Ask A Scientist Molecular Biology Archive

name        Vanessa S.
status       student
age          18

Question -   How is the sex of a human baby determined on the
sub-cellular and molecular levels of structural complexity?
-----------------
"How is the sex of a human baby determined on the
sub-cellular and molecular levels of structural complexity?" 

This is an exceedingly complex question!

The question has also some ambiguity...Is it asking how the organism
"determines" or directs its "sexual" destiny...or how do we determine what
the sex of the organism is at the molecular/sub-cellular level.  Defining
sex in humans alone is a difficult task.  At first one might think it rather
simple...phenotype (physical traits) of females and males being quite
obvious...(to most).  At the cellular level...the sex chromosomes can reveal
the sex and at the molecular level the presence and levels of certain
proteins that differ in males and females is also well established.  All of
these have problems when we look at some of the less usual individuals of
our species who might have all or nearly all female phenotypes but have an X
chromosome or some unusual compliment of sex chromosomes...If we use
chromosomes as a measure of sex, are females with an extra X chromosome more
female?  Moreover, in all the somatic (non-sex cells) of a female on of the
two x chromosomes is inactive...50% paternal and 50% maternal...making
females a sexual mosaic...at least compared to males.  What of those females
with an inactive Y chromosome...?  

Now if you are asking how sex is instructed to develop through the action of
the various proteins in development...ah...now there is a greater question
to which we have assembled some of the pieces of the puzzle but have very
little understanding.   Our human genome project reveals our ignorance of
genes and their products by its listing of the genes of our chromosomes not
based typically on what they do but upon a syndrome or disease that is
related to the malfunction of the gene.

As to the term "complexity"...I look at living systems as "incomprehensibly
complex".  I do not think we will ever fully understand our bodies, how they
work and our genetics.  We will make slow progress towards a functional
understanding and hopefully enable us to relieve some of the human misery
associated with disease. 

pf
=====================================================
I'm not sure what you mean.  If you are talking about chromosomes-males are XY and females 
are XX, so a simple karyotype, or picture of the chromosome pairs, can determine the sex of 
a child before it is born.  But if you are talking about genes that is more complicated.  
Even though a child's ultimate sex is determined at conception, an embryo doesn't start 
developing into a boy or girl until about 6 weeks of pregnancy.  If the child inherited a 
Y chromosome from his father, a gene on the Y called the SRY gene turns on.  This gene is 
kind of a master switch that turns on all the other genes necessary to become a boy.  If 
this gene is not present, the embryo develops as a girl.  That sounds very simple, but 
there a many things that can go wrong in this process.  Even though the SRY turns on the 
other genes, if any of them are mutated, full sexual phenotype may not develop.  For 
example, testosterone, a male hormone, is responsible for beginning the development of male 
characteristics.  If this gene is mutated or the gene for their cell receptors is mutated, 
male development won't begin and the child will be born looking like a female.  
 
vanhoeck
=====================================================