Essay on the Determination of Sex in Living Beings

Groups of genes in a cell often cooperate in controlling a single composite trait. One of the best illustrations is the trait of sexuality, which in numerous organisms is controlled not by individual genes acting separately but by whole chromosomes acting as functionally integrated units.

Each organism is believed to have genes for the production of both male and female traits. Such genes need not necessarily be specialized “sex genes” but could be of a type that, among other effects, also happen to influence sexual development.

Organisms thus are considered to have a genetic potential for both maleness and femaleness, and two categories can be distinguished according to how this genetic potential is translated to actual sexual traits. In one category, comprising probably the majority of all types of organism, the masculinizing genes are exactly equal in effect, or “strength”, to the feminizing genes.

In the absence of other influences an organism will then develop as a hermaphrodite. If other sex-determining factors do exert an influence, they are nongenetic and environmental: different conditions in the external or internal environment affect an organism in such a way that it develops either as a male or as a female. In most cases the precise identity of these environmental conditions has not yet been discovered. The genetic nature of a species determines whether nongenetic influences will or will not play a role. Also, the genetic nature of the species determines when during the life cycle the sex of such organisms becomes fixed by nongenetic means. In hydras, for example, sex determination does not occur until an adult is ready to produce gametes.

Up to that time the animal is potentially bisexual. In other animals sex is determined during the larval phase. After metamorphosis, therefore, such animals are already males or females. In at least one case, the echiuroid worm Bonellia, the sex-determining factor is known to be environmental C0₂. The relatively low concentrations of C0₂ in sea water cause free-swimming larvae of this worm to develop as females.

But if a sexually still undetermined larva happens to make contact with an adult female or with a larva already determined as a female, then the added respiratory C0₂ produced by that animal causes the undetermined larva to develop as a male; it becomes a small, sperm-producing, structurally simplified parasitic animal, permanently attached inside the excretory organ of the female.

In still other animal groups, nongenetic sex determination occurs even earlier during the life cycle-for example, after cleavage or in the fertilized egg itself. In general, the portion of the life cycle before the stage of determination is always potentially bisexual, and if the determination occurs no later than the time of gamete production in the adult, the organism will produce either sperms or eggs.

But if by then a determination has not taken place, the organism will be a hermaphrodite. An altogether different form of sex determination occurs in a second category of organisms, including some protists, some plants, and some animals (particularly insects and vertebrates).

In these the masculinizing genes are not equal in their effects to the feminizing genes, and the Primary determination of sex has a purely genetic basis.

Every individual becomes either a male or a female; hermaphroditism does not occur except as an abnormality. Also, sex always becomes fixed at the time of fertilization, and every cell later formed is genetically male or female.

Organisms of this type contain special sex chromosomes, different in size and shape from all other chromosomes, the autosomes. Sex chromosomes are of two kinds, X and Y. In some of these organisms, chromosomal differences control sex distinctions in the haploid phase of the life cycle.

For example, in the liverwort Sphaerocarpos each cell of a female gametophyte and each egg contain one X chromosome. Similarly cells of the male gametophytes and sperms contain Y chromosomes. Fertilization then produces diploid XY zygotes, and each cell of the resulting saprophyte inherits the XY chromosomes (and is therefore sexually undetermined).

When a spore-producing cell of the saprophyte later undergoes meiosis, the two sex chromosomes become segregated in different spore cells. The result is that, of the four mature spores formed, two contain an X chromosome each and two a Y chromosome each.

Thus although all spores look alike, the}’ are genetically of two different sex types. Spores with X chromosomes subsequently mature as female gametophytes and spores with Y chromosomes, as male gametophytes. An entirely similar sex-determining mechanism exists in a number of other bryophytes and also in several protests.

The pattern is somewhat different where both the haploid and the diploid phases of the life cycle exhibit chromosomal sex distinction. Each diploid cell of such organisms contains a pair of sex chromosomes, either XX or XY, and all such cells are genetically either male or female. For example, XY cells are genetically female and XX cells genetically male in strawberry saprophytes butterflies, most moths, some fishes, and birds.

