X-autosome translocation, manifesting female carrier
46,X,t(X;4)(p21;p16)
Also known as: t(X;4), X;autosome translocation, manifesting carrier, skewed X inactivation, Duchenne in a girl
46,X,t(X;4)(p21;p16) is balanced, yet the carrier is affected. In a balanced X-autosome carrier the normal X is the one silenced, because silencing the derivative would spread inactivation into the attached chromosome 4 material. That leaves the der(X) as her only active X, so the dystrophin gene broken at the Xp21 breakpoint has no working copy behind it, and she presents with a muscular dystrophy that is otherwise almost exclusive to boys. Translocations like this one are how several X-linked genes were first mapped.
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What the notation means
46- total chromosome count (the normal human number)
X- sex chromosomes: one X, listed alone because the other X is named in the rearrangement below. This is not monosomy X
t(X;4)(p21;p16)- a reciprocal TRANSLOCATION: chromosomes X and 4 break (at Xp21 and 4p16) and swap the pieces beyond those breaks, giving two derivative chromosomes der(X) and der(4). Expected X inactivation is skewed: the normal X is silenced, and both derivatives stay active. Silencing the der(X), which carries the center, would spread inactivation into the attached autosomal segment and leave it functionally monosomic, so those cells are selected against. Because the intact X is the silenced one, a gene disrupted at the X breakpoint is unmasked, and a balanced female carrier can still manifest an X-linked recessive disorder
Clinical notes
Turner syndrome (45,X and variants)
Loss of all or part of the second sex chromosome. 45,X (monosomy X) is classic; variants include an isochromosome i(Xq), a ring r(X), an idic(Y), and 45,X mosaicism (e.g. 45,X/46,XX). Short stature, ovarian dysgenesis/streak gonads, webbed neck, coarctation/bicuspid aortic valve, lymphedema.