109m (*P* < 10^{−7}) and 217m (*P* < 10^{−10}) progeny. The ratio in 246m progeny also significantly differed from ratio in 109m (*P* < 10^{−4}) and 217m (*P* = 0.017) progeny. (C) *Pctk1* in progeny of control *vs.* RX2 mice. The ratio in CastXm and 183m progeny did not differ (*P* = 0.35). The ratio in CastXm significantly differed from the ratio in 137m (*P* < 10^{−8}), 2173m (*P* < 10^{−6}), 3695m (*P* < 10^{−11}), 6443m (*P* < 10^{−3}), 2181m (*P* < 10^{−12}), 5005m (*P* < 10^{−7}), 6570m (*P* < 10^{−4}), 830m (*P* < 10^{−4}), 800m (*P* = 0.011), and 1114m (*P* = 0.012) progeny. The ratio in 246m progeny significantly differed from the ratio in 137m (*P* < 10^{−3}), 2173m (*P* < 10^{−8}), 183m(*P* < 10^{−9}), 3695m (*P* < 10^{−6}), 6443m (*P* < 10^{−7}), 2181m (*P* = 0.013), 5005m (*P* = 0.011), 6570m (*P* = 0.031), 830m (*P* < 10^{−7}), 800m (*P* < 10^{−6}), and 1114m (*P* < 10^{−6}) progeny.

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