Human sexuality and Sex Chromosomes Interactions

Chromosomes.Gene.SRY Gene.Genetics.Sex Anomaly.Sex Determination Systems

Human sexuality is a complex topic. It has many perspectives: historical, biological, evolutionary, cross-species, cross-cultural, physiological, and socio-cultural among others.^[1] Nevertheless, regardless of position one's stand for, the final and defining moment of human sexuality is sex interactions and its ultimate purpose is the reproduction. In humans and other organisms, sexual reproduction is a norm for the perpetuation of the species from generation to generation. Without sexual reproduction, the genetic endowment of the species is lost thru time and eventually the species becomes extinct, But, what is really passed from generation to generation during sexual interaction?

This article is a product of literature search on human sexuality. The vastness of human sexuality could not be contained in this article alone, Attendant to this, are two major areas of human sexuality i.e. nature (biological) and nurture (environmental) that need be stressed. To discussed even anyone of these areas requires great efforts and this article focussed only on the genetic nature of human sexuality i.e. genes and sex chromosomes and how sex chromosome interacts that strongly affects human sexuality. It has been known already that Sex chromosomes determine one's sexuality. In humans, the X and Y chromosomes are attributed to the Biological or Genetic sex of a person. During fertilization, if the fetus received two X chromosomes become a female, but, if it received X and Y chromosomes it became a male, moreover, this universal classification does not always work assuredly as expected since paucities in the expression of the Sex chromosomes exist in various degrees and correspondingly affects Phenotypic expression vis a vis the sexuality of the person.

From female idols to phallic worship. Art in Stone Age suggests the worship of women's ability to bear children and perpetuate the species. Primitive statues and cave drawings portray women with large pendulous breasts, rounded hips, and prominent sex organs. Experts believed that these figurines are fertility symbols.It appears though that stone age people may have been unaware of the contribution of the males to reproduction. As climate improves after the last Glacial Age, about 11,000 BCE, human society turned from nomadic hunter- gatherer to a sedentary farmer -herders to Agrarian form of society and Villages sprung around fields. ushering the era of Neolithic revolution.^[Ibid]

As people grew in numbers, awareness of male role in reproduction become prominent exemplified by Phallic worship (worship of the penis). Knowledge of paternity role is believed to have developed around 9,000 BCE, resulting from observation of livestock. When people began to observe animals throughout the year, they also started to understand that predictable period of time elapsed between copulation and birth of offspring^[Ibid]. In other words, at this juncture, the role of males in sexual reproduction was recognized.

• 3.1 The mammalian Y chromosome. Previously, according to experts, the mammalian Y chromosome is a "genomic wasteland", usually shrinking over the course of evolution and largely bereft of pertinent information. Because of this belief, one's sexuality is the sole responsibility of the X chromosome. Dr. Page's and co-workers, however, changed this perception thru their research finding by revealing that Y chromosome contains remarkable patterns of repeating sequences that appear dozens to hundred times. His colleagues explored the Y chromosome carried by males of several species, mapping structures of repetitive DNA in unprecedented details that clarified a long-standing clash for supremacy between the sex chromosomes^[2].
• The Y chromosome is one of the two sex chromosomes in mammals including humans. The other is the X chromosome.
• The Y chromosome contains about 59 million base pairs. The Y chromosome is passed only from father to son.
• With 30% difference between humans and chimpanzees, the Y chromosome is one of the fastest evolving parts of the human genome. presently there are about 80 Y-linked genes identified.
• All Y-genes are expressed as hemizygous (present only in one chromosome) except in cases of aneuploidy such as XYY syndrome or XXYY syndrome^[3].
• It is now fully accepted that the presence or absence of Y chromosome determines one's sexuality

3.2 The Mammalian X Chromosome. The X chromosome is one of the sex chromosomes in humans, the other is the Y chromosome.

• The X chromosome spans about 155 million base pairs and represents 5 percent of the total DNA in a cell.
• It looks like the X chromosome is bigger than the Y chromosome and therefore contains more genes than the Y chromosome.
• The X chromosome carries hundred of genes but few, if any, has anything to do directly with the sex of the person. however,
• Its genetic expression follows certain rules: males have only a single X chromosome,
• Almost all genes on the X chromosome have no counterpart in the Y chromosome,

and any gene in the X chromosome even recessive in females will be expressed in males. This is because the male received one copy only of the X chromosome.

After fertilization, the human embryos were genetically similar and assume the hermaphroditic constitution as both males structure(Wolffian ducts) and females structure(Mullerian ducts) were present. It was only in the seventh week of conception that the male and female genotype is expressed. If the fetus received two X chromosomes becomes female but, if it received one X and one Y chromosome it yields male offspring. Moreover, this condition is not always true as we learn later that many of the dreaded human abnormalities are associated with the anomalous expression of the sex chromosomes^[4].

4.1 Are persons of 46,XX karyotype always a female? the answer is No, here's the evidence:

• A 14-year old male with macroorchidism and mild bilateral gynecomastia sought medical attention due to abnormal gender characteristics. The results of gonadal biopsy showed hyalinization of the seminiferous tubules, but there was no evidence of spermatogenesis. Karyotype analysis of the patient confirmed 46,XX karyotype ^{[5 ][6]}
• In another study, XX males are the most common condition in the absence of cytogenetically detectable Y chromosome. Using the Fluorescence InSitu Hybridization (FISH) technique or PCR, it was detected the transfer of Yp fragments including SRY gene to the terminal part of X chromosome in the majority of XX males^[7].It appeared that the X chromosomes were masculinized instead of expressing feminine traits in the presence of SRY gene.
  

