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Mendelian Disorders

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Mendelian Disorders - Lesson Summary

All of us are carriers of potentially hazardous genes. We can broadly classify genetic disorders into two types – Mendelian and chromosomal.
Mendelian disorders include genetic disorders caused by alterations or mutations in a single gene as in thalassaemia  and sickle cell anaemia while chromosomal disorders include genetic disorders caused by an excess, absence or abnormal arrangement of chromosomes as in Down’s Syndrome and Turner’s Syndrome.
The mutated genes that cause Mendelian disorders follow the Laws of Mendelian Inheritance.
These disorders can be autosomal recessive, autosomal dominant, X-linked dominant or X-linked recessive. Some Mendelian disorders are cystic fibrosis, haemophilia , thalassaemia, sickle cell anaemia and phenylketonuria .
Haemophilia is a genetic disorder that impairs the body’s ability to coagulate  blood when a blood vessel breaks. A male offspring inherits this disorder from an unaffected carrier or a heterozygous  female parent and these males are generally infertile in later stages of their lives. The possibility of a female offspring being haemophilic is very rare and occurs only if both the X-chromosomes in the offspring are in recessive form. That is, the father must be a haemophilic, carrying the recessive X-chromosome, and the mother must be a carrier or a haemophilic too.
Another widely studied Mendelian disorder is sickle cell anaemia, which affects RBCs . Sickle-cell anaemia is an autosome-linked recessive trait or a trait inherited through non-sex chromosomes and it is passed on to the progeny when both parents are carriers of the gene, which means they are heterozygous for the trait. The disorder is controlled by the two alleles, HbA  and HbS . Sickle cell anaemia is caused when Glutamic  acid is substituted with Valine  at the sixth position of the beta globin chain of a haemoglobin molecule.
phenylketonuria is also an autosomal recessive trait. It is an inborn error of metabolism inherited during birth and is characterised by the deficiency of the enzyme phenylalanine  hydroxylase , which is required to convert the amino acid phenylalanine into tyrosine. As a result, phenylalanine accumulates in the body and gets converted into phenylpyruvic  acid and other derivatives. Phenylpyruvic acid is even detected in the urine as the kidneys fail to absorb it. If phenylpyruvic acid and its derivatives accumulate in the brain, it could lead to mental retardation. To avoid such a situation, a diet low in phenylalanine is usually recommended for patients suffering from phenylketonuria.


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