Question

1. Describe how normal sex determination occurs in humans from fertilization to puberty; include in your discussion the role that chromosomes, hormones and gonads play in this process.

2. Describe 2 ways in which this process can be altered due to abnormalities. Describe each abnormality and the result in the offspring.

Answer 1

1. Chromosomal sex is determined at the time of fertilization; a chromosome from the sperm cell, either X or Y, fuses with the X chromosome in the egg cell. If X chromosome fuses with X chromosome of ova it forms a female and when the Y chromosome of sperm fuses with the X chromosomes of ova then the zygote develops into a male.

Gonadal sex refers to the gonads, that is the testis or ovaries, depending on which genes are expressed. Phenotypic sex refers to the structures of the external and internal genitalia.

In mammals, sex is determined by the Y chromosome, which encodes a testis-determining factor (TDF). This factor causes the undifferentiated embryonic gonads to develop as testes rather than ovaries. The testes subsequently produce the male sex hormones that are responsible for all male sexual characteristics.

2. some males are born 46XX due to the translocation of a tiny section of the sex-determining region of the Y chromosome. Similarly, some females are also born 46XY due to mutations in the Y chromosome. Clearly, there are not only females who are XX and males who are XY, but rather, there is a range of chromosome complements, hormone balances, and phenotypic variations that determine the sex. The Y chromosome acts as a dominant inducer of male phenotype and individuals having four X chromosomes and one Y chromosome (49XXXXY) are phenotypically male. When a Y chromosome is present, early embryonic testes develop around the 10th week of pregnancy. In the absence of both a Y chromosome and the influence of a testis-determining factor (TDF), ovaries develop.