How is one born girl or boy ?

From conception, everything is decided. If the egg is fertilized by a Y sperm, it will be a boy (XY). If it's an X, it will be a girl (XX).

The embryo develops the drafts of its genitals at the end of the first month of pregnancy. They are the same for both sexes. The boys are the first to show the difference. Seven weeks after fertilization, a gene carried by the Y chromosome, called SRY, is activated like a switch. It commands the two undifferentiated sexual glands to transform into testicles. These will therefore perform two essential roles : they will support the germ cells which will give rise to sperm, and will manufacture the male sex hormone, testosterone. This hormone will lead to the sexualization of the embryo, which then measures five centimeters. It drives the transformation of the genitals into prostate, urethra, penis, testicles.

The first gene for femininity was not identified until 2007

The sexualization of the female embryo begins in the eighth week. Do you become a "default" girl because you don't have the SRY gene? It was believed until recently.

If the SRY gene was discovered in 1990, the first gene for femininity, R-Spondin-1 (Rspo-1), was not identified until 2007. A second, FOXL2, was discovered in 2011. But we are far from having understood precisely how they work. Still, the two undifferentiated sexual glands turn into ovaries. They will take care of the germ cells which will give the eggs and produce the female sex hormones, estrogen. These hormones are involved in the differentiation of female sexual organs: the uterus, tubes, vagina, labia minora and labia majora as well as the clitoris.

The internal and external genitalia are in place at the end of the first trimester of pregnancy for both sexes. It is not until much later, during puberty, that their development is completed. However, this cascade of cleverly orchestrated phenomena which results in sexual differentiation can get out of hand at any time and irreversibly.

Exposure to chemicals, in particular endocrine disruptors such as bisphenol A or phthalates, which are widely present in the environment, can affect the development of the reproductive system.

Defects of the genitals are just the tip of the iceberg. Germ cells are also susceptible to damage and future fertility can therefore be compromised.

what are some genetic disorders that can affect the sex of a baby ?

There are several genetic disorders that can affect the sex of a baby. These disorders are caused by changes or mutations in genes that are involved in sex development. Here are some examples :

Androgen Insensitivity Syndrome (AIS): This is a condition where individuals with XY chromosomes (genetically male) are unable to respond to male sex hormones called androgens. As a result, they may have mostly female external sex characteristics or signs of both male and female sexual development.

Swyer Syndrome: This is a condition that affects sex development and is characterized by individuals with XY chromosomes (genetically male) developing female reproductive structures. It is caused by variants in genes such as DHH and NR5A1, which disrupt the process of sex development.

Sex-Linked Disorders: These are genetic disorders caused by changes in genes located on the sex chromosomes. For example, hemophilia is a sex-linked disorder caused by a faulty gene on the X chromosome.

Chromosomal Abnormalities: Certain chromosomal abnormalities can affect the sex development of a baby. For example, individuals with Turner syndrome have only one X chromosome (45,X) instead of the usual two (46,XX), which can result in underdeveloped or absent ovaries and infertility.

It's important to note that these genetic disorders are relatively rare and not all babies with these conditions will have ambiguous genitalia or atypical sex development. The diagnosis and management of these disorders require medical evaluation and genetic testing by healthcare professionals