As long as there has been human life there has been a search for who and what we are. Things from such where do we come to where are we going have been at the forefront of science. In this paper human variation, specifically height will be discussed. This paper will then cover the extremes of height, dwarfism and gigantism. Finally, I will attempt to gauge where are and in the understanding of human variation
There re several factors that interact to define human height. The two predominant factors are genetics and environmental effects. The question many are asking is “ What effects do genetics and environmental play in determining height?”. The studies vary but as little as sixty percent to as much as eighty percent of height is a product of genetics. This leaves as much as forty percent to as little as twenty percent of height as environment induced (Chao, 2005). Using the concept of heritability, the impact of genetics on a a given individual height can be determined. Heritability is defined by Merriam Webster as “the proportion of observed variation in a particular trait (such as height) that can be attributed to inherited in contrast to environmental ones”(Webster). So if there is a male that is 165 cm and a female that is 150cm, and both are from a population where the average male is 175 cm and the average female is 155 cm, the heritability can be predicted as long as we know the heritability factor (for this population it is 65 percent for males and 60 percent for females). If a male child is conceived, the heritability is equal to 0.65 x (175-165) + 155-150)/2 giving us an expected height without the influence of environmental factors. Some environmental factors that influence height are diet, culture and atmosphere. Wealthier countries conversely have better diets (as far as access to food variety, this does not mean that they utilize it) and the opposite is true. Protein intake has been linked to height. Atmosphere is a product of how polluted an environment is. Pollution can effect plant and animal growth which are used as fuel. For the purpose of this paper our primarily focus will be genetically, and more specifically on the effects of human growth hormone.
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The assumption that an excess of human growth hormone would increase height and a lack of human growth hormone could result in decreased height is basically true. The Pituitary gland is divided into an anterior and posterior lobe. The anterior lobe, closely associated the hypothalamus, a section of the brain. The pituitary gland secretes, among many other hormones, growth hormone, This hormone is the most abundant. It plays a pivotal role in growth, influencing linear bone growth. Growth hormone levels are highest in childhood, spiking during puberty, then began steadily to decline into adulthood. Growth hormone works by binding to GH receptor molecules, which then began a series of reactions that result in the biological effects characterized by Growth hormone secretion, or lack there of. This brings us to a discussion of the extremes.
Too much growth hormone can lead to excess height. Gigantism is a condition which has excessive height as its main symptom. Height is divided into sections such as before two years of age, pre-puberty, and then the final push at puberty. It has been established that a person that is more than two standard deviations above the height of a comparable group which includes sex, age, and ethnicity are thought to have attributes leading to gigantism. Excessive height brings along a certain possibility of illness and disease since it is caused predominantly by abnormal hormone secretion. One of the most common reasons for excessive growth hormone release comes from a benign tumor of the pituitary gland. Weakness, double vision, delayed puberty, excessive sweating, as well as irregular periods in women. A MRI or CT scan as well as blood tests can be used as diagnosis.
Too little growth hormone secretion can lead to a paucity in height. This is often referred to as dwarfism. Growth hormone deficiency is one of several hundred reasons for dwarfism, which is defined as height of less than 4 feet, ten inches tall in adulthood. which itself has several different variations. If it is noticed earlier and it can be treated with growth hormone injections. Unfortunately, individuals with dwarfism are beset with a large amount of illness which unfortunately is beyond the scope of this paper.
Human variation is the spice of life. The urban myth that everyone has an identical twin somewhere in the world is intriguing. From Mendel to McClintock, researchers have always taken an interest in the genetics of all life. This paper dealt with human variation as it relates to height. The paper examined one of many causes of extreme variation, as well as exposed the readers to sources in the bibliography for further study. We have mapped the human genome so there may one day be that extremes in height will be a thing of the past.
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