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Sex

From Academic Kids

The members of many species of living things are divided into two or more categories called sexes (or loosely speaking, genders). These refer to complementary groups that combine genetic material in order to reproduce, a process called sexual reproduction. Typically, a species will have two sexes: male and female. The female sex is defined as the one which produces the larger gamete (i.e., reproductive cell) and which bears the offspring. The categories of sex are, therefore, reflective of the reproductive functions that an individual is capable of performing at some point during its life cycle, and not of the mating types, which genetically can be more than two.

Sex in the plant kingdom

Plants are generally hermaphrodites, but this terminology is quickly complicated by variations in the degree of sexuality. As with animals, there are only two types of gametes. These are generally called male and female based on their relative sizes and motility. In flowering plants, flowers bear the gametes. In some cases, flowers may contain only one type of gamete while in others they may contain both.

Sex in the animal kingdom

Some species, such as earthworms, honeybees, and geckos, are capable of both sexual and asexual reproduction. In the insect order Hymenoptera, which includes honeybees, the queen (i.e., fully functional female) can decide to fertilize an egg or to lay it without its being fertilized. Fertilized eggs will develop into females -- workers if given standard nutrition in their larval stages and queens if lavishly fed with royal jelly. Unfertilized eggs, which have only half the number of chromosomes as fertilized eggs, develop into drones, i.e., male bees. In other species (e.g. earthworms), all individuals are hermaphrodites, that is, individuals that have male and female sex organs.

In mammals, birds, and many other species, sex is determined by the sex chromosomes, called X and Y in mammals, and Z and W in birds. For mammals, males typically have one of each (XY), while females typically have two X chromosomes (XX). All individuals have at least one X chromosome, the Y chromosome is generally shorter than the X chromosome with which it is paired, and is absent in some species, this pattern admitting some considerable variation. One interesting variation is in the platypus, a rather unusual mammal in many other ways, where sex is determined by 10 chromosomes. Males are XYXYXYXYXY and females XXXXXXXXXX. In birds, males have two of the same kind of sex chromosome (ZZ) and females have one of each type (ZW). In other species, including crocodiles, and most insects, sex may be determined by various other sex-determination systems, including those controlled by environmental factors such as temperature. Yet other species change sex during their lifetime.


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