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Name: Sandra
Status: Educator
Age: 40s
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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.



Replies:
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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