ÐÓ°ÉÔ­´´

We have long known that most men have XY sex chromosomes, and most women
are XX. But about one in a thousand men have three sex chromosomes, XXY,
while a smaller proportion of women have only a single X chromosome. This
suggests that the presence of a Y chromosome is associated with male sex
and its absence with female sex. But how does the Y chromosome function
to turn an embryo into a male? Many researchers now believe that the Y chromosome
carries a single dominant male-determining gene, whose presence or absence
alone determines the sex of an individual. The available evidence, however,
makes it unlikely that this concept is correct.

The steps that decide whether an embryo becomes a male or a female are
well understood. In the earliest stages, the embryo is flexible, and can
develop down either of two pathways. A five-week-old human embryo possesses
a pair of glands – that is, either testes or ovaries.

These gonadal ridges contain germ cells, which can become either male
sperm cells or female egg cells, and other cells that will form the supportive
tissue of either a testis or an ovary and secrete the appropriate sex hormones
(see ‘The race to be male’, New ÐÓ°ÉÔ­´´, 22 October 1988).

In the 1940s, Alfred Jost in Paris showed that the hormones secreted
by a fetus’s testis have two functions: first, they demolish the rudiments
of the femal reproductive tract, and secondly, they cause the development
of the male reproductive tract, including that of the external genitalia.
If the fetus has no testes, the male duct regresses, while the female duct
develops and the external genitalia become female. The female duct can develop
even if the fetus lacks ovaries and this is perhaps hardly surprising given
that the fetus will be exposed to the mother’s ‘female’ hormones. It is
significant that testes develop earlier than ovaries.

So the decision ‘male’ or ‘female’ depends on whether the fetus has
testes, which is why the hypothetical male-determining gene on the Y chromosome
has been called ‘testis-determining’ factor, or TDF. Much effort, skill
and ingenuity has gone into attempts to identify this gene.

In the past three years, two candidates have emerged. In 1987, David
Page and his colleagues at the Whitehead Institute announced that they had
isolated a ‘zinc-finger’ gene, named ZFY, located near the tip of the Y
chromosome, and the authors proposed that this was the long-sought-after
testis-determining gene. Recently, however, ZFY has fallen out of favour
– it turned out to be a non-sex chromosome in marsupials – and earlier this
year a new candidate gene was announced by Peter Goodfellow’s group at the
Imperial Cancer Research Fund Laboratories in London. The new gene is located
even nearer the tip of the Y chromosome, and its name, SRY, signifies the
‘sex-determining region’ of the Y chromosome. But the abnormalities of sexual
development, which formed the bases for the search for SRY, argue against
the idea that ‘maleness’ is determined by a single gene sitting on the Y
chromosome.

The rule that male development requires the presence of a Y chromosome
has exceptions. About 1 in 20,000 men have two X chromosomes and no Y. The
existence of XX males, once a total enigma, has been considerably clarified
by recent advances in molecular genetics. Their sex chromosomes may superficially
appear to be the same as those of women, but experiments have shown that
most of such men carry DNA sequences normally present on the short arm of
the Y chromosome (which they do not possess) on one of their X chromosomes.
The presence of these DNA sequences could explain why testes develop. Yet
about a third of these men appear to lack these Y-derived DNA sequences,
and so their origin remains unexplained.

There are also clear physical differences between XX men with and without
Y-derived DNA sequences. With the sequences, masculinisation tends to be
more or less complete. But men without them have abnormal, or ‘ambiguous’,
genitals. Typically these are underdeveloped, with the urethra failing to
reach the end of the penis (‘hypospadias’), and testes are often undescended
– the result of the fetal testes failing to produce sufficient levels of
testosterone.

Men with two X chromosomes are sterile, and the same applies to the
males of other mammals. In adult life, XX men have low levels of the male
hormone testosterone, and usually sparse beard growth. But this cannot be
entirely blamed on the missing Y-chromosomal DNA, as low levels of testosterone
are also a feature of men with Klinefelter’s syndrome – these men have an
intact Y chromosome in addition to two X chromosomes. They have very few
sperm-forming cells in infancy and not at all by the time puberty arrives.
Although men with Klinefelter’s syndrome are usually well masculinised,
careful studies show that the incidence of undescended testes is higher
than in the XY population.

The deleterious effect of extra X chromosomes on male development becomes
more apparent as the number of X chromosomes increases. Men with four X
chromosomes and a Y have severely underdeveloped genitalia, with undescended
testes the norm. So the current dogma of the dominance of the Y chromosome
is not entirely supported by the available evidence.

Sexual ambiguity is a particular feature of true hermaphroditism, where
one individual has both testicular and ovarian tissues. Hermaphroditism
is the norm for some invertebrate animals, such as earthworms, as well as
in most flowering plants (which, of course, have stamens and carpels rather
than testes and ovaries); but in humans and other mammals, it reflects a
path of development, which, in a world whose inhabitants are classified
into males and females, is regarded as abnormal. Yet the observed facts
of hermaphroditism are highly relevant to sexual development, and any acceptable
hypothesis of the genetic basis of sex must account for them, rather than
simply ignore them.