In such organisms maleness appears to be controlled by the genes of the autonomies (and in part perhaps also by those of the Y chromosomes). By contrast, XY cells are male and XX cells female in the saprophytes of, for example, holly and Elodea and in flies and mammals. Femaleness here is controlled by the genes of the X chromosomes; maleness is known to be determined by the autonomies in fruit flies, but to a large extent by the Y chromosomes in mammals.

In man, for example, each adult cell contain 22 pairs of autonomies, plus either an X\’ or an XX pair. Female cells, 44A + XX, thus have two female-determining chromosomes, whereas male cells, 44A + XY, contain one female- determining and one male-determining chromosome. This difference of one whole chromosome lies at the root of the sexual differences between males and females.

More specifically, in a female cell the feminizing effect of the two X chromosomes outweighs any masculinizing influence the autonomies might have; and in a male cell, the masculinizing effect of the Y chromosome (and probably also the autonomies) outweighs the feminizing influence of the single X chromosome.

These relations suggest that the sexual nature of an individual might depend on a particular numerical ratio or balance between different chromosomes. That this is actually the case has been shown by experiments in fruit flies. In these animals, in which maleness is controlled by the autonomies, it is possible and autonomies that occur in sperm and eggs.

One can then’ obtain offspring with, for example, normal paired sets of autonomies but three X chromosomes instead of two. Such individuals grow into so-called super females; all sexual traits are accentuated in the directions of femaleness.

Other chromosome balances give rise to supermales and intersexes, the matter with sexual traits intermediate between those of normal males and females. Paradoxically, super sexes and also intersexes are generally sterile, for as a result of the abnormal chromosome numbers meiosis occurs abnormally, and the sperms and eggs then produced are defective.

Every individual becomes either a male or a female; hermaphroditism does not occur except as an abnormality. Also, sex always becomes fixed at the time of fertilization, and every cell later formed is genetically male or female.

Organisms of this type contain special sex chromosomes, different in size and shape from all other chromosomes, the autonomies. Sex chromosomes are of two kinds, X and Y. In some of these organisms, chromosomal differences control sex distinctions in the haploid phase of the life cycle.

For example, in the liverwort Sphaerocarpos each cell of a female gametophyte and each egg contain one X chromosome. Similarly cells of the male gametophytes and sperms contain Y chromosomes. Fertilization then produces diploid XY zygotes, and each cell of the resulting saprophyte inherits the XY chromosomes (and is therefore sexually undetermined).

When a spore-producing cell of the sporophyte later undergoes meiosis, the two sex chromosomes become segregated in different spore cells. The result is that, of the four mature spores formed, two contain an X chromosome each and two a Y chromosome each.

Thus although all spores look alike, they are genetically of two different sex types. Spores with X chromosomes subsequently mature as female gametophytes and spores with Y chromosomes, as male gametophytes. An entirely similar sex-determining mechanism exists in a number of other bryophytes and also in several protests.

The pattern is somewhat different where both the haploid and the diploid phases of the life cycle exhibit chromosomal sex distinction. Each diploid cell of such organisms contains a pair of sex chromosomes, either XX or XY, and all such cells are genetically either male or female. For example, XY cells are genetically female and XX cells genetically male in strawberry saprophytes, butterflies, most moths, some fishes, and birds.

In such organisms maleness appears to be controlled by the genes of the autonomies (and in part perhaps also by those of the Y chromosomes). By contrast, XV cells are male and XX cells female in the saprophytes of, for example, holly and Elodea and in flies and mammals.

Femaleness here is controlled by the genes of the X chromosomes; maleness is known to be determined by the autonomies in fruit flies, but to a large extent by the Y chromosomes in mammals.

In man, for example, each adult cell contain 22 pairs of autonomies, plus either an XV’ or an XX pair. Female cells, 44A + XX, thus have two female-determining chromosomes, whereas male cells, 44A + XY, contain one female- determining and one male-determining chromosome.

This difference of one whole chromosome lies at the root of the sexual differences between males and females. More specifically, in a female cell the feminizing effect of the two X chromosomes outweighs any masculinizing influence the autonomies might have; and in a male cell, the masculinizing effect of the Y chromosome (and probably also the autonomies) outweighs the feminizing influence of the single X chromosome.

