X-linked Disorder Research Paper
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Example Research Paper on X-linked Disorder
One should start by saying that genes are passed from the biological parents to their offspring. These genes are inherited in various specific ways, with one pattern of inheritance being called X-linked inheritance.
Prior to speaking about the x-linked disorders, I would like to note that x-linked inheritance means that the genes that bring a trait or cause a certain disorder are located on the X chromosome. One should not forget that females have two X-chromosomes, while males have one Y-chromosome and one X-chromosome. The X-chromosome genes can be either dominant or recessive while the genes expression in males and females are different since the Y-chromosome genes do not pair up with the X-chromosome genes identically.
The x-linked recessive genes find expression in females only WHEN there are two same copies of the gene, with each of them being located on the X-chromosome. Speaking about males who have only one X-chromosome, one needs only one copy of X-linked recessive gene in order for a specific trait or disorder stipulated or caused by that gene to be expressed. For instance, a female can carry a certain recessive gene on one of her X-chromosome and unknowingly pass it to her son (X, Y chromosomes), who will then express the trait or disorder (Stone 78).
If a female has daughters, than the chances are that 50% of her daughters will inherit the gene, while the other 50% of daughters will not inherit the gene. Thus, the first 50% of females will pass the gene further to their offspring, while the other half will not pass the gene further to their offspring. By the same token, half of the sons will not have that gene and will not pass it on to their offspring, while the other half of the sons will inherit that gene, express the trait or disorder and pass it on to their offspring (Fletcher, 50).
Speaking about some disorders caused by the x-linked recessive genes, I would like to note that red-green color blindness and hemophilia A are passed on by x-recessive chromosomes. Since this essay is concerned with color-blindness, it is necessary to point out that it takes many forms:
- Protanopia: People are unable to distinguish green-yellow-red spectrum. They also have a difficulty distinguishing the colors in the orange-red spectrum. This is a rare form of colorblindness (Fletcher, 53).
- Deuteranopia: People are unable to distinguish green-yellow-red section of the spectrum. This form is called Daltonism. This form is also rather rare.
- Protanomaly: People are less sensitive to red light than normal and oftentimes perceive it as black. Just like Deuteranopia, Protanomaly is also rare.
- Deuteranomaly: People are unable to distinguish the red and green color due to their low sensitivity to these colors. Deuteranomaly is the most common form of colorblindness and is present in about 8% of the male population.
The red-green color blindness, in simpler terms means that a person who has it cannot distinguish the colors and the shades of red and green. Typically one cannot also distinguish blue and green. At the same time their visual ability is normal and does not express any abnormalities (Miller, 201).
I have to add here that there are typically no serious complications, yet people with color blindness are usually not considered for certain employment where the ability to recognize colors is vital. Various transportation occupations or the US Armed Forces typically prevent these people from holding certain posts. One should understand that since the color blindness gene is located on the x-chromosome, males are affected 16 times more often with color blindness than females who require the presence of two genes on 2 chromosomes (defective) present for this trait to be expressed (Rosenthal, 140).
Certain genes are responsible for the formation of three types of cones that are responsible for color vision in humans and are active in day light. Rod cells provide black-and-white vision and are active in low light. There are three types of cones responsible for red, green and blue color perception. Apparently, the genes that are responsible for the proper formation, construction and functioning of these red, green and blue cones are located on the x-chromosome. Thus, the color-blind disorder as caused by defective genes on the x-chromosome are passed from the parents to the offspring (Cos-Gayon, 22).
One should also point out the fact that these genes for red-green color blindness in males are passed on to ALL his daughters, who are heterozygote carriers and are typically unaffected. The female will then pass on the gene, as noted earlier to 50% of males. The sons of colorblind male do not receive/inherit the colorblind trait, since they receive only Y-chromosome (rather than the defective x-chromosome that causes the disorder) which causes them to become males.
Cos-Gayon, C.S.R., Aspects of Human Genetics With Special Reference to X-Linked Disorders (Monographs in Human Genetics), Prentice Hall, 2002.
Rosenthal, Odeda, Coping With Color-Blindness, NY Random House, 2002.
Miller, James, X-Linked Traits : A Catalog of Loci in Non-human Mammals, McGraw Hill, 2003.
Fletcher, R., Defective Colour Vision: Fundamentals, Diagnosis and Management, Penguin books, 2002.
Stone, Rhonda, The Light Barrier: A Color Solution to Your Child’s Light-based Reading Difficulties, Wiley and sons press, 2000.