True hermaphrodites have both ovarian and testicular tissue, and ambiguous
genitals, which are intermediate between the male and female state. Most
hermaphrodites, who in the past were usually raised as males, nonetheless
have XX chromosomes like normal females and apparently no Y-derived DNA
sequences. Another puzzle is that the two types of gonadal tissues are not
distributed equally on the two sides of the body: ovaries are more frequently
found on the left, and testes on the right. Hermaphrodites with XY chromosomes
are also known, raising the question as to why ovarian tissue should form
in spite of the presence of a Y chromosome.

The recent discovery of the SRY gene stems from the study of four patients
with XX chromosomes: three had testes but evidence of incomplete masculinisation,
while one had ovotestes and was, therefore, a true hermaphrodite. These
people were referred by different authors, as ‘four XX males’, ‘three men
and one intersex’, ‘four sex-reversed individuals’. All four had Y-derived
DNA sequences, including the SRY gene, but they lacked the ZFY gene. The
findings are compatible with the hypothesis that SRY plays an important
role in the development of testes, but in view of the incomplete masculinisation
of the patients, the gene cannot by itself direct the development of normal
testes. Furthermore, the four patients belonged to a group of 14, in 10
of whom no Y-derived DNA sequences were found, which suggests that testes
can develop without SRY.

True hermaphroditism in humans is, in fact, strong evidence against
the view that the sex of an individual is determined by the presence or
absence of a single ‘testis-determining’ gene. The manifestation of this
condition does not resemble that of a single gene defect, such as sickle
cell anaemia, but has the characteristics of the results of several interacting
genes with several environmental factors – as, interestingly, in cleft lip,
which like hermaphroditism exhibits bilateral asymmetry, the cleft occuring
more often on the left than on the right side.

The mechanism of what is known as ‘multifactorial inheritance’ has been
baffling geneticists. In spite of its clinical importance, this is not an
area in which much progress has been achieved. But the manifestations of
true hermaphroditism offer some important clues. The bilateral asymmetry
of testes and ovaries actually mirrors the growth of the gonads in human
fetuses. In both XX and XY fetuses, the gonad on the right tends to be a
little more advanced than the left-hand gonad. Could this mean that accelerated
development leads to testes, while a slower rate of development results
in ovaries? This view is supported by some curious differences in geographical
distribution of hermaphroditism and some other reproductive characteristics.

XX hermaphrodites are common among African blacks, but in Japan XY hermaphrodites
seem to be more frequent. Another difference between these populations is
that nonidentical twins (from two eggs) are commonly born to African women
but very rarely to their Oriental counterparts. It is also known that Oriental
men have relatively small testes.

Although these facts seem at first unrelated, we know that in animals
ovulation rate is correlated to the size of male’s testes. Small testes
in adults suggest that the gonads grew slowly in the fetus, something that
could occasionally result in an XY gonad being left behind in the race to
become masculinised, and developing some ovarian tissue instead. If this
happened, the fetus could become an XY hermaphrodite. By contrast, when
gonads develop fast, as is likely among Africans, an occasional XX gonad
may develop testicular tissue, so giving rise to an XX hermaphrodite.

The rate at which gonads develop is under genetic control, and in mice
there are several genes on different chromosomes known to affect this. These
genes are likely to play a supporting role in determining sex. In the normal
course of development, however, genes on the Y chromosome play the lead
role. How, then, do they manage to outperform all other genetic and environmental
factors?

The newly discovered SRY gene is thought to encode a protein with a
potential DNA-binding domain. A similar gene and gene product are present
in yeast. As testes are conspiciously lacking in yeast, any gene that is
common to humans and yeast cannot have a direct testis-determining function.
Yeast are unicellular organisms, and so their genes must act at the level
of a single cell. So, an involvement in cell division is by no means an
unlikely function for SRY. The same applies to ZFY, which produces another
DNA-binding protein belonging to the zinc-finger family. Even the so-called
H-Y antigen, which was once regarded as a sex determiner, is now thought
to be a growth regulator.

Time is not on the side of the developing male, for any delay in the
growth of the fetus’s gonad increases the risk of its ‘missing the boat’
and becoming an ovary. To deal with this, the Y chromosome must be carrying
an array of genes, and perhaps other DNA sequences, virtuallly to guarantee
that the developing gonad grows steadily to become a fully functional testis,
and the individual male capable of begetting offspring. In normal development,
the Y chromosome is inherited as a unit, so this arrangement simulates the
pattern of Mendelian inheritance of a single dominant gene.

Professor Ursula Mittwoch works in the anatomy department at Queen Mary
and Westfield College of the University of London.