Chromosomes are thread-like structures, located in the nucleus, which carry hereditary information in the form of genes. Structurally, it is composed of ‘packaging proteins and DNA molecules’, and it is represented in different shapes and numbers which vary among different living organisms.
For example, bacteria have circular chromosomes, whereas organisms like humans and other animals have linear chromosomes (humans have linear chromosomes in the nucleus and circular chromosomes in mitochondria).
Similarly, the number of chromosomes also varies among different organisms:
| Name of organisms | Number of Chromosomes |
|---|---|
|
Human |
46 (23 pairs) |
|
Fruit fly |
8 (4 pairs) |
|
Monkey |
48 (24 pairs) |
|
Rat |
41 (21 pairs) |
Chromosomes are of two types: autosomes and sex chromosomes. This article is focused on the theory and facts of human sex chromosomes.
It includes every aspect of human sex chromosomes from the mechanism of sex determination and X-inactivation in humans to the sex chromosomes aneuploidy. It also provides a glimpse of how the mechanisms in humans differ from other organisms.
Ever wondered how you developed into a girl or a boy? Well, that is determined by the sex chromosomes of your parents.
Humans have a total of 23 pairs of chromosomes (two sets of chromosomes). One set of the chromosome is inherited from the mother and the other set from the father. Out of 23 pairs of chromosomes, 22 pairs are autosomes and 1 pair is sex chromosomes.
Autosomes function in the regulation of somatic characteristics of the individual (e.g. body weight, length of the body, etc.) whereas sex chromosomes are involved in the “sex determination” of the organisms and regulation of sex-linked traits.
In humans, sex is determined by two types of chromosomes, X and Y chromosomes. The X chromosome is three times larger than the Y chromosome. Moreover, it contains over 900 genes and the Y chromosome has only about 55 genes on it.
Figure: The illustration of the 23 pairs of chromosomes in humans (22 pairs autosomes and one pair sex chromosome)[6].
Source: https://www.genome.gov/genetics-glossary/Sex-Chromosome
Hermann Henking was the first to discover the X chromosome (in 1981). He named it the X element but he could not understand its role. In 1905, two scientists, Nettie Stevens and Edmund Beecher Wilson worked individually to study the role of sex chromosomes (on insects).
Stevens reported the presence of two large chromosomes in the beetle gonads (female), and one small and one large chromosome in the male sperm. She concluded that the small chromosomes carried by the sperm, determine the sex of the male[5].
Steven’s study was later supported by Wilson, that both large and small chromosomes are sex determinants and the small chromosome determines the maleness of an organism. He named the large chromosome “X” and the small one “Y”[5].
Later, scientists observed the absence of the small chromosome in wasps. It caused a dilemma, whether the presence or absence of the Y-chromosome is responsible for the maleness in organisms! Wilson resolved this by studying data from different labs. He concluded that two types of ‘sex determination systems’ exists among organisms.
Humans have an “XY system” of sex determination. The individual, having two X chromosomes (XX) develops into a female while the one with a pair of XY chromosomes will develop into a male.
In a few human cases, sex chromosome aneuploidy can also be observed. For example, an individual can be born with 47, XXY; 47, XYY; 47, XXX; and 45, X[4]– these kinds of chromosomal arrangements cause several abnormalities in the organisms.
But, an individual having a Y-chromosome (no matter how many X chromosomes are present) will always develop into a male. On the other hand, an individual without Y chromosomes will develop into a female.
Moreover, in females, one X chromosome is inherited from the mother and the other X from the father while in males X chromosome is always inherited from the mother[7].
Genes are the key regulators of all the features of the organisms so their role in sex determination cannot be neglected. However, in a few other organisms, there are some other factors, other than sex chromosomes, that decide the sex of the organisms. For example, in European turtles, sex is determined by the temperature of the surroundings.
The human embryo contains two types of duct, Mullerian duct, and Wolffian duct, in the seventh month of development. These ducts can develop a female or male reproductive tract. But, these ducts can not do it alone (by themselves). So how does it happen?
The SRY (Sex determining Region Y) gene, present on the short arm of the Y chromosome, encodes the Testes Determining Factor (TDF) which induces the formation of the testes.
