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| Genetic Diseases |
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| A large number of diseases are known to be inherited from the parents to the offspring. Such diseases are known as genetic diseases. Most of these diseases are caused by the expression of recessive genes. |
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| The genetic diseases can be broadly classified into two types: |
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 Autosomal disorders |
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 Allosomal disorders |
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| These are metabolic disorders caused by the expression of some genes present on somatic chromosomes. Such disorders express equally in both the sexes. Following are some common autosomal disorders in man: |
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| Albinism |
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| Albinism is a disorder associated with a skin pigment called melanin. In this condition, there is either very little production of melanin or, melanin is totally absent. As a result, the skin becomes pale and the hair is totally white. Such persons are called as albinos. The albinos cannot expose themselves to bright sunlight since they easily get sunburns. The absence of melanin is due to the expression of a recessive gene in the homozygous condition. |
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| Fig. 20.3 Albinism |
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| Phenyl Ketonuria (PKU) |
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| Phenyl Ketonuria (PKU) is a common metabolic disorder in growing children. A child suffering from PKU has defective legs and is usually unable to stand. Such a child may also be mentally retarded. This condition is due to a defect in the amino acid metabolism. In the course of amino acid metabolism there is a step in which the amino acid phenyl-alanine gets converted into another amino acid called tyrosine. This conversion requires specific enzyme called phenyl pyruvic hydroxylase. In the child suffering from PKU this particular enzyme will be absent and as a result phenyl-alanine gets converted into a ketone body phenyl pyruvic acid. It starts circulating throughout the body through the blood resulting in the disorder. The absence of the enzyme phenyl pyruvic hydroxylase is due to the expression of a recessive autosomal gene in the homozygous condition. |
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| PKU can be detected by a simple urine test. Such children are usually treated with a diet, which is free from the amino acid phenyl-alanine. |
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| Sickle-cell Anemia |
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| It is a metabolic disorder characterised by a decreased oxygen transport in the blood. The decrease is due to the presence of a defective haemoglobin (HbS) in place of the normal haemoglobin (Hb). |
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| HbS has a very low affinity for oxygen. As a result, there is a decrease in the blood supply to vital organs such as heart, brain, liver and kidney. This may lead to various disorders. In addition to this, RBC become sickle shaped with pointed edges. The change in the nature of haemoglobin and the shape of RBC is due to mutation in the autosomal genes, which are responsible for the production of haemoglobin. This mutation is characterised by a change in a single amino acid in the sequence. This defect arises from the substitution of valine for glutamine at the sixth position in chain of globin molecule of haemoglobin. |
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| fig. 20.4 Sickle Cell |
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| Sickle cell anaemia is very common in Africa and in the American blacks. Due to the high incidence of diseases, the average life span in this population is only about 40 to 45 years. The disease is controlled by a single pair of alleles. Though heterozygotes are apparently unaffected as they carry at least one normal allele, they are carriers of the defective gene and can transmit the same to 50% of their offspring. |
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| These disorders are caused by genes present on the sex chromosomes. The abnormal disorders express more commonly in males than females. Some common examples are colour blindness, haemophilia and congenital night blindness. |
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| fig. 20.5 Colour Blindness |
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| Disorders Due to Aneuploidy |
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| Aneuploidy is a condition, which represents a change in chromosome number. Such a change is most commonly caused in the reproductive cells during the process of gametogenesis. The most common cause for aneuploidy is a phenomenon called non-disjunction. It represents a failure in the separation of the paternal and maternal chromosomes following synapsis and crossing over during prophase I of meiosis. |
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| fig. 20.6 Non-disjunction |
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| Non-disjunction in the autosomes may result in some metabolic disorders. However, if non-disjunction occurs in the sex chromosomes, it affects the formation of gametes and results in the production of abnormal gametes. Such abnormal gametes are known as non-disjunct gametes. Non-disjunction of sex chromosomes in females results in the formation of two types of non-disjunct gametes 22A+XX and 22A+O. Similarly non-disjunction of the sex chromosomes in the males, results in the formation of two types of non-disjunct sperms 22A+XY and 22A+O. The fusion of such abnormal gametes results in the formation of defective zygotes carrying genetic disorders. Such zygotes may or may not undergo further development. If such zygotes undergo development, the resulting individuals will be defective. |
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| Two very common genetic disorders resulting from non-disjunction are Klinefelters syndrome and Turners syndrome. |
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 Klinefelters syndrome results from the fusion of a normal ovum 22A+X with a non-disjunct sperm 22A+XY or a non-disjunct ovum 22A+XX with normal sperm 22A+Y. The zygote has the genotype 44A+XXY. It develops into a male individual with degenerate sex organs. The individual is therefore |
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| sterile. In such a person, a few feminine characters may also be present. |
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 Turners Syndrome results from the fusion of an ovum 22A + X with an sperm with 22A. So the genetic composition is 44+ X0. It develops into a female individual with rudimentary ovaries. They have a short stature, webbed neck, broad chest, lack of secondary sexual characteristics and sterility. |
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| Downs Syndrome |
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| It was first reported in 1866 by Longolon Down. It is caused by the presence of an extra chromosome, No.21. Both the chromosomes of the pair 21 pass into a single egg due to non-disjunction during oogenesis in the mothers ovary. Thus, the egg has 24 chromosomes instead of 23 and the offspring has 47 chromosomes (45 + XY male) or (45 +XX female). |
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| The presence of 3 chromosomes of the same kind is called trisomy. |
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| Symptoms: |
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| It is characterised by rounded face, broad forehead, a mongolian type of eyelid fold, projecting lower lip, protruding tongue, short neck, flat hands and stubby fingers. Victim has little intelligence due to malformation of the brain. Gonads and genitalia are underdeveloped. This disease is also called Mongolian Idiocy and Mongolism. |
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| fig. 20.7 Downs Syndrome |
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| Alzheimers Disease |
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| It is a case of loss of memory and ability of judgement as well as general physical impairment. It is due to the accumulation of amyloid protein plaques in the brain resulting in the degeneration of neurons. Different genes have been linked to Alzheimers disease but these genes only predict susceptibility to disease. |
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| fig. 20.8 Alzheimers Disease |
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