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The phenomenon in which a single gene has multiple effects is called pleiotropy. This means a single gene often influences more than one phenotypic characters. The gene having multiple phenotypic effects because of its ability to control the expression of a number of characters is called pleiotropic genes.
Sometimes, the pleiotropic gene effect may produce various abnormal phenotypic features which are collectively called syndromes. The pleiotropic gene may be lethal when one of its effects becomes the cause of the death of an individual. Such type of gene is known as the lethal gene. The lethal genes cause a great deviation from the normal development of an individual. Hence, that individual does not survive. Due to the lethal gene, the abnormal ratio 2:1 is brought about. This lethal gene is seen either in the homozygous dominant condition or homozygous recessive condition. As a result of the lethal effect, Mendel's monohybrid ratio of 3:1 gets modified and changed into 2:1.
In 1905, Lucien Cuenot reported a peculiar pattern of inheritance in mice. Lethal gene in mice causes death at an early stage of development, often before birth. The effect of lethal gene is illustrated by the inheritance of fur (coat) color in mice,
In mice, yellow fur is dominant over non-yellow fur color. The dominant homozygous organism dies because of a lethal combination. Therefore, the dominant homozygous organism with yellow fur color will never survive. Therefore, instead of ratio 3:1 ratio, 2:1 ratio is obtained. It is modified monohybrid ratio. A cross was made between two heterozygous yellow fur mice (Yy and Yy). Normally, this cross would be expected to produce 1YY, 2 Yy and 1 yy. The 'Y' is responsible for yellow fur color. However, the homozygous dominant (YY) yellow fur mice are conceived but never complete developed and hence dies before birth. It leaves a 2:1 ratio of Yy(yellow) mice survived. Therefore, all living yellow fur mice are heterozygous(Yy).
Here the homozygous dominant (YY) is a lethal gene and therefore, yellow mouse (YY) never survives and dies in the embryonic stage. Here, gene 'Y' is recessive in relation to its effect on viability but dominant in relation to fur color.
In human pleiotropic gene shows an effect on RBC causing sickle cell anemia. It is a hereditary disease common among certain African populations, It is caused by a pleiotropic gene with a lethal effect in the homozygous recessive condition (HbS/HbS). A person dies when its genome carries (HbS/HbS) in homozygous condition.
The gene causes the production of abnormal hemoglobin. As a result, the shape of RBC containing Hb gets distorted and becomes sickle-shaped. Hence these carry less amount of oxygen leading to anemia. The homozygous recessive individual dies before they attain maturity due to severe anaemia.
The heterozygous individuals survive but show mild anaemia. They possess both normal and abnormal haemoglobin so obviously, some of the cells are sickle-shaped. Such heterozygous individuals are naturally protected against malaria as the malarial parasite cannot live within these abnormal or distorted cells. Malarial parasite survives in normal blood cells and hence affects the normal homozygous individuals.
The gene for sickle-celled RBC is represented by HbSwhile that of normal erythrocytes is written as HbA. The homozygous for two types are HbSHbSand HbA HbA. The heterozygote is written as HbA HbSwhich is called as the carrier.
When two sickle-cell carriers or heterozygotes (HbA HbS) marry, then they can produce three types of children- homozygous normal (HbA HbA), heterozygous carrier (HbAHbS) and homozygous sickle celled (HbS HbS) in ratio 1:2:1.However, homozygous sickle celled individuals (HbSHbS) show the lethal effect and ultimately die in childhood due to severe anaemia. Therefore, the ratio becomes 2:1.
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