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Y Chromosome: Definition, Structure, Functions and Forensic Significance.

Y Chromosome Definition

The Y chromosome is one of 46 (23 pairs) chromosomes found in humans, as well as many other animals. The Y chromosome is one of the two sex chromosomes, with the X chromosome being the other.

Individuals usually have two types of sex chromosomes. The Y chromosome is only found in men, and they also get one X chromosome (XY). Females possess two X chromosomes in general (XX). There are, however, several exceptions, which are mentioned further below.

Y Chromosome Genetics

Inheritance of the Y chromosome

The Y chromosome, like the other chromosomes, is passed down to individuals during fertilization, when two gametes (eggs in females, sperm in males) combine. Both gametes have one copy of every chromosome (23 chromosomes). When they combine, they form a zygote with 23 pairs of chromosomes (a total of 46 chromosomes) that will grow into a new person.

An XX female can exclusively pass on an X chromosome to her kids, while an XY man may pass on either an X or a Y chromosome. The person will be XX and typically female, only when the sperm that fertilizes the egg contains an X chromosome; if the sperm has a Y chromosome that fertilizes the egg, the individual will be XY and typically male.

Structure of the Y chromosome

The Y chromosome is still the smallest of all the human chromosomes. It comprises 59 million nucleotides of DNA and accounts for around 2% of a man’s total DNA. To put things in perspective, humans have 249 million nucleotides on their biggest chromosome (chromosome 1).

On the Y chromosome, only about 70 protein-coding genes are found. The SRY (sex-determining region Y) gene, which commences male development, is one of the most important.

Y Chromosomal Adam

Y chromosome Adam is the name offered to the most recent common ancestor of all human men, who lived around 200,000 years ago. He wasn’t the only man living at the time, of course. All males living today, however, have a Y chromosome that can be linked back to this one man.

Haplogroups are defined by mutations in the Y-DNA. Such haplogroups represent an individual’s paternal lineage’s ancestral beginnings.

Functions of the Y chromosome

Determining the Male Phenotype

The Y chromosome basically only exists to determine the male phenotype, while the X chromosome has many distinct genes with varied roles.

A growing embryo’s default state is female. The SRY gene begins to activate about six weeks into embryonic development. This gene is known as the “master switch” in mammalian sex determination. The gene product, the SRY protein (also known as the testis-determining factor (TDF)), is produced when the gene is turned on. This protein is a DNA-binding factor that connects to other transcription factors and increases their expression, triggering a signalling cascade that initiates male development.

SOX9 is the most essential SRY-controlled factor. When this gene is turned on, it starts the crucial events that occur throughout male development. The production of the sex cords that grow into the testes, as well as the inhibition of the development of feminine traits,

Even if the person had a Y chromosome, elimination of the SRY gene (or any means of its inactivation) would result in female development.

Y Chromosome Other Roles

Other functions of the Y chromosome include male fertility and, maybe unexpectedly, hearing.

Aziospermia factors a, b, and c (AZFDa, AZFDb, and AZFDc) are three areas on the Y chromosome that are required for normal sperm formation. Infertility is linked to gene deletions in these locations owing to the lack of sperm as well as other problems in germ cell development.

Furthermore, mutations in the DFNY1 gene have been related to male-specific deafness, which is coupled with a complicated chromosomal and chromosome 1 rearrangement.

Y Chromosome Clinical Relevance

Among the remaining chromosomes, changes to the Y chromosome seem to have very minor functional implications (outside a few key genes, such as SRY). The Y chromosome, on the other hand, is linked to a number of sex-chromosomal diseases. 

Sex-Chromosome Disorders

In humans, having the wrong number of chromosomes is extremely unusual. There are some exceptions to this concept, such as Down Syndrome, which is caused by three copies of chromosome 21, and Edward Syndrome, which is caused by three copies of chromosome 18.

Multiple copies of the sex chromosomes, on the other hand, are considerably more likely to end in a live delivery. As a result, there are many distinct genotypes and phenotypes that may exist both between and within these illnesses. The Y chromosome is shown in the following illustrations.

