Sex Chromosomes (Allosome): Definition, Facts & Examples

All living things are classified as either prokaryotes or eukaryotes.

Prokaryotes, which include bacteria, usually consist of only a single cell and contain only a small amount of deoxyribonucleic acid (DNA), nature's universal genetic material. Eukaryotes (animals, plants and fungi) are almost all multicellular with cells rich in specialized organelles and genetic material divided into distinct chromosomes, or long strands of DNA.

Perhaps the most notable difference between prokaryotes and eukaryotes is that the former reproduce asexually while the latter reproduce sexually.

This is not a behavioral distinction, but a biochemical one. When prokaryotes reproduce, they simply divide in two to create two daughter cells identical to the parent while eukaryotes produce special cells called _gametes_ that fuse during fertilization to create a zygote that ultimately develops into a unique organism (aside from instances of identical twins, triplets and so on, whose DNA is very similar to each other's).

Chromosomes consist of a material called _chromatin, which is composed of a mixture of DNA and structural proteins called histones_. The DNA in a given chromosome is one long, unbroken nucleic acid molecule.

(The other nucleic acid found in nature is RNA, or ribonucleic acid.)

Chromosomes consist largely of _genes_, which are lengths of DNA that carry the chemical code for a particular protein product that is synthesized elsewhere in the cell. For the most part, genes on the same chromosome bear no relationship to one another in terms of the proteins they code for.

That is, individual chromosomes are not devoted entirely to specific traits such as height, eye color and so forth. There are, however, special chromosomes called sex chromosomes that carry genes that determine biological sex (male or female).

Each species has a characteristic number of chromosomes. Humans have 46, with one inherited from each parent. 46 is the diploid number in humans, whereas 23 – the number of unpaired chromosomes contributed by each gamete – is the _haploid number_.

"Everyday" cells, which account for the overwhelming majority of cells in the body, carry the diploid number in their nuclei, while only gametes have 23.

In haploid cells, 22 of the 23 chromosomes are called autosomes and are simply numbered 1 through 22. The remaining chromosome is a sex chromosome and is labeled either X or Y, referring to its general appearance under a microscope during cell division.

• If gametes carried a diploid number of chromosomes, their fusion during fertilization would create a zygote with 92 chromosomes and grind the process to a halt. The mechanism of meiosis ensures that the haploid number is maintained in organisms across generations.

Sex chromosomes, also called allosomes, are the chromosomes that contain the genes that determine biological sex. Note that this is different from the unqualified statement, "sex chromosomes determine biological sex," which is a common misconception that fails to convey the entire picture.

These chromosomes do create pairs, just as autosomes form homologous pairs. That is, the human X chromosome and the human Y chromosome become physically adjoined during meiosis, the process in which gametes (egg cells, or oocytes, in females; sperm cells, or spermatocytes, in males) are formed.

There is, however, little to no recombination between these pairs during the prophase stage of meiosis, as this would cause fatal mutations or other serious developmental issues.

The XY system is the sex-chromosome scheme used by humans, other primates and many other mammalian as well as non-mammalian animal species. Males normally contain one X and one Y chromosome, whereas genetically typical females have two X chromosomes.

By convention, normal human males are labeled 46,XY and females are labeled 46,XX, with the number being the normal diploid number.

The reason for the apparently unnecessary inclusion of this number is explained in a subsequent section.

• A label such as 46,XY is called a karyotype. Some sources omit the comma between the number and the letters (e.g., 46XY). 

In other species that employ the XY system, the situation is less straightforward, and the ratio of X chromosomes to Y chromosomes determines sex.

For example, in the species Drosophila melanogaster, XX, XXY and XXYY individuals are females, while XY and XO individuals are males. (The odd-looking combinations featuring more than two sex chromosomes or an "O" in this example are explained in a later section.)

Other animals, including many birds, reptiles, moths, butterflies and other insects, make use of the ZW system. In this sex-determination system, males are ZZ whereas females are ZW. In these organisms, then, in contrast to what is observed in the XY system, it is that males that have two copies of the same kind of sex chromosome.

The Y chromosome in males is significantly smaller than the X chromosome. The X contains approximately 1,000 specific genes, while the Y contains fewer than 80 functional genes. The end of the Y chromosome contains a critical gene known as SRY (sex-determining region of Y).

This gene is a "switch" that is responsible for activating other genes that lead to development of the testis, and thus male sex. Lack of the SRY gene leads to female development.

