Determinism of sex is explained by the existence of special chromosomes, sex chromosomes or heterocromosomas, which are distinctly represented in individuals ofeither sex. The human chromosome comprises 46 chromosomes: 22 pairs of autosomes and one of heterocromosomas (XX in women and XY in males), theinheritance of sex in man takes place, then, as in the case of the fruit fly.
The same occurs in those species where the female has two heterocromosomas (XX) while the male has only one (X). Conversely, if the female has the heterocromosomasXY and XX male has the female, being heterogametic (eggs with eggs with X and Y), is the one condition the sex of their offspring according to the type of fertilized egg.
Of course, all those characters whose genes are located determinantsheterocromosomas be transmitted in conjunction with sex; we can talk of a sex-linked inheritance. It should be noted, however, that the Y chromosome is usually quite inert, which means that usually is very poor in genes.
Due to this fact, sex-linked characters may follow a cross inheritance. Thus, when the male has one X chromosome and the female two, and that is homozygous genes andantagonistic to those carrying the X chromosome of male characters linked to the lattermay only be transmitted to their grandchildren through daughters, a son never directly or through it.

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