The gender (male or female) in organisms is normally determined by the presence or absence of a so-called Y-chromosome in the genome (the genome is the genetical make-up, present in every cell). Males and females differ in their morphological and physiological constitution, and the determination of that constitution is thus exerted by the DNA.
There are however some striking exceptions. So for example the Anglerfish. Here the gender is not determined by the genetic make-up, despite the fact that male Anglerfish have less than one thousandth of the volume of a female. When an Anglerfish larva encounters an adult female then some chemicals, produced by that female, ' switch ' the larva into the ' male-mode '. The larva attaches itself to that female, stops growing (when that larva is still very small), becomes a parasite, and concentrates on the production of sperm. All other larvae which did not encounter an adult female become females. It is an ingenious system, that garantees that every male possesss a mature female, without any waste in the form of too many males (COHEN & STEWART, 1995, The Collapse of Chaos ).
A second example is Bonellia .
For a long time male and female Bonellia's were thought to belong to different species. This is not surprising, because the males are small ciliated micro-organisms consisting of about 150 cells, while the females have the size of a wal-nut, submerged in mud, and to which is attached a tube of about 15 cm long. At its tip this tube splits into forks with ciliated grooves. The males parasitize in those grooves, and in so doing become males. When they do not settle in such a groove they become females.
Again it is the case that the genetic make-up is the same for males and females. The gender is determined by context, not by content ( Ibidem ).
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