the presence of characteristics of both sexes in a dioecious organism. These characteristics are not fully developed, that is, they are of intermediate character and are manifested jointly in the same parts of the body. The embryonic development of such an organism, which is called an intersex, begins normally, but from a certain moment it proceeds according to the pattern of the other sexual type. The earlier the organism’s direction of development changes, the more sharply expressed is its intersexuality. Several types of intersexuality are distinguished.

Zygotic, or genetically conditioned, intersexuality is the result of deviation from the norm of the set of sex chromosomes and genes, which are predetermined at the moment of fertilization, when the gametes were combined in the zygote. Depending on the type of disorder, one may distinguish triploid (or aneuploid) intersexuality, which is produced by a deviation from the norm in the number of chromosomes in the zygote, and diploid intersexuality, which is caused by a disruption of the interrelationship of the genes that are brought to the zygote. Triploid (aneuploid) intersexuality was first studied in Drosophila. It was shown that in Drosophila intersexes the interrelationship between the number of sex chromosomes and autosomes is disrupted; the degree of intersexuality of the individual is determined by the chromosome (or gene) balance—that is, by the ratio of the number of sex chromosomes to the number of autosomes (and the sex-determining genes included in them).

The various forms of intersexuality, or so-called pseudohermaphroditism, found in humans are also caused by a disruption of the normal number of sex chromosomes. Depending on which of the chromosomes determining (respectively) the male or female sex are found in excess, one distinguishes “male” or “female” pseudohermaphroditism. Diploid intersexuality is observed in the gypsy moth after the interbreeding of various geographic races. Depending on the type of interbreeding, the intersexuality is noted either in the females or the males. Since in such cases no disruption of the normal number of chromosomes has been discovered, the German biologist R. Goldschmidt put forward the theory in 1912 of the varying “strength” of the genes that determine sex in different races (possibly determined by the qualitative differences in alleles or by the presence of other sex-determining genes).

Hormonal intersexuality is observed in animals, in which the sex glands secrete female or male sex hormones that determine the development of the secondary sex characteristics. When such an animal is castrated and the gonads of the other sex are transplanted to it, a respective masculinization or feminization occurs—that is, the animal becomes an intersex. Similar phenomena are observed in the so-called parasitic castration of crustaceans (found, for example, in the crab Inachus, which is parasitized by the cirripede Sacculina).