After fertilization up to seven weeks of pregnancy whether male or female is possible. This sexual dimorphism in humans has been the subject of controversies for centuries. In 335BC, Aristotle postulated that sexual dimorphism arose from differences in temperature during copulation. In his scheme, hot semen generates male while cold semen made female.[8] Today, we know that this is not true. In fact, it's the SRY gene located on the short arm (11.3Yp) of Y chromosome determines one's sexuality. Research results showed that SRY gene served as the master regulator of sex determination^[9].

Previously, it was believed that the X chromosomes determines one's sexuality, but this belief was changed in 1905 when Stevens and Wilson, hundred eleven years ago independently discovered that Chromosome Y determines human sexuality^[10] Later studies revealed that Y-chromosome has a unique region or domain at the terminal end of the short arm of the Y chromosome(Yp) called the sex determining region Y that contains the SRY (sex-determining region of Y chromosome) gene. This gene is highly specific to Y chromosome only. There is no equivalent copy of this gene on the X chromosome. The SRY gene main role is to confer maleness. The unique role of SRY gene of the Y chromosome was further substantiated by medical reports, the existence of 46,XX males and 46, XY females.^[11][12] In the former, a segment of SRY gene was translocated in the X chromosome during spermatogenesis while in the later, the SRY gene failed to express strongly in the embryo and subsequent sexual development. The person having this anomaly, possessed both male and female phenotypic characteristics.

For a long time, it was believed that the Y chromosome was a genomic wasteland, inert, and lack genomic attributes, in fact, Y chromosome was a very active sex chromosome. During gamete formation, though, not a homolog of X chromosome pairing occurred in the especial region found at the terminal ends of both chromosomes called the pseudoautosomal region. This unique region carries the "intelligent mark" of homology from their ancestral autosomes million of years ago before these identical autosomes developed "sexual locus" and subsequently separated during the course of evolution. Since then, the Y chromosome has undergone drastic changes. Today, chromosome Y is much smaller and shorter than it's counter -part the X chromosome. During gamete formation, it is not unusual to observe the crossing-over between X and Y chromosomes at the specialized region called the pseudoautosomal region. The interactions of these segment allow both chromosomes to counter check each other on the fidelity of the genetic information they carry and once shared and possibly do repair and eliminate deleterious mutations if found thus, maintaining the integrity of the sex chromosomes generation after generation.It appeared, however, that this system function as a sophisticated biological security system.

During gamete formation in humans, females produced ova containing X chromosomes only (homogametic), while males produced sperms carrying X and Y sex chromosome (heterogametic). If the egg cell carrying X chromosome is fertilized by sperm cell with X chromosome, a female is born, however, if, the egg is fertilized by Y-bearing chromosome, a male is born. The XY sex determination system is not only found in humans, but also in some animal species. Other systems also exist for example the ZW sex determination system among birds, insects, and reptiles.^{[13 ]} This video presents some of the well-known sex determination systems.

It can be surmised that human sexuality is immensely affected by the interactions of the sex chromosomes. Chromosome Y has a region called the sex-determining region of Y chromosome officially designated as the SRY gene, Collectively, human sexuality is a complex topic and genetic sex or referring to X and Y sex chromosome is a vast topic to deal with, but regardless, of perspective one assume, the final and defining moment of human sexuality is sexual reproduction. It is thru this process that sex chromosomes were passed during sexual interactions to the offspring of males and females of the species. The expression of sex chromosomes was not always perfect. This anomalous behavior sex chromosomes and it's interactions often lead to serious human disabilities vis a vis the sexuality of the individual.

1.What is human sexuality? December 23, 2009. Chapter 1. Retrieved-Dec. 12, 2015 [PDF]

2.Brendan, M. A battle of the sexes is waged in the genes of humans, bulls, and mice. June 29, 2015. Natures Magazine.

3.X chromosome. November 30, 2015. Genetics Home Reference. The U.S. National Library of Medicine.

4.Prokop, J.W.and C.F. Deschepper.2015. Chromosome Y genetic variants: impact in animal models and on human diseases. Physiol. Genetics 47(11):525-37.

5.XY sex determination system. Wikipedia, the free encyclopedia.

6.Gregorescu-Sido, A.et.al. 2005.Three new 46,XX male patients: a clinical, cytogenetic and molecular analysis. J. Pediatr. Endocrinol. Metab. (2):197-203.

7.Banayed Abdelmoulo, N. et. al. 2003. Skewed X-Chromosome inactivation pattern in SRY-positive XX males: a case report and review of literature Ann. Genet. 46(1):11-18.

8.Haqq,C, nd P.K. Donahoe.1998. Regulation of sexual dimorphism in mammals. Physiol.Review. 78:1-33.

9.Hake, L.and C. O'Connor. 2008. Genetic mechanism of sex determinism. Nature Education 1(1):25.

10. Y Chromosome. Wikipedia, the free encyclopedia.

11.Ayesha, A. et.al. 2012.Is 46,XX karyotype always a female? BMJ Case Report 292[PMC]

12.Berta, P. et.al. 1990. Genetic evidence equating SRY and the testis -determining factor. Nature 348:448-450.

13.XY sex determination system. Wikipedia, the free encyclopedia.

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