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3. Do all women with a karyotype 45, XO qualify for a diagnosis of Turner
   syndrome?

The diagnosis of Turner syndrome requires the presence of characteristic fea-

tures in a phenotypic female along with karyotypic abnormalities. Hence, pres-

ence of karyotypic abnormalities without phenotypic features does not qualify

for a diagnosis of Turner syndrome.

4. What is SHOX?

Homeobox-containing genes are involved in growth, differentiation, and organ-

ogenesis. SHOX (short-stature homeobox) is one of the homeobox- containing

genes and is located on the pseudoautosomal region (PAR) of short arm of X

and Y chromosomes. SHOX is expressed as early as 6 weeks of intrauterine life

in various tissues including humerus, radius, ulna, wrist, and first and second

pharyngeal arches and is responsible for chondrocyte growth and differentia-

tion in these tissues.

5. What are pseudoautosomal genes?

Pseudoautosomal genes are genes which are present on sex chromosomes but

behave like autosomal genes. Normally, an autosomal gene works in tandem

with its homologous pair on the corresponding chromosome for its expression,

whereas genes in the sex chromosomes need only one copy for their expression

(either in X or Y chromosome). Genes in the pseudoautosomal region (e.g.,

SHOX) require both the copies for their optimal expression and do not undergo

lyonization. These genes are present in the distal ends of X and Y chromo-

somes. The pseudoautosomal region in sex chromosomes is depicted in the

figure given below (Fig. 8.2).

PAR

MSY

NPX

X

Y

PAR - Pseudoautosomal region

MSY - Male Specific-region of Y- chromosome

NPX - Nonpseudoautosomal region of X-chromosome

Fig. 8.2 Pseudoautosomal
region in sex chromosomes

8 Turner Syndrome