Ask A Scientist Molecular Biology Archive

status       educator
age          40s
 
Question -   is it true that some men can only father girl babies?  I
heard that some men cannot conceive a male child.

For mammals, the father does determine the sex of the baby, since the
mother always provides an X chromosome and the father provides either an
X or a Y.  Scientists have found differences in the behavior and
survival of sperm carrying X and Y chromosomes.  These differences can
be greater or less for different men.  In humans, there are generally
more boys born than girls.

Recently, studies in a number of industrialized countries have reported
that the proportion of male babies has decreased.  Studies are on-going
to explore this issue and people have theorized links to pollutants,
timing of conception, smoking, stress and other factors.  Additionally,
some types of mutations in either the X or Y chromosome have been found
to lead to more female births.  Probably the biggest effect is just
random variation.  Many smaller families would, by chance, have just
boys or just girls.

Laura Hungerford DVM, MPH, PhD
University of Maryland
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I think you are talking about sex reversals.  Genetic testing due to infertility concerns
and biotechnology has led to the discovery that some females are actually XY rather than XX.  
There are males that are XX rather than XY.  These individuals are rare to say the least
and the details of why they exist lies with mutations and abnormal chromosomal structures.  The gene
for maleness is either misplaced in the sex chromosomes or this gene, called the SRY gene, is not
turned on to set up the chemistry to produce a male.  The female is the default in our species.
However, many of the XX males are sterile by what I have read.  If any are fertile, yes, they would
produce only girls since they only have X chromosomes, unless they past on the reason that they are
male rather than female to the next generation.  There is not a lot of information as yet on this
topic.

Steve Sample
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