Determining sex in living creatures who have two dissimilar sexes is done through certain mechanisms.Before explaining the mechanism involved in determining sex in human beings,two points should be dealt with in connection with the transfer of traits from parents to offsprings and cell division.
A - inside the nucleus of the cells of all living creatures there exist tiny bodies called chromosomes,which are in fact sites for genes.The types and orders of genes,like codes,also keep in themselves characteristics of creatures.Chromosomes are usually in pairs and their number differs in various creatures.In human each somatic cell has 64 or 23 pairs of chromosomes.22 pairs of these are non-sexual and the remaining pair is human sexual chromosomes human . Sexual chromosomes are clearly distinct from other ones.In females both sexual chromosomes are identical and are shown as XX,but in males one of them is much smaller than the other one )Y(,and is shown as XY.
B - generally,two types of cell divisions are distinguishable in living creatures : 1.mitosis division,in this kind of division which is generally seen in non-sexual cells (somatic(,each cell producess two cells of its own type , which from all respects, including the number of chromosomes,are similar to the original cell.
2 - meiosis division,in this kind of division which is generally carried out in sexual cells and when male and femal gamets are produced,each original cell produces 4 cells with half as many chromosomes as that of the original one.In man,as a result of such division,each gamet will have 23 chromosomes. Otherwise,following the pairing of male and female gamets,the person evolved would have a cellular chromosomes twice as many as that of his / her parents'.
Considering the a foresaid,the mechanism with which human sex is determined is taken up. During the process of sexual gamets production,men produce two kinds of gamets (sperms(,half of which carry X chromosomes and the other half carrying Y chromosomes. Unlike men,gamets produced by women) ovum) all have X sexual chromosomes . During fertilization if an X carrying chromosome fertilizes an ovum,the offspring would be a girl) XX(;and if the ovum is fertilized by a Y carrying chromosome,the baby would become a boy )XY( . In human being sex is determined during the fertilization process, and man's sperm has a decisive role.
Here although in normal situations fertilization and sex determination seems quite accidental ,now a days in the light of vast studies carried out in this regard, many factors have been considered to be effective indetermining whether the offspring would be a girl or a boy . For example scientists believe that the size and volume of the head of the sperms carrying X sexual chromosomes differs from that of those carrying Y sexual chromosomes.Therefore,the desired sperm can be fecundated with ovum. Sometimes in meiosis division,all or more chromosomes accumulate at one pole instead of being evenly distributed at the two cell poles;therefore the person evolved is different from normal man from the point of view of chromosomes;the result of which is the occurance of severe changes in the person and usually leads to infertility . Such abnormalities as Down's syndrome )mongolism) and Turner's syndrome are all complications of this irregularity in chromosome numbers. It should be noted that not all living creatures follow this sex determining mechanism.For instance in birds and butterflies,unlike in man,the female produces two gamets.In addition,in some creatures sex is determined through having one or two sexual chromosomes.In locusts,for example, males have one sexual chromosome and 32 chromosomes altogether,while females have 2 sexual chromosomes,24 altogether.