Sex Chromosomes

Topics: Chromosome, Aneuploidy, Y chromosome Pages: 8 (3096 words) Published: January 19, 2013
Sex Chromosomes

Few people every wonder or care about their gender and why they are male or female.. Sex-determination system is a complex but fascinating progress, especially within reptiles, birds and insects. All living organisms begin with one eukaryotic cell; inside the nucleus of each cell contain thread-like structures called chromosomes. Chromosomes are once theorized to be derived from autosomes (chromosomes that are not sex chromosomes) and have changed into specific chromosomes throughout evolution. All living organisms contain chromosomes, but they can range widely between different organisms. For example, inside the nuclei of human cells contain 46 chromosomes (22 pairs of autosomes and one pair of sex chromosomes) while butterflies have 380 chromosomes. There are two types of sex chromosomes present in the human body, X (female) and Y (male) chromosomes. The combination of these two chromosomes decides the gender of the offspring. Interestingly, the X and Y in certain species mammals are backwards, where the male would have the X chromosome and the female with a different combination. This unusual system is believed to be derived from the ancestral X and Y chromosome. The X and Y chromosome is essential for living organisms to function but the presence and absence of either the X or Y chromosome can be related to numerous disorders and abnormalities.

X Chromosome
The X chromosome is one of the two sex-determining chromosomes in a variety of species. Each individual will always have one X chromosome present. The X chromosome contains essential genes that the human body cannot live without. Cells in the female body contain two XX chromosomes, whereas males only have one X and one Y chromosome. In females, the initial X chromosome is retained from their mother, and the second X chromosome comes from their father. The presence of the X chromosome in combination with the X or Y chromosome differentiates the two genders. As mentioned above, both genders must have at least one chromosome in order to survive. This is because the X chromosome contains roughly 2000 genes and 150 million base pairs. X chromosomes make up approximately five percent of the total DNA in human cells. The X chromosome was discovered by German scientist Hermann Henking in 1891. When he was studying the sperm of wasps, he had noticed that some sperms contained eleven chromosomes, while others contained twelve. Because the newly discovered chromosome was unknown to science, Henking gave it the name X element, indicating that X can be anything. X Inactivation

In 1959, Asian-American geneticist Susumu Ohno noticed that two X chromosome in certain female mammals were different: one appeared similar to an autosome, but the second one was heterochromatic (has a pattern of mixed colors). In 1961, Mary Lyon hypothesized that X-inactivation is responsible for different color coatings in female mice. After multiple studies and experiments, her hypothesis was confirmed right. X inactivation requires two X chromosomes, thus only present in females.  In the early stages of embryonic development in females, one of the two X chromosomes is randomly inactivated in female cells instead of the egg cells. This unusual process is called lyonization, or commonly known as X chromosome inactivation.

For X chromosome inactivation (XCI) to happen, there must be a particular arrangement on the X chromosome, this is called the X-inactivation center. Inactivation occurs when a certain type of RNA binds itself to one of the two X chromosomes, preventing transcription of genes on the bound chromosome. During this process, some genes on the X chromosome may escape inactivation. A very common example of X inactivation occurs in female tortoiseshell cats where some patches of the coating are black or orange (see Appendix 1). There are two types (XCI) that occur in the development of the female embryo : random XCI and imprinted...
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