CHROMOSOMAL ABNORMALITIES

BY- SREELAKSHMI (MSIWM012)

Chromosomal abnormalities are seen in many people. These are generally aneuploidy (presence of abnormal number of cells). it’s either autosomal or sex-chromosome. Translocation, duplication and deficiency in chromosome can also be the reason for this abnormality. It is found that certain disorders or diseases in man are associated with the abnormal number and nature of chromosome.

Abnormalities in Sex Chromosome

A normal male and female has XY and XX chromosome respectively.Klinefelter’s syndrome and Turner’s syndrome are an example for the anomalies in the number of sex chromosomes. Other important abnormal sex in man is the XYY male. They are usually tall and are more aggressive. Many cases it is observed they are mental retarded, but they are fertile.

Abnormalities of the Autosome

In some cases abnormal number of some of the autosomes or other autosomal aberrations. First autosomal anomaly that was described in man is Down’s syndrome. This abnormality is also called mongolism, as the features of such individuals resemble those of Mongolian races. This syndrome was first described by Seguin in 1844.The symptoms include: round face with small skull, short body, swollen tongue, eyelid fold resembling those of Mongolian’s and also mental retardation. In some cases congenital heart diseases is present in these individuals and mostly death occurs early in childhood. But some of them live upto young adult age with the low IQ.

Studies showed that Down’s syndrome is due to trisomy of the 21^st chromosome. This trisomy is due to non-disjunction during meiosis. It may occur either during spermatogenesis or oogenesis. Sometimes the third chromosome in the set 21 may be attached to another autosome which is most commonly paired to .This is called translocation trisomy. Down’s syndrome can be related to the age of mother. Frequency of this syndrome increases with increase in mother’s age. In most cases, Down’s syndrome births occur in women conceiving after the age of 30.The trisomy’s of two slightly larger chromosome number 13 and number 18 which leads to early death.

13-Trisomy: It is observed less in number. Such people will have severe mental and physical deformities. The head and the eyes are small.in some cases eyes are absent. Such individuals have congