The X and Y Chromosomes in Mammalian Development 
Nothing was known as to the number or nature of 
these hypothetical Turner genes. 
We began to focus our attention on this dis- 
order when it was noticed that certain XY females 
exhibit the same anatomic abnormalities as XO 
females. A pivotal finding was that all such XY 
Turner females lacked a portion of the Y chromo- 
some. We postulated that the Y chromosomal de- 
letions in these individuals might encompass not 
only a sex-determining gene or genes, but also a 
nearby Turner gene or genes. 
Pursuing this hunch, we discovered two candi- 
date Turner genes, one on the Y chromosome and 
one on the X. These genes, named RPS4Y and 
RPS4X, appear to encode slightly different forms 
of a protein constituent of the ribosome, a struc- 
ture required for protein synthesis and vital to all 
cells. In embryos lacking a second RPS4 gene 
(i.e., having a single RPS4), the rate at which 
ribosomes are constructed may be slowed, in turn 
reducing the embryo's capacity to synthesize 
other proteins. We are currently testing the 
highly speculative hypothesis that such a reduc- 
tion in protein synthetic capacity is the cause of 
at least some of the physical features of Turner 
syndrome. 
An interesting analogy can be found in the fruit 
fly Drosophila melanogaster. There, deficien- 
cies in ribosomal protein genes are associated 
with a particular "syndrome" called the Minute 
(pronounced mi-NUTE) phenotype, which 
includes reduced body size, diminished via- 
bility and fertility, and specific anatomic 
abnormalities. 
The very existence of related but nonidentical 
ribosomal protein genes on the X and Y chromo- 
somes suggests that the ribosomes of human 
males may differ slightly from those of females. It 
will be a surprise to many if the differences be- 
tween the sexes extend all the way down to the 
most fundamental and vital of intracellular 
machines! 
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