Sex linked Inheritance. Sex is determined by the non-autosomal sex chromosomes. In humans there are two sex chromosomes in non gametic cells. Females possess two X chromosomes, XX and are said to be homogametic . When gametes are formed in the female all the ova must contain an X-chromosome.
Fertilisation therefore provides approximately equal numbers of male and female offspring.
It is the male gamete that ultimately determines the sex of the offspring in humans. Genes carried on the sex chromosomes are said to be sex-linked. Interestingly the male Y chromosome is smaller than the X chromosome. Therefore many genes present on the X chromosome cannot be found on the Y chromosome. Many important genes are carried on the X chromosome and XX females have two alleles for these characteristics, whereas XY males have only one.