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Name: Ronnie J.
Status: Student
Age: 20s
Location: N/A
Country: N/A
Date: N/A 

I need to know if all embryos are girls at conception. I overherd someone talking about this subject and i brought up the fact that i thought that they were. Am I right or wrong?

Embryos are genotypically boy or girl from the time of conception, ie. the chromosomes they inherit make them either XX-a girl or XY-a boy. However they do not start to differentiate into sexes until about 6 weeks of gestation. The gonads and tubes are undifferentiated at first and could become either male or female. The Y chromosome provides instructions for the male phenotype.

If the Y is present testosterone is produced and the structures become male. Until recently it was believed that if the Y is not present, the "default" phenotype occurs which is female. However, there is some research that shows that there are triggers for female development as well.


No, all embryos are not female at conception. The egg recieves one sex chromosome from the mother, and since the mother has two X chromosomes, the egg will get one of these X chromosomes. At that point, it is neither male nor female, just an unfertilized egg. When a sperm joins with the egg, it will donate one sex chromosome -- either an X or a Y chromosome. The newly concieved embryo will be a girl if the father's sperm cell donates an X, (the embryo will be XX), and it will be a male if the sperm donates a Y (the embryo will be XY). It's the luck of the draw whether the sperm that fertilizes the egg has an X or Y chromosome in it from the father.


Dear Ronnie:

Assuming that you are talking about HUMAN embryos, then the answer is no. The gender of the embryo is determined by one pair of chromosomes -- XX for females, XY for males. An X chromosome is always inherited from the mother, but the father may contribute either an X or a Y. Once the egg and sperm unite, the gender is set.

For birds, it is females that have two different kinds of sex chromosomes, and males that have two of the same kind.

For some species, like alligators, gender actually depends on the temperature during early development.

So, nature has many answers to this question -- it depends on which species.

Tom Douglas

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