Saturday, March 21, 2009


Picture:The sex chromosomes as the basis of human sex determination
Source: William F.Ganong.MEDICAL PHYSIOLOGY, California,1971, p 308

Sex is determined genetically by 2 chromosomes called the sex chromosomes to distinguish them from the other somatic chromosomes [autosomes]. It is now clear that in man there are 46 chromosomes : in males, there are 22 pairs of autosomes plus a large X chromosome and a small Y chromosome ; in females, there are 22 pairs of autosomes plus 2 X chromosomes.
When a sperm containing a Y chromosome fertilize an ovum, an XY pattern results an the zygote develops into a genetic male. When fertilization occurs with an X-containing sperm, an XX pattern and a genetic female result.
Because the human Y chromosome is smaller than the X chromosome, it has been hypothesized that the sperms containing the Y are lighter and able to “swim” faster up the female genital tract, thus reaching the ovum more rapidly. This supposedly accounts for the fact that the number of males born is slightly greater than the number of females.

No comments:

Post a Comment