To understand how the sex of any individual is determined we need to start at the formation of the gametes (eggs and sperm). Gametes are produced by a type of cell division known as meiosis, or reduction division. Every cell (except the gametes) in our bodies contains 23 pairs of chromosomes. One of each pair came from our mother and one from our father. There are 22 pairs of autosomes and one pair of sex chromosomes.
These chromosomes contain the 20,000 – 25,000 genes that control our appearance and physiology. Between the genes lie non-coding segments which form the bulk of the DNA and used to be thought of as mostly “junk”. However, it is now known that at least 80% of these non-coding segments have essential controlling roles (An integrated encyclopedia of DNA)
Meiosis separates the chromosome pairs – one chromosome from each pair is allocated to “intermediate cells” (primary oocytes and spermatocytes), but not before pieces of information have been swapped between the pairs (known as crossing over – see below). The primary oocytes and spermatocytes which contain a mix of chromosomes from our mother and father divide again. The result is that each gamete has half the genetic information found in the parent cells (23 chromosomes) and this set of information is different from other gametes, no two sperm are identical, neither are any two ova.
Meiosis
The stages of meiosis are shown in the image below. A detailed description of the process can be found here.
meiosis_yourgenomeImage from: www.yourgenome.org/facts/what-is-meiosis
In this greatly simplified version there is only one chromosome pair represented by the dark blue and light blue chromosomes. Humans have 23 pairs of chromosomes, the alignment of the pairs in metaphase I (Independent Assortment) is random. There are 8,324,608 possible combinations of the 23 chromosome pairs. This plus crossing over and random fertilization are key to variation and result in a unique combination of genes in each fertilized egg.
The process of meiosis is carefully controlled and coordinated to ensure that the gametes have just one copy of each of the chromosomes, but errors can happen (see Abnormalities in Gametogenesis).
Crossing Over
Each of the chromosomes in primary oocytes and spermatocytes is duplicated before meiosis begins – given the familiar “X” shape that we associate with chromosomes. These identical chromatids remain joined together throughout the first meiotic division (meiosis I), but are separated in the final stages of meiosis. When the homologous pairs come together in the Prophase I of meiosis, crossing over occurs between chromatids of the homologous pairs. Crossing over normally only occurs between equivalent sections of the chromosomes (shown in the diagram as A & a). Each chromosome contains dozens to thousands of different genes. The total possible combination of different versions of these genes (alleles) in humans is approximately 70,368,744,177,664. .
Crossing Over in the Sex Chromosomes
xx-male1Crossing over usually only occurs at 2 regions on the sex chromosomes, these are labelled PAR1 and PAR2 and are located at the ends of both the chromosomes. These regions do not contain any sex-specific chromosome. In very rare circumstances (1:20.000), the SRY region of the Y chromosome crosses over to the X, this may result in an males are phenotypically and psychosexually male 46,XX male syndrome is www.urology-textbook.com/46XX-males.html
Abnormalities in Gametogenesis
Normally, oogenesis produces ova with 22 autosomes and 1 X-chromosome, and spermatogenesis produces sperm with 22 autosomes and either an X-chromosome or Y-chromosome. Abnormalities in segregation of the chromosomes, known as nondisjunction, can result in gametes with either too many or too few of a particular chromosome. Embryos which have more than or fewer than 46 chromosomes are usually not viable, however there are exceptions such as Down’s syndrome in which individuals have 3 copies (trisomy) of chromosome 21; Patau syndrome, trisomy of chromosome 13; or Edwards syndrome; trisomy of chromosome 18. Nondisjunction of the sex chromosomes can also occur leading to anomalies such as:
Klinefelter syndrome – extra X chromosomes in males – i.e. XXY, XXXY, XXXXY, etc.
Turner syndrome – lacking one X chromosome in females – i.e. X0
Triple X syndrome – an extra X chromosome in females
XYY syndrome – an extra Y chromosome in males.
More information can be found here
Gamete Formation
In females, the total number of eggs ever to be produced are present in the newborn female. These are arrested at an early stage of meiosis I from foetal life through childhood until puberty, when surges in luteinizing hormone (LH) stimulate the resumption of meiosis. Following puberty, during each menstrual cycle, Follicle Stimulating Hormone (FSH) stimulates completion of meiosis I (usually in 1 but occasionally in more oocytes) the day before ovulation. The second part of meiosis which results in a mature ovum does not occur unless the ovum is fertilized. In females the meiotic divisions are uneven resulting in the production of a single ovum from each primary oocyte, and 1-3 polar bodies. In males, sperm production starts at puberty and continues throughout life. Division of each Spermatocyte will produce 4 spermatozoa (sperm).
gametogenesis
Fertilization restores the number of chromosomes to 46; 44 autosomes and either XX or XY. Hence the sex of the offspring is determined at fertilization. The zygote (fertilized egg) is totipotent (capable of forming any body cell) and is the progenitor of all of the cells in the off-springs body (plus the placenta and foetal membranes).
To summarise:
Sex is determined at fertilisation
99% human zygotes are XX or XY
The totipotent zygote is the progenitor of all cells in the individual
All ♂ cells have XY chromosomes
All ♀ cells have XX chromosomes
X cannot be changed to Y
Men cannot become women
If an egg is fertilised by an X-bearing chromosome, the offspring will be ♀.
If the sperm is Y-bearing, the offspring will be ♂.
This occurs >99% of the time. X & Y chromosomes are totally different & in no way interchangeable
“Primary sex determination is the determination of the gonads. In mammals, primary sex determination is strictly chromosomal and is not usually influenced by the environment. In most cases, the female is XX and the male is XY. Every individual must have at least one X chromosome. Since the female is XX, each of her eggs has a single X chromosome. The male, being XY, can generate two types of sperm: half bear the X chromosome, half the Y. If the egg receives another X chromosome from the sperm, the resulting individual is XX, forms ovaries, and is female; if the egg receives a Y chromosome from the sperm, the individual is XY, forms testes, and is male.” (From Developmental Biology by Scott Gilbert)
Detailed current information can be found here: embryology.med.unsw.edu.au/embryology/index.php/Fertilization