These relations suggest that the sexual nature of an individual might depend on a particular numerical ratio or balance between different chromosomes. That this is actually the case has been shown by experiments in fruit flies. In these animals, in which maleness is controlled by the autonomies, it is possible and autonomies that occur in sperm and eggs.

One can then obtain offspring with, for example, normal paired sets of autonomies but three X chromosomes instead of two. Such individuals grow into so-called super females: all sexual traits are accentuated in the directions of femaleness.

Other chromosome balances give rise to super males and intersexes, the matter with sexual traits intermediate between those of normal males and females. Paradoxically, super sexes and also intersexes are generally sterile, for as a result of the abnormal chromosome numbers meiosis occurs abnormally, and the sperms and eggs then produced are defective.

Every individual becomes either a male or a female; hermaphrodites do not occur except as an abnormality. Also, sex always becomes fixed at the time of fertilization, and every cell later formed is genetically male or female.

Organisms of this type contain special sex chromosomes, different in size and shape from all other chromosomes, the autonomies. Sex chromosomes are of two kinds, X and Y. In some of these organisms, chromosomal differences control sex distinctions in the haploid phase of the life cycle.

For example, in the liverwort Sphaerocarpos each cell of a female gametophyte and each egg contain one X chromosome. Similarly cells of the male gametophytes and sperms contain Y chromosomes. Fertilization then produces diploid XY zygotes, and each cell of the resulting sporophyte inherits the XY chromosomes (and is therefore sexually undetermined).

When a spore-producing cell of the sporophyte later undergoes meiosis, the two sex chromosomes become segregated in different spore cells. The result is that, of the four mature spores formed, two contain an X chromosome each and two a Y chromosome each.

Thus although all spores look alike, the}’ are genetically of two different sex types. Spores with X chromosomes subsequently mature as female gametophytes and spores with Y chromosomes, as male gametophytes. An entirely similar sex-determining mechanism exists in a number of other bryophytes and also in several protists.

The pattern is somewhat different where both the haploid and the diploid phases of the life cycle exhibit chromosomal sex distinction. Each diploid cell of such organisms contains a pair of sex chromosomes, either XX or XY, and all such cells are genetically either male or female.

For example, XY cells are genetically female and XX cells genetically male in strawberry sporophytes, butterflies, most moths, some fishes, and birds. In such organisms maleness appears to be controlled by the genes of the autonomies (and in part perhaps also by those of the Y chromosomes).

By contrast, XY cells are male and XX cells female in the sporophytes of, for example, holly and Elodea and in flies and mammals. Femaleness here is controlled by the genes of the X chromosomes; maleness is known to be determined by the autonomies in fruit flies, but to a large extent by the Y chromosomes in mammals.

In man, for example, each adult cell contain 22 pairs of autonomies, plus either an XV’ or an XX pair. Female cells, 44A + XX, thus have two female-determining chromosomes, whereas male cells, 44A + XY, contain one female- determining and one male-determining chromosome.

This difference of one whole chromosome lies at the root of the sexual differences between males and females. More specifically, in a female cell the feminizing effect of the two X chromosomes outweighs any masculinizing influence the autonomies might have; and in a male cell, the masculinizing effect of the Y chromosome (and probably also the autonomies) outweighs the feminizing influence of the single X chromosome.

These relations suggest that the sexual nature of an individual might depend on a particular numerical ratio or balance between different chromosomes. That this is actually the case has been shown by experiments in fruit flies. In these animals, in which maleness is controlled by the autonomies, it is possible and autonomies that occur in sperm and eggs.

One can then’ obtain offspring with, for example, normal paired sets of autonomies but three X chromosomes instead of two. Such individuals grow into so-called super females; all sexual traits are accentuated in the directions of femaleness.

Other chromosome balances give rise to super males and intersexes, the matter with sexual traits intermediate between those of normal males and females. Paradoxically, super sexes and also intersexes are generally sterile, for as a result of the abnormal chromosome numbers meiosis occurs abnormally, and the sperms and eggs then produced are defective.