Genetic Mutations

Human beings are composed of many cells and to reproduce, in this case to make offspring, these cells divide and then merge with another set of cells. Most times this division and conjunction happens uneventfully, but sometimes, in fact three percent of births every year, a mishap occurs, a mutation. Not only do they occur before birth, they also occur later in life due to external stimuli like radiation and chemical exposure. This essay will examine some different categories of abnormalities showing specific mutations, how they occur and how they manifest themselves, with their effects on individuals.

Mutations occur in several different fashions. Some abnormalities are sex-linked disorders and therefore depend upon the chromosomes received from each parent. An example of this disorder is Hemophilia. Hemophilia is the lack of clotting factor VIII in the hemophilia A sufferers or clotting factor IX in hemophiliac B sufferers. These clotting factors are necessary to properly seal wounds as they activate prothrombin into thrombin which causes the reaction that eventually heals cuts, bruises and the like. Females have two X chromosomes (written XX) and males have one X and one Y (written XY). This disease results from a defect in one of the parents X chromosome(s) that causes the child to become affected by the disease. If a father has a sex-linked disorder, he can pass the gene along only to their daughters because almost all sex-linked diseases originate in the X chromosome. Therefore since a son receives a Y chromosome from his father he cannot receive a diseased X. For this same reason a woman is the only person capable of passing a sex-linked disease to her son. There are more male hemophiliacs in the world than females because the male chromosomes are XY meaning he receives an X chromosome from his mother and a Y from his father. If his mother is a carrier of the disease then he has a fifty percent chance of becoming a hemophiliac because he does not have another X chromosome, unlike a female, to counteract the diseased chromosome with a healthy one. For a female to suffer from the disease, her father must suffer from hemophilia and her mother must carry the disease, and even then there is still a seventy-five percent chance that she will not be afflicted with the disease, but a fifty percent chance that she will be a carrier of the disease. There will only be a twenty-five percent chance that the daughter will receive both hemophilia-causing chromosomes. A male in the same position would have a fifty percent chance of becoming a hemophiliac. Other sex-linked disorders following the same pattern is colourblindness, where people are afflicted with the inability to distinguish between light shades of red and green, Lesch-Nyhan syndrome where the victim’s nervous system is affected causing stuttered, shaky and uncoordinated movements, violent tendencies, self-mutilation using teeth, sharp and blunt objects. Another common sex-linked disease is Retinitis pigmentosa, where a patch of skin covers the light sensitive area at the back of the eye. And finally Duchenne muscular dystrophy which creates a weakness in the skeletal muscles at an early age. Another disease called Hypophosphatemia, that causes a deficiency of phosphates in the blood., differs from the others as it is caused by a dominant X chromosome, not a recessive. In this disease, a male will transmit it to all of his daughters, but none of his sons. Also a woman will pass this disease onto half her children because those are the odds that they will receive the diseased X chromosome as opposed to the normal one.

Disjunction abnormalities are also related to the X and Y abnormalities, except these abnormalities occur during meiosis. During this time the cells are supposed to divide into two separate cells. Sometimes the cells fail to separate and this creates one cell with a pair of homologous chromosomes and one cell with none. When these cells merge with other cells to create an embryo, a variety of syndromes. Turner Syndrome occurs when a normal cell merges with a cell containing no chromosomes. This syndrome has a genetic make-up that is written XO, the O shows the absence of the second chromosome. This syndrome afflicts females and causes sterility, webbing of the back of the neck, they are generally shorter than average, they have enlarged feet, a lowered hairline and abnormalities of the aorta. Klinefelter syndrome on the other hand occurs in males. These males were created from cells that had merged, one containing two homologous chromosomes, the other, a normal cell. This creates a genetic make-up written XXY. These males tend to be sterile, abnormally tall, and they develop breast tissue at puberty. They also tend to have a stature reminiscent of that of a female and they also tend to be mentally retarded, though the severity varies with each case. There are also more severe cases of Klinefelter syndrome, these have a genetic makeup of XXXY and XXXXY. In these cases, mental retardation is more pronounced. There are several other variations that occur a male with the genetic makeup of XYY tend to be sterile, unusually tall and a vast majority of the sufferers of this abnormality are in prisons or in mental institutions as they have problems controlling their behavior. Also, for women, there are much debated mutations that occur. There are three additional mutations with the makeups XXX, XXXX and XXXXX. These mutations show symptoms similar to that of someone with Turner syndrome, although the pattern is not consistent enough to draw concrete conclusions. Another abnormalities are caused by what is called Autosomal Recessive Inheritance. The most easily explained abnormality is Albinism. Albinism is the inability to synthesize melanin. Albinos tend to have extremely white skin, more colourless than anything else, also they exhibit an extreme sensitivity to light that affects their skin and eyes and makes it impossible for them to stay in bright sunlight for any length of time. This is caused when both parents have at least one of the recessive genes causing that particular disease and they pass on both the recessive genes to the child. This means that they are homozygous for it. Most people who carry the gene have a normal gene that dominates the other. If two people, both one gene causing albinism and the other normal (written Aa), were to have a child, there would be a twenty-five percent chance that they would give birth to an albino (aa). There would be a seventy-five percent chance that the child would appear normal, though there would be a sixty-six percent hance that the child would carry one copy of the defective gene, same as the parents, and a thirty-three percent chance that the child would receive neither recessive genes and be entirely normal (AA). One abnormality that acts like Albinism is Cystic Fibrosis. This mutation is characterized by increased mucus production that blocks air passages and digestive disorders. This disease has a high mortality rate for children. Another mutation that follow this pattern is Tay-Sachs disease. This disease is characterized by the appearance of a reddish patch on the retina at the age of six months caused by the inability to dispose of fatty tissue in the nervous system. The child later suffer from blindness, deafness and convulsions and normally they succumb to the effects of the mutation by age three. This disease is found predominantly in Jewish families that originated in eastern Europe. This shows that when people of a similar heritage produce a child the odds of a disease occurring is increased. This is due to the fact that through reproducing closely to ones own gene pool, it [the gene pool] becomes weakened and more susceptible to the appearance of certain abnormalities. Two other abnormalities, lactosemia, the inability to digest milk sugar, and sickle-cell anemia, a fatal disorder that affects the oxygen-carrying proteins of the blood, are not caused by recessive alleles, but rather defective versions of normal ones. Although these two disorders are different in this respect, they are inherited in the same fashion as the other abnormalities under this classification.