Experiments toward the end of the 20th century resulted not only in the clarification of SRY, but to the discovery that SRY in rare cases can be translocated to an X chromosome and trigger male development even in individuals with two X chromosomes.

This is one rationale for the word of caution about sex chromosomes per se being determinants of biological sex; it is actually a sex-determining gene almost always, but not universally, found on the Y chromosome.

Often as a result of extra or missing sex chromosomes, humans can have a chromosome number other than 46, which usually, though not always, produces recognizable clinical features, including serious health problems. A chromosome number other than 46 is known as aneuploidy.

Because the Y chromosome is sex-determining in almost all cases, its presence or absence determines the sex of individuals with sex chromosome aneuploidies. Thus the karyotypes 47,XXY and 47,XYY indicate males, while individuals with 45,X and 47,XXX are females.

Turner Syndrome (45,X): These individuals have only one sex chromosome, an X. This condition leads to abnormal growth, infertility, short stature and a lack of many typical "female" characteristics. They possess genitalia that look female, but experience stalled puberty and other abnormal forms of sexual development.

Klinefelter Syndrome (47,XXY): Because of the presence of a Y chromosome and its SRY gene, these people develop generally as males, but with altered or abnormal development of the testes development, low testosterone levels, hypogonadism, infertility (an inability to father children), hormone imbalances and other issues.

47,XXX: This version of aneuploidy causes few or no obvious symptoms except that these women are usually taller and thinner than the average woman. Because it is usually benign, this condition often goes undetected for the woman's lifetime. Some individuals, however, do experience mild learning difficulties.

As noted, this chromosome is much smaller than the X chromosome. Many of its genes are believed to have either been deleted, moved to a different chromosome, or simply deteriorated over the course of human evolution, explaining the chromosome's small size.

Some researchers theorize that this is because at some point, the Y chromosome lost its ability to recombine properly with other chromosomes, leading to its ongoing and, at least in evolutionary terms, rapid degradation.

In addition to the SRY region, some of the Y chromosome's 80 or so operational genes are responsible for fertility and – like the SRY gene itself – the regulation of other genes.

The X chromosome contains about 1,000 functional genes.

Many of these are not related to sex development and are referred to as _X-linked genes_. The inheritance of these genes differs from genes found on autosomes. The X chromosome in males always comes from the mother, since as males they have a Y chromosome that must have been passed from the father. Females, of course, have two X chromosomes.

This has powerful implications in the inheritance and manifestation of numerous conditions.

Because females have an X chromosome from each parent, if they get a recessive X-linked gene (the recessive gene being the one resulting in a given condition) from one parent and a dominant X-linked gene (the "normal" gene) from the other, they will not display that trait, because the chromosome carrying the dominant gene masks the recessive one.

If a male offspring receives the recessive X-linked gene, however, then he will show it since his Y chromosome cannot prevent its manifestation.

X-linked traits and disorders show a "skip-generational" inheritance pattern, because even though they are masked in women, they can reappear in the male descendants (i.e., sons, grandsons, and so on) of those women.

This pattern is seen in male-specific disorders and traits that run in families, such as the blood-clotting disorder hemophilia A, red-green color blindness and male pattern baldness.

Because females have twice the number of X-linked genes as males owing to have twice the number of X chromosomes, and these genes contribute to normal functioning equally in males and females, a system exists for preventing or minimizing the over-expression or different expression in females of the traits coded for by these X-linked genes. This is called X-inactivation, or lyonization.

In X-inactivation, during embryonic development, one X chromosome is randomly "turned off" in every cell, thereby ensuring that the number of X-linked genes in women is lower then that observed in men. Importantly, though, because the process is random, it is not always the same copy of the X chromosome.

The result is that different X-linked genes can be expressed in different cells in the same females.

Calico cats represent a classic example of the visible results of X-inactivation and its random nature. These cats are all female and have a patchwork of colors including white, orange and black.

The genes for black and orange are X-linked and co-dominant, meaning that neither predominates over the other and when both are present, both will be expressed.

Since females have two X chromosomes, if one contains the orange gene and one contains the black gene, X-inactivation results in a random distribution of orange and black, along with the white coloration that occurs as a result of color inactivation specified elsewhere in the cat's genome.

This "calico" pattern does not indicate a particular breed of cat, merely a trait.

Male cats that carry these color genes, because they have only one X chromosome, will display whatever color their X chromosome specifies. Thus they are either solid black or solid orange, with none of mottling observed in females of this type. That said, it is possible, though rare, for males to be calico in color; this requires aneuploidy of the sort seen in Klinefelter syndrome in humans (47,XXY).