Expert: Dana Krempels, Ph.D. - 5/8/2010

Question
Is a only women is responsible for born a male or female child.

Answer
Dear Om,

The sex of a mammal (including a human) is determined by two sex chromosomes.  Every person has two sex chromosomes, and there are two different types:  the X chromosome (which is large and carries much genetic information) and the Y chromosome (which is much smaller, carries fewer genes, and contains many of the genes that determine the "maleness" of a fetus).

Male mammals (including humans) have both an X and a Y chromosome (their sex chromosome genotype is XY), and females have the XX genotype.  When cell division in the gonads (testes or ovaries) occurs, the two sex chromosomes separate and migrate into different cells, so any sperm or any ovum (egg) will get only one sex chromosome, either X or Y.

Because females have only X chromosomes, all ova have an X chromosome.  Males, however, have both X and Y, so half of a man's sperm will carry the X chromosome, and half will carry the Y chromosome.  The X-carrying sperm that fuse with an ovum will produce a fertilized egg with an XX genotype, and it will become female.  The Y-carrying sperm that fuse with an ovum will produce a fertilized egg with an XY genotype, and it will become male.

So to finally answer your question, it is the *male* in mammals that determines the sex of the offspring, not the female.  (Interestingly, the situation is reversed in birds and certain insects:  it is the female (genotype ZW) who determines the sex, not the male (ZZ).

Hope that helps.

Dana