Ask A Scientist© Molecular Biology Archive

name        Sandra
status      educator
age         40s

Question -  I was in discussion today with a fellow student who had said 
that they had learned in their science class that all fetuses(?) 
are  female until 3 months then the determination is
made by a chemical  change. I had always been under the understanding that 
sex is determined
at the time of conception. Could you please
clarify.


The way I understand it you are both right. The sex is
determined by the chromosomes of the male sperm cell,
at the time of conception. However, development of a
fertilized egg is the same for male and female, until
at a certain stage, due to the genes on the
Y-chromosome, male fetuses start to produce substances
that change the development to follow the 'male'
course.

In fact, you are hitting a difference in 'genotype'
versus 'phenotype' here. The fertilized egg has the
genotype of male or female, but the phenotype (the
'form' you see) is not yet apparent. The difference
between genotype and phenotype is crucial in biology
but it is often ignored. You have just identified a
beautiful example.

Trudy Wassenaarr
=========================================================
I believe it is somewhat earlier...around day 27 as I  remember for the SRY
gene to be expressed which then allows the fetus to express androgen
resposive receptors on its cells and then go on to develop primary male
characteristics of testicles etc.  If the SRY gene is prevented from turning
on then the gonads (testicular or ovarian primordial tissue) will develop
into neither the male or female.  In this case, the person will have all the
appearances of a female but will not have some of the internal
characteristics of most females, since the gonads will not be ovaries and
the uterus will usually be "blind"  if I remember my embryology correctly.
These women are now considered "androgen insensitive"  and although they
have a "y" chromosome are women.  It raises philosophical questions on how
we define what a male or female is....As a scientists I define a sex by its
genetic expression...not whether it has the gene or not

Peter F
========================================================
The way I understand it you are both right. The sex is
determined by the chromosomes of the male sperm cell,
at the time of conception. However, development of a
fertilized egg is the same for male and female, until
at a certain stage, due to the genes on the
Y-chromosome, male fetuses start to produce substances
that change the development to follow the 'male'
course.

In fact, you are hitting a difference in 'genotype'
versus 'phenotype' here. The fertilized egg has the
genotype of male or female, but the phenotype (the
'form' you see) is not yet apparent. The difference
between genotype and phenotype is crucial in biology
but it is often ignored. You have just identified a
beautiful example.
 
Trudy Wassenaarr
=========================================================
You're both right-sort of.  Yes, sex is determined genetically at conception
by the inheritance of either two X chromosomes (female) or an X and a Y
(male).  The expression of the sex characteristics doesn't begin until about
6 weeks gestation.  Before that time the fetus is not really male or female,
but non-specific.  The fetus has two non-specific gonads and two sets of
tubes.  If the fetus has a Y chromosome, at 6 weeks a gene called the SRY
(sex determining region of the Y) turns on.  This gene causes the
degeneration of the female characters and causes the male characters to
develop.  It causes the non-specific gonads to become testes and the tubes to
become the vas deferens.  Once the testes are mature enough another gene
turns on to start producing testosterone.  This causes the internal sex
organs to develop.  Part of the testosterone turns into another hormone which
then causes the external sex organs to develop.  If the Y chromosome isn't
present the female pattern of development occurs; the female pattern is the
default pattern so-to-speak.  The gonads become ovaries and the tubes become
fallopian tubes.  So even though the gene for maleness is inherited at
conception, the expression of the trait doesn't begin until about 6 weeks.


Van Hoeck
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