Chromosomes are microscopic structures made up of nucleic acids and proteins; they are found in the nucleus of most living cells and are responsible for carrying a person’s genetic information. The human cell contains a total of 23 pairs of chromosomes, 22 of which are autosomes and 1 pair is the sex chromosomes. Autosomes are those chromosomes that are not sex chromosomes (Veeramah and Krishna 2267) The sex chromosomes are designated as X and Y where females have two X chromosomes, while males have one X and one Y chromosome. The female ovum only contains X chromosomes; on the other hand, the male’s sperm carries Y chromosome on one half, while the other half carries X chromosome. When the sperm meets the ovum, only one chromosome will be used, either the X or the Y chromosome. The chromosomal arrangement gotten after mating determines the sex of the child. The Y chromosome holds a very limited number of chromosomes when compared to other human chromosomes. This has been attributed to the fact that the Y chromosome has been known to degenerate during evolution. What is more, 50% of the sequences in the Y chromosome are made up of repeated elements. The Y chromosome is composed of two parts: the pseudo autosomal region and the euchromatic and heterochromatic regions. The pseudoautosomal region is located on the short arm of the chromosome, and this is where the Y chromosome exchanges genetic material with the X chromosome (Mangs and Morris 129). The euchromatic and heterochromatic region, on the other hand, makes up 95% of the Y chromosome, but it is genetically inert since it contains highly repetitive genetic sequences. Most of the genes carried by the Y chromosome are therefore related to sex and fertility. However, the X chromosome carries more genes than the Y chromosome, and very few of these genes are related to sex. Morphologically, the X chromosome is big with one short arm and a long arm; it makes up about 5% of the total cells in DNA. The X chromosome carries several genes which are responsible for protein synthesis; these proteins then perform different activities in the body (Cunningham et al. 1090).
As earlier discussed, females have two sets of the X chromosome. If both X chromosomes were left active, females would have more gene expression than normal; thus, X-inactivation occurs to prevent this excessive expression of genes. Moreover, X-inactivation is important to strike a balance between the variations in gene quantity between XY and XX; hence, we conclude that the sole purpose of X-inactivation is to correct the imbalance of genes linked to the X chromosome which otherwise would be toxic if left unchecked. Therefore, only one X chromosome is active in the cells of females. The inactivated X chromosome remains inactive throughout the lifetime of the cell and also in all the descendants of that cell (Kalantry 2).
The mechanism through which the X chromosome is inactivated is highly coordinated and complex and results in the formation of an inactive and densely packed X chromosome, also referred to as the Barr body. The X chromosome inactivation (XCI) system is made up of numerous gene elements that are crucial to initiating the transcription of long non-coding RNAs involved in X-inactivation. In humans, XCI requires an accumulation of non-translated XIST-RNA which is found as a coating on the X chromosome (Peeters and Cotton 746). The XIST-RNA accumulation is followed by several epigenetic changes on the X chromosome hence resulting in a silent X chromosome. Two different forms of XCI exist, random XCI and imprinted XCI. Imprinted XCI is not found in humans but rather in female marsupials and mice placental tissues; here, the inactivation is selective where the X chromosome from the paternal side is inactivated. However, this paternal X chromosome is later activated in the development cycle. Random XCI is found in all eutherian mammals including humans. In random XCI, there is no preference for inactivation, and, therefore, either the maternal or paternal X chromosome may be inactivated (I. M. van den Berg and J. S.E. Laven 2009).
Consequently, the XIST-RNA is the key player in XCI. XIST-RNA recruits numerous silencing proteins which work together to produce the dormant X chromosome. However, a number of genes in the X chromosome flee from the inactivation, several of which are situated at the terminal of every section of the X chromosomes in regions regarded as the pseudoautosomal areas (I. M. van den Berg and J. S.E. Laven 2009).
X-linked Diseases (Cardiovascular Diseases)
The inheritance of genes on the X chromosome follows special rules due to a number of factors. These special rules arise since males have only one X chromosome, and almost all the genes on the X chromosome have no counterpart on the Y chromosome. Therefore, genes on the X chromosome even if recessive in females will be expressed in males. Genes that are inherited through the Y and X chromosome are said to be sex-linked genes.
An example of an X-linked disease is cardiac valvular dysplasia. This disease is characterized by abnormal development of heart valves. The illness has been linked to mutations in the FLNA gene carried on the X chromosome (Corrado et al. 442). As such, females transfer this defective gene to their offspring’s who then develop the cardiac valvular dysplasia. Males have only one copy of the X chromosome; as a result, a mutation in this chromosome is enough to cause the condition. In females, however, the defective gene has to be found in both copies of the X chromosome for it to cause the condition. Females with the defective gene on one chromosome fail to show any severe symptoms, and, hence, they act as carriers of the disease. In this way, such a female carrier can transfer the condition to her offspring. Males, on the other hand, cannot pass X-linked traits to their sons since they inherit the Y chromosome (Crespi 449).
Other diseases linked to the X-chromosome include hyperlipidemia and the Turners syndrome (Rao et al. 590). Familial combined hyperlipidemia (FHCL) has been linked to the X chromosome through studied conducted on Dutch families (Balderman and Marshall 5). Turner’s syndrome, on the other hand, is a defect experienced by females where one of their X chromosomes is altered or missing entirely hence leading to abnormal growth.
MSY (Male-Specific Region of the Y chromosome) and Y-linked Diseases
The male-specific region of the Y chromosome (MSY) makes up to 95% of the Y chromosome’s length. This region is responsible for the sequence of events that lead to the formation of a male embryo. However, very few genes in the MSY region have been found to be functional; most of the region is barren in terms of genetic functions (heterochromatin). Functional genes are found in the euchromatic region where some of these genes are encoded for proteins used by all cells, while the others seem to encode functions only carried out in the male testes. A key part of the euchromatic region is the SRY (sex-determining region Y) located on the short arm of the Y chromosome just below the pseudoautosomal region (Charlesworth 1339).
A number of diseases have been linked to the Y chromosome. They include hypertension as well as other coronary diseases. A study conducted by Fadi (2012) demonstrated that men having a certain mutation on the Y chromosome were more likely to develop heart disease compared to other men. This was independent of other risk factors such as smoking and hypertension. Furthermore, more data have shown that the Y chromosome is linked with inheritance of high blood pressure, the concentrations of LDL and HDL in the blood as well as a paternal history of coronary artery disease.
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