Human genetic makeup is a sophisticated phenomenon because it introduces unique traits of an individual which combines genetic information received from parents. Talking about a chromosome, one should consider the role of an X and a Y chromosome in shaping the genome of a human. At this point, Ridley (2006) assumes that a gene is the major carrier of imprinted information from a mother and a father. It uses memories of the parents and creates a genetic makeup which differentiates a human in terms of physical appearance and character traits. Therefore, the X and Y chromosomes play role both in sex determination and genetic evolution of a person due to the constantly developing genetic profiles.
An in-depth exploration of the X and Y chromosomes and their related conditions has revealed a number of contradictions which introduce new theories regarding genetic variations. Apparently, X chromosomes might contain DAX gene, which produces new theories about sex determination. For instance, sometimes, people are born with two copies of DAX genes, and as a result, they are developed into normal women, while their genome is male. As a result of the antagonism, there is a danger of developing the theory about the necessity of presence of an X and a Y chromosome in shaping the sex of a future child. Nonetheless, both chromosomes should be present in the development of the male genome. However, the presence of different ratio between genes varies from species to species.
The evolution of new genes is supported by many facts that deliberate on the sex ratio among females and males. Specifically, it is logically that the ratio of X chromosomes equals 75%, whereas Y chromosomes are only 25%. In other words, the X chromosome spends the most of its time in females and only some time in males. As Ridley (2006) assumes, “the X chromosome is three times as likely to evolve the ability to take pot shots at the Y as the Y is to evolve the ability to take pot shots at the X” (p. 111). Therefore, the Y chromosome is more vulnerable to the attacks of the newly developed X chromosome. Moreover, there is a threat of the extinction of the Y chromosome because it will serve no purpose in determining gender. As a result, it is highly essential to explore other alternative routes to evolution that the nature has invented to avoid the problem.
In order to define how the evolution of chromosomes affects sex determination and genetic profile, the attention should be given to the evolutionary consequences of sex-biased gene expression. Specifically, Parsh and Ellergen (2013) have focused on the physical traits that define male and female affiliation. The scholars attached much importance to sexual dimorphism and have revealed the extent and nature of sex-biased gene expression in different species, including the development of sexual antagonism as well as insufficient dosage compensation. The researchers have also introduced the sex-biased advances which can foster the evolution of sex-related genes.
Some of the scholars have paid specific attention to the study of Y chromosome evolution which introduces the emerging insights into the process of degeneration of the Y chromosome. Specifically, Bachtrog (2013) assumes that the Y chromosome requires detailed consideration because it focuses on the “master switch” gene defining sex as well as possesses a complicated evolutionary history. In particular, the Y chromosome originates from an autosome and its development has been featured by significant gene decay. The current image of a genome and transcriptome evaluation of Y chromosomes has introduced new advances of the current genome analysis of the Y chromosome, including its long-term development and nature. Furthermore, a comparative evaluation of the molecular and evolutionary forces triggers Y chromosome degradation, leading to the evolutionary destiny of the chromosome.
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The analysis of Y chromosome is essential because it provides more accurate predictions with regard to the ways in which sex determination in males develops. The role of the Y chromosome is important because it is the only gene which takes part in sperm production. In this respect, Hughes and Rozen (2012) have explored the evolution of Y chromosomes in mammals to show that their genomic landscapes are repetitive, creating difficulties in defining their development. At this point, the knowledge of the sophisticated structural organization of the Y chromosome in humans is essential because it can provide a deeper understanding of the mechanisms taking part in disease-causing mutations. Variations and mutations of human Y chromosome have been a valuable tool for defining the relationships among humans. The in-depth comparison of human Y chromosome sequences and other primates have highlighted the aspects of the chromosome development. The scholars have also discovered that the future sequencing of chromosomes will create a foundation for more comprehensive studies of Y chromosome evolution as well as their importance in reproduction.
With regard to the combination of the X and Y chromosome, there should be the chromosomal arm, which is also called the pseudo-autosomal region (PAR), which contains a dozen of genes existing as the copies of both chromosomes. By means of the region, the X and Y chromosomes are connected to form the male meiosis. In case the exchange between the chromosomes does not take place, further deletion of the PAR could lead to male sterility. The discussion of Y chromosome evolution should also be supported with the discussion of the differences in number of gender-determining chromosomes in males and females. According to Richards, Hawley, and Mori (2005), “there are few X chromosome genes, including those few X genes that have counterparts on the Y, that escape inactivation, but the vast majority of genes on the inactivated copy of the X chromosome are shut down” (p. 249). Therefore, each cell has equal likelihood of inactivating the X chromosome originated from mother and that of father.
While deliberating on the importance of understanding the equality of sex determination, it is essential to comprehend whether X and Y chromosomes play an equal role in gender formation. Although the male and female heterogamety is essential in defining biological differences, there are also important theoretical predictions and empirical objections revealing that these different properties uniquely develop each system. In the studies by Bachtrog, Kirkpatrick, Mank, McDaniel, Pires, Rice, and Valenzuela (2011), the attention has been paid to the research which facilitates the understanding of the drivers that trigger sex chromosome evolution in various organisms. These genetic advances can be exploited for highlighting the differences in the evolution of drivers of genome structure that predict sex differentiation.
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The evolution of the X and Y chromosomes is an important aspect that defines further exploration in the field of genes that determine gender variation as well as the ratio of male and female systems. Specific attention should also be paid to SRY gene which plays a pivotal role in sex determination because of the constantly modifying nature of this gene. The cascade of sex determining genes is still underexplored, but they should be considered in more detail to determine male and female specific expressions in these genes. As Ridley (2006) notes, “the SRY gene is peculiar. Its sequence is remarkably consistent between different men: there are virtually no point mutations in the human race” (p. 112). In this context, SRY is a variation-free gene that has introduced serious shifts to the human genome. Interestingly, in mammals, SRY differs significantly due to its rapid evolution affecting genomes of various species. Apparently, this paradox is based on the process of hiding and fleeing. In fact, a driving gene appears from time to time on the X chromosome attacking the Y chromosome that recognizes protein produces by SRY. There is a selective benefit for any SRY mutant that differs significantly from the unrecognized one. The mutation starts spreading at the expense of other male genomes. Therefore, the dominating X chromosome destroys the X percentage in favor of women; however, the spread of the mutated SRY restores the created balance. As a result, there is a new form of SRY gene which further affects the created variation. Consequently, the rapid evolution leads to the production of new forms of SRY genes that differ significantly among species. However, this difference is further compensated by chromosome ratios, which prevent the SRY gene from destroying the Y chromosome. The presence of sexual antagonism is also due to the multiple research studies and theories of sex division and determination. However, the problem of sex ratio is currently the major one, which should be taken into consideration.
The evolution of the X and Y chromosome among the species has given rise to numerous questions regarding the equality of distribution of genes on the X and Y chromosome. Due to the fact that X chromosome takes part in sex determination in 75% of cases, 25% of Y chromosomes should somehow be compensated. However, not chromosomes but the number of genes they contain play a crucial role in sex determination and genetic evolution. In particular, there is preudo-autosome region, which plays an important role in shaping a male genome that could mutate leading to genetically acquired disease. The presence of SRY in a gene chromosome also creates variations among the mammal species as well as the fast-evolving gene, which introduces shifts to the human genome.
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