Mechanisms[ edit ] All animals have a set of DNA coding for genes present on chromosomes. In humans, most mammals, and some other species, two of the chromosomes , called the X chromosome and Y chromosome , code for sex.
In these species, one or more genes are present on their Y chromosome that determine maleness. In this process, an X chromosome and a Y chromosome act to determine the sex of offspring, often due to genes located on the Y chromosome that code for maleness.
Offspring have two sex chromosomes: Humans[ edit ] Human male XY chromosomes after G-banding In humans, half of spermatozoons carry X chromosome and the other half Y chromosome. Presence of this gene starts off the process of virilization. This and other factors result in the sex differences in humans. The inactivated X chromosome remains within a cell as a Barr body. Humans, as well as some other organisms, can have a chromosomal arrangement that is contrary to their phenotypic sex; for example, XX males or XY females see androgen insensitivity syndrome.
Additionally, an abnormal number of sex chromosomes aneuploidy may be present, such as Turner's syndrome , in which a single X chromosome is present, and Klinefelter's syndrome , in which two X chromosomes and a Y chromosome are present, XYY syndrome and XXYY syndrome.
Other animals[ edit ] In most mammals, sex is determined by presence of the Y chromosome. Not all male-specific genes are located on the Y chromosome. Other species including most Drosophila species use the presence of two X chromosomes to determine femaleness. One X chromosome gives putative maleness. The presence of Y chromosome genes is required for normal male development.
Sex determination system Birds and many insects have a similar system of sex determination ZW sex-determination system , in which it is the females that are heterogametic ZW , while males are homogametic ZZ. Many insects of the order Hymenoptera instead have a system the haplo-diploid sex-determination system , where the males are haploid individuals which have just one chromosome of each type , while the females are diploid with chromosomes appearing in pairs.
Some other insects have the X0 sex-determination system , where just one chromosome type appears in pairs for the female but alone in the males, while all other chromosomes appear in pairs in both sexes. After the discovery of the testis-determining gene SRY , many scientists shifted to the theory that the genetic mechanism that causes a fetus to develop into a male form was initiated by the SRY gene, which was thought to be responsible for the production of testosterone and its overall effects on body and brain development.
This perspective still shares the classical way of thinking; that in order to produce two sexes, nature has developed a default female pathway and an active pathway by which male genes would initiate the process of determining a male sex, as something that is developed in addition to and based on the default female form.
However, In an interview for the Rediscovering Biology website,  researcher Eric Vilain described how the paradigm changed since the discovery of the SRY gene: For a long time we thought that SRY would activate a cascade of male genes.
It turns out that the sex determination pathway is probably more complicated and SRY may in fact inhibit some anti-male genes. The idea is instead of having a simplistic mechanism by which you have pro-male genes going all the way to make a male, in fact there is a solid balance between pro-male genes and anti-male genes and if there is a little too much of anti-male genes, there may be a female born and if there is a little too much of pro-male genes then there will be a male born.
We [are] entering this new era in molecular biology of sex determination where it's a more subtle dosage of genes, some pro-males, some pro-females, some anti-males, some anti-females that all interplay with each other rather than a simple linear pathway of genes going one after the other, which makes it very fascinating but very complicated to study.
In mammals, including humans, the SRY gene is responsible with triggering the development of non-differentiated gonads into testes, rather than ovaries. However, there are cases in which testes can develop in the absence of an SRY gene see sex reversal. In these cases, the SOX9 gene, involved in the development of testes, can induce their development without the aid of SRY. Even so, the absence of the SRY gene or the silencing of the SOX9 gene are not enough to trigger sexual differentiation of a fetus in the female direction.
A recent finding suggests that ovary development and maintenance is an active process,  regulated by the expression of a "pro-female" gene, FOXL2. In an interview  for the TimesOnline edition, study co-author Robin Lovell-Badge explained the significance of the discovery: We take it for granted that we maintain the sex we are born with, including whether we have testes or ovaries. But this work shows that the activity of a single gene, FOXL2, is all that prevents adult ovary cells turning into cells found in testes.
Implications[ edit ] Looking into the genetic determinants of human sex can have wide-ranging consequences. Scientists have been studying different sex determination systems in fruit flies and animal models to attempt an understanding of how the genetics of sexual differentiation can influence biological processes like reproduction, ageing  and disease.
Maternal[ edit ] In humans and many other species of animals, the father determines the sex of the child. In the XY sex-determination system, the female-provided ovum contributes an X chromosome and the male-provided sperm contributes either an X chromosome or a Y chromosome, resulting in female XX or male XY offspring, respectively.
Hormone levels in the male parent affect the sex ratio of sperm in humans. Human ova, like those of other mammals, are covered with a thick translucent layer called the zona pellucida , which the sperm must penetrate to fertilize the egg. Once viewed simply as an impediment to fertilization , recent research indicates the zona pellucida may instead function as a sophisticated biological security system that chemically controls the entry of the sperm into the egg and protects the fertilized egg from additional sperm.
However, not all sperm are positively impacted; some appear to remain uninfluenced and some actually move away from the egg. If, then, the male element prevails it draws the female element into itself, but if it is prevailed over it changes into the opposite or is destroyed.
Aristotle claimed that the male principle was the driver behind sex determination,  such that if the male principle was insufficiently expressed during reproduction, the fetus would develop as a female.
Ford and his team, in the wake of Jost's experiments, discovered  that the Y chromosome was needed for a fetus to develop as male when they examined patients with Turner's syndrome , who grew up as phenotypic females, and found them to be X0 hemizygous for X and no Y.