  • Turner Syndrome (XO): Turner Syndrome is characterized by the lack of one of the sex chromosomes, resulting in a total of 45 chromosomes in the afflicted individual. These people are phenotypically female, but their development is abnormal, resulting in small height, infertility, and various health issues.
  • Klinefelter Syndrome (XXY): Klinefelter Syndrome is characterized by the inclusion of an additional X, resulting in a total of 47 chromosomes. The person still has a Y chromosome and those who are phenotypically masculine, but their development has been disrupted, and they may have extra health issues.
  • XYY Syndrome: XXY syndrome, also known as YY syndrome, is a rare condition in which a man has an additional copy of the Y chromosome. As a result, they have 47 chromosomes, much like people with Klinefelter syndrome. Individuals who are affected are often tall and may have intellectual difficulties. During their adolescent years, they are also prone to severe acne. In other situations, however, the signs are not visible, and the afflicted people grow normally. As a consequence, many people who are impacted do not get treatment till later in life.
  • XX male syndrome: XX male syndrome is a rare condition caused by abnormal recombination of the X and Y chromosomes in the gametes of the father. As a consequence, one of the X chromosomes has a part of the Y chromosome attached to it. Male development will begin if this region has the SRY gene. As a consequence, despite having an XX genotype, the person will be phenotypically male.

Androgen Insensitivity Syndrome

Androgen insensitivity syndrome is an unusual condition in which a person carries a male genotype (XY) but does not completely develop the male phenotype, instead displaying exclusively feminine traits. This occurs because their cells do not react to the effects of male sex hormones in the way that they should.

This condition may be of varying degrees of severity. Male genitalia, female genitalia, or genitalia with mixed male and female traits are all possible outcomes of partial androgen insensitivity. Complete androgen insensitivity, on the other hand, complete androgen insensitivity results in people developing outward feminine traits but with abnormally small male testes and no uterus. It’s fairly uncommon for complete androgen insensitivity to be undetected until adolescence.

Loss of Y chromosome (LOY)

Some men’s cells may lose their Y chromosomes as they age. This causes a sort of genetic mosaicism, in which cells from the same person have various genotypes. Men who smoke are likely to be at a higher risk, and LOY has been related to a shorter life expectancy.

Y Chromosome Forensic Significance

In forensic research, the Y chromosome may be valuable. When investigators are looking into sexually motivated crimes committed by a man against a woman, Y-chromosome analysis is often employed. This is due to the Y chromosome, which permits male DNA to be isolated from the DNA of the female victim.

Because sex chromosomes do not undergo recombination, the Y chromosome has less potential to discriminate between people than autosomal DNA. This indicates that the sequences are passed down in the family and are only modified over time by mutations. The haplotype refers to these hereditary mutations. Men with similar haplotypes are members of the same haplogroup. This sort of study may aid in tracing ancestral roots all the way back to Adam’s Y chromosome.

While this is useful for tracking lineage and discovering previously undiscovered links, it is very hard to distinguish between related people based just on Y chromosome sequencing. In forensic investigations, however, it may be used to rule out suspects, provide fresh leads, and establish the perpetrator’s paternal ancestry.

References

  1. Jobling, M. A., & Tyler-Smith, C. (2017, August 1). Human Y-chromosome variation in the genome-sequencing era. Nature Reviews Genetics, Vol. 18, pp. 485–497. https://doi.org/10.1038/nrg.2017.36
  2. Kayser, M. (2017). Forensic use of Y-chromosome DNA: a general overview Making the case with a case. Human Genetics, 136, 621–635. https://doi.org/10.1007/s00439-017-1776-9
  3. Quintana-Murci, L., & Fellous, M. (2001). The human Y chromosome: The biological role of a “functional wasteland.” Journal of Biomedicine and Biotechnology, Vol. 2001, pp. 18–24. https://doi.org/10.1155/S1110724301000080.
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