The testis produces two types of hormone, anti-mullerian hormone (produced from Sertoli cells of the testis), and testosterone (from Leydig cells which are induced by Sertoli cells)[4].
The testosterone and its derivative dihydrotestosterone further induce the formation of organs of the male reproductive system whereas the anti-mullerian hormone (AMH) suppresses the development of the female reproductive tract.
When there is an absence of the Y chromosomes (in females), the SRY gene is not expressed and the pathway of ovary formation is activated. The ovary then secretes the female sex hormone, the estrogen, which activates the Mullerian duct that leads to the development of female reproductive organs (uterus, cervix, and oviducts)[4].
Figure: An illustration of a similar sex-determination system in humans and Drosophila melanogaster.
When the X:A ratio is ≥ 1.0 (1X:1A, 2X:2A, 3X:3A, and 3A:2A) then the fly will develop into a female[3]. On the other hand, when the ratio of X:A is ≤ 0.5, then a male will be developed.
An important fact about sex chromosomes is that the X chromosome is larger than the Y chromosome in size and contains 1000 genes more than the Y chromosome.
You know that a female has two X chromosomes and a male has only one. So, it is expected to see significant differences in gene expression, in both the sexes. But, the picture is quite the opposite! So, the question arises, “why is this difference not observed?”
The answer to the above question is “dosage compensation”.
When one sex of organism contains two dosages of one sex chromosome and the other sex has only one, then the difference in the expression level is achieved by a mechanism called dosage compensation. Variant mechanisms of dosage compensation, of X-linked genes, are evolved by different organisms.
In Humans (and other mammals as well), dosage compensation is achieved by the inactivation of the majority of genes, localized on one of the two X chromosomes in females, during early development. The inactivation of the X chromosome is a random process, either maternal or paternal X-chromosome (in female) is randomly inactivated.
The key gene that is involved in the process of the X-inactivation gene is the Xist (X-inactive-specific transcript) gene, present in the Xic region on the X chromosome[2]. The expression of Xist is asymmetrical in two X chromosomes.
The chromosomes having a higher expression of genes produce higher transcripts of Xist RNA[2]. The RNA, with some binding proteins, coats one of the two X chromosomes along its length and causes repression.
The inactivated X chromosome structurally becomes small, dense, and highly compact (condensed chromatin). This structure is called “Barr bodies”, named after its discoverer, Murray Barr. The feature of the presence and absence of Barr bodies is used in labs for the sex determination of the individual.
Figure: The random X chromosome inactivation in the mouse during embryogenesis[2].
Source: DOI:10.1016/bs.ircmb.2015.03.001.
Not all organisms follow a similar mechanism of dosage compensation. Researchers have found two other mechanisms of dosage compensation in other organisms which completely differ from humans!
Figure: Illustration of difference in the mechanisms of dosage compensation in different organisms[1].
Aneuploidy is a condition of the presence of less than or more than the normal number of chromosomes. Sex chromosome aneuploidy is a condition in which the number of sex chromosomes alters in an organism.
For example, normally an organism has XX (in females) and XY (in males) sex chromosomes while during aneuploidy condition he/she can have XXX, XXY, XYY, XO, etc. type of chromosomal arrangements. The most common cause of sex chromosome aneuploidy is non-disjunction during the process of cell division (meiosis).
The aneuploidy condition leads to several abnormalities in the organism. A few examples of diseases[9] caused due to sex chromosome aneuploidy are given below:
Sex chromosomes are involved in the sex determination of organisms and the regulation of sex-linked traits. Various studies have also shown the involvement of other factors in the sex determination of other organisms.
However, humans have X and Y sex chromosomes, which are the only determining factors of the sex of a newborn human.
The presence and/or absence of the Y chromosome determine the maleness of the organism, while the inequality of the gene expression between males and females is balanced by the mechanism of X-inactivation.
If the normal number of sex chromosomes (XX or XY) is altered in an individual, it leads to several abnormalities in the affected human. Further research in this area will add more understanding to the sex linked genes, their inheritance, and the mechanism behind sex determination.
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