This next category of mutations is the easiest to explain, this is the category of Autosomal dominant inheritance. A simple mutation is the Darwin Tubercle, a small thickening of the cartilage on the upper rim of the ear. Abnormalities occurring in this category can happen easily, only one parent needs to carry the defective gene, whether active or inactive. The gene can affect the child easily because it is dominant (E) and overpowers the recessive (e), therefore it makes no difference if they are heterozygous or homozygous for it. If a child has a genetic mutation of Ee, and the dominant allele is abnormal, then the child will display the mutation. One example of an exception is Polydactyly, which causes extra fingers and toes. This is because it shows partial penetrance, which means that even if the child has the gene, it may not manifest itself and the usual number of fingers and toes will appear. Another mutation is Huntington’s disease which appears in individuals in their late thirties and forties and causes the slow degeneration of the nervous system. This abnormality follows the same pattern as the first abnormality. In this category, although it may appear that if one of the parents has one of these mutations, then the child will always get that abnormality, that is not true. There is always the possibility of the parent giving an unaffected dominant gene, and therefore the child will not have the mutation. Also if two dominant genes are given, there is a fifty percent chance that the healthy gene will overpower the defective gene and the mutation will not display itself, but the individual would still carry the chromosome and could pass it on to his or her children. There is also a fifty percent chance that the defective gene will overpower the healthy gene and the mutation will manifest itself as normal.

This final paragraph deals with mutations occurring after birth. These mutations manifest themselves after exposure to specific substances. One common mutation is cancer. Cancer is caused by the exposure to carcinogens, radiation, or viruses. Sometimes it is even inherited usually because of a deletion in a chromosome. Radiation is also a major cause, everything from ultraviolet light in the sunlight, whose effects are easily guarded against, to gamma rays from nuclear explosions, which needs seven centimeters of lead to block it out, can cause cancer. But the most potent of the causes of cancer are the chemical carcinogens. The largest killer of them all is cigarette smoke which causes cancer in the mouth, esophagus, lungs, bladder, pancreas and breasts. Close behind cigarette smoke is high-energy radiation which causes cancer in every organ, muscle and bone in the body. Radiation is not the number one killer due to the fact that there is not as much widespread exposure to it as cigarette smoke. Cancer develops when one normal cell is damaged and the cell is freed from its natural restraints on growth and reproduction. This one cell can produce millions of similar cells creating tumors that may only differ by a single gene. The cancerous cell can also spread itself to neighbouring cells. Tumors are classified under two titles. The first are benign tumors, whose cell growth is restricted to the area of the tumor itself although they can cause other problems relating to pressure on vital organs and other integral systems of the body if they develop in delicate areas. The second type of tumors are malignant, their cell growth tends to extend past and break off of the original tumor causing possibly dozens of other tumors which tend to almost always kill the host due to the difficulty of the removal of the entire tumor, which is necessary due to the fact only a sliver of cancer is needed for the cells to begin to replicate again. Treatments for cancer vary greatly, some cancers require surgery, while others are treated with chemotherapy, a combination of drugs and chemicals that target areas of cancerous cells. This type of therapy is preferable to radiation therapy because of the lack of healthy cell damage. Radiation therapy is still used often because of the reason that a cancer cell must divide quickly and therefore does not have enough time to repair radiation damage allowing mutations to take hold of the cancer and kill the organism. New techniques are being researched to allow the body’s own immune system to defeat the cancer on its own. Leukemia is another mutation although it only develops from exposure to high-energy radiation. Leukemia is another form of cancer that attacks the blood.

This essay has examined several different varieties of mutations and abnormalities in detail. It has also hopefully led you to understand the effects of these mutations and their appearance in everyday life. Also it shows how easy it is for these mutations to occur.

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