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Thursday, 19 April 2012

Human Genetic diseases: Abnormal Genetic permutation, combination during Human Cell-Division

List of Human Genetic Diseases

Human Have 23 pairs of Chromosomes (22 pairs chromosomes are known as Autosomes and one pair X and Y known as Sex chromosomes) 

This is an example of autosomal recessive inheritance which has the following characteristic:
1) Both male and female carry the recessive allele on an autosome.

2) Heterozygotes of either sex are symtom free.Homozygotes of both sexes are affected.

3) When the both parents are heterozygous, there is a 50% chance that each child born to them will be heterozygous also.
    There is a 25% chance that each child will be homozygous recessive.

4) When both parents are homzygous for the recessive allele, all of their children will be affected.

3D Picture of Chromosome Pair

About 1 in 10,000 individuals is affected by achondroplasia , a type of dwarfism that is caused by an autosomal dominant allele. When long bones develop in affected children , cartilage forms in ways that lead to disproportionately  short arms and legs.Affected persons end up being less than four feet , four inches tall.The dominant allele has no other phenotypic effects in heterozygotes. The heterozygotes normally are fertile; however, homozygous fetuses usually are stillborn.

This is less common form of autosomal dominant inheritance.It is characterized by progressive degeneration of nervous system, and it eventually is lethal . In about half the cases symtoms are expressed  from age 40 onwards, after most people already had children .In time movements are convulsive and brain function deteriorates rapidly. Although this disorder is rare worldwide, its frequency is much higher in South Africa, whre about 500 cases have been identified.Researcher discovered  that all effected persons were directly or indirectly descended from the same Dutchmen who settled there in 1658.

Changes  in Chromosome Number due to Non-Disjunction during Cell-Division

A mutated form of one gene which normally codes for clotting factor VIII, give rise to hemophilia A. Males who inherit a recessive allele on their one X-Chromosome  are always affected. Without medical attention, cuts, bruises  or internal bleeding could lead to death. The blood clotting time is more or less normal in heterozygous females; the non mutated gene on their other X-chromosome code for enough of factor VIII to cover the required function.Hemophilia A is an example of X-linked recessive inheritance. Recessive allele for hemophilia A is rare .Only about  1 in 7000 human male are effected, and unless hemophilic males marry close relatives , it is not likely that they will have affected daughters.

Human Syndromes occur within Human Sex Chromosome Pair

Sometimes gamete receives two copies of chromosome 21 as a result of non-disjunction.Fusion of the abnormal gamete with a normal one produces an individual with 3 copies of chromosome. The condition called trisomy 21 leads to Down syndrome. Trisomic 21 embroys are often lost through miscarriage; however about 1 of every 1000 liveborn in North America alone is destined to develop the disorder. Most  children with this syndrome show moderate to severe mental retardation, and about forty percent have serious heart defects. This is not usually a heritable disorder , however as women age they run a much greater risk of giving birth to a trisomic 21 child than do women in their early twenties.

Nondisjunction leads to other sex-chromosome abnormalities, including XXY males who show Klinefelter syndrome. Affected males are sterile and mentally retarded. Their testes develop to only about a third of normal size, body hair in sparse, and there may be some breast enlargement.Injectios of the hormone testosterone can reverse the feminized phenotype but not the sterility or mental retardation.

About 1 in every 5000 newborns is destined to have turner syndrome a sex chromosomal abnormality that arises through non-disjunction.Affected persons have only 45 chromosomes instead of 46; only one sex chromosome (XO) is present in their somatic cells. The frequency of turner syndrome is lower than for other sex chromosome abnormalities,probably because most affected embryos are miscarried early in pregnancy. This leads to a female phenotype, but the phenotypic distortions  are pronounced. The females are sterile . Their ovaries are non functional, and secondery sexual traits fail to develop puberty. Often they age prematurely, and they are shortened life expectancies.

Human Have 23 pairs of Chromosomes (22 pairs chromosomes are known as Autosomes and one pair X and Y known as Sex chromosomes)     
3D Model of Person with Albino  Genetic Disorder.

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