At what stage in the human reproductive system is the sex (i.e. XY or XX) determined?
When the sperm cell joins with the egg.
The sperm cells come in two types, X and Y-cells.
The egg cell only carries the X-chromosome.
I thought X and Y are associated with chromosomes only. Do you mean two types of cells, one carrying the X chromosome and the other Y chromosome?
So this would imply each cell of a female and male is different? All these tiny differences at the cell level make up the macro difference we observe.
That's what he means. The male's sperm determines the sex of the fertilized egg by virtue of the fact is carries either an X or Y chromosome, which then pairs with the X chromosome in the egg.
Yes, chromosome pairs in males are XY, while in females they are XX.
You may want to read about meiosis.
In short, when sperm is produce, the XY chromosome of the father are seperated in two different sperm and you end up with a femal sperm (the one carrying the X chromosome) and a male sperm (the one carrying the X chromosome. A female has two XX chromsome, therefore all the egg will carry one copy of the X chromosome (i.e. both egg are female). Therefore, the "sex" of the sperm will determine the sex (XY or XX) in mammal.
I don't have time to find a link for the following.
Normally, that is the case, but it is possible for a person with two "X" chromosomes to develop as an anatomically correct male. The individual is not able to produce viable sperm -- or at least those XX males who discovered their genetic defect because of infertility are.
It is also possible for someone with an XY pair to develop as a female. This condition can run in families and often the vagina does not have an adequate connection to the uterous. It appears a genetic defect prevents the fetus from knowing how to develop male sexual features.
These individuals think of themselves as being of their anatomical sex rather than their genetic sex.
Other individuals are born with both male and female genitals. Some experts suggest that premature decisions about which physical sex these individuals are may create problems later if they belief they are of the sex associated with the genitals that were removed. In other cases the reason a person believes himself or herself to be of the other anatomical sex is unclear, but there is a theory that it involves develpment of the brain.
Here is an interesting article that explains fetal sexual development and the differences between phenotypic and genotypic sex.
Excellent discussion -- sex is not a matter of conscious choice. Well before human conscious is developed within neurons, sex was determined during fetal development.
A key point is 'normally'. I wonder, it this because one X chromosome in the XX pair is faulty, or the Y in and XY pair is faulty, which certainly could happen.
Is the development (e.g. hormone development) along the lines of genetic or anatomical?
I imagine that is how most people develop, unless one's genetics are tested.
The term with which I am familiar is 'hermaphroditism'. And there are genetic anomalies such as XXY and XYY, but I believe such occurrences are rare.
Also, recent studies have revealed differences between the brain structures of hetero- and homosexual men and between hetero- and homosexual females. There is some difference(s) in the amygdala, and apparently the "anterior commisure", and perhaps other areas.
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