We have seen that in sexual reproduction we have male and female gametes. These are produced by organisms that are male or female respectively. But how is sex determined? In general the determination of sex is genotypic, that is, it depends on the chromosome set. Equally in general, the phenotypic gender corresponds to genotypic sex. In either case, however, there may be exceptions. Genetic (or chromosomal) sex is determined by the genome. In each species there is a characteristic number of chromosomes (karyotype) of which only some are in charge of the determination of sex (gonosomes) while the others are called autosomes. There are at most two gonosomes in the normal diploid genome: one for haploid, that is, one for gamete.
In humans the 46 chromosomes of the diploid kit comprise two series of autosomes (22 + 22 = 44) and two gonosomes. In other species the relationships are variable.
The female has two X gonosomes and the male one X and a Y.
The female gametes will always receive an X, while the spermatozoa are equally likely to carry an X or a Y: in the first
case they will give zygotes XX (females), in the second zygotes XY (males). Thus we speak of female homogametia and male heterogamy, since the gametes are not all equal in the male.
The determination of the sex of the new organism takes place at the moment of fertilization (singular determination of sex). In other species, however, different phenomena may occur.
PHENOTYPICAL SEX
Generally, but not always, phenotypic sex corresponds to genotypic sex. There are species in which the phenotypic sex is determined by the environment (in the well-known Bonellia viridis the embryos that are implanted on the maternal organism become males, those that are implanted on the bottom become females: one speaks then of metagamic determination of sex). In other species the individual may behave first as a female and then as a male: phenotypic sex varies with age.
Phenotypic sex generally derives from the action of hormones. Also in humans an alteration of the normal level of masculinizing or feminizing hormones (due to illness, malformation or external administration) can determine phenotypic sexual characteristics that differ from genotypic sex.
NUCLEAR SEX. BODIES OF BARR. THEORY OF MARY LYON
In the female microscope observation of cells treated with nuclear dyes reveals the presence of a chromatin mass leaning against the nuclear membrane, which is missing in the male cells, called Barr's body, from the name of the discoverer. The explanation of this phenomenon came with the "theory of Mary Lyon", according to which a cell contains only one X chromosome in metabolic activity; any excess X chromosome is "inactivated" and remains spiraled even during interchanesis, and therefore can be observed under a microscope.
This is confirmed by individuals with caríotipo 47 [XXY (Klinefelter syndrome: abnormal and sterile male phenotype)], who present Barr's body while appearing masculine.
The determination of nuclear sex can therefore be used in several cases: it can reveal a Turner syndrome (45; X0, female phenotype, Barr negative) or Klinefelter; may indicate genotypic sex in the case of an indeterminate phenotype (to address any hormonal treatment in the same sense); can reveal a male individual who disguises himself as a woman to win athletic competitions in the female categories; etc.
CHARACTERS RELATED TO SEX
In some species the determination of sex is linked to the relationship between autosomes and gonosomes, which shows that autosomes also contribute to determining sex. The same can be said in the opposite direction: even gonosomes contain genes that determine non-sexual characters.
In the case of man this is especially true for the X chromosome and the characters whose locus is on that chromosome. In fact a recessive character brought by a heterozygous female does not manifest itself, but, if it is transmitted to a male child, it manifests itself. This happens because in the male the X chromosome is lacking, at least in most of its length, of a homologue that "covers" the mutations. In these conditions we speak of a hemizygous character, which evidently manifests itself in the phenotype. If the frequency of such a recessive trait in the population is low, the case of a homozygous female will be very rare. Then the character will be manifested only in 50% of the heterozygous carriers' male children, who will never transmit it to their children, while the daughters, generally heterozygous, will not show it. Classic examples of similar characters are haemophilia (which affected only the males of the imperial family of the Habsburgs), as well as the color blindness normally known as color blindness.
Edited by: Lorenzo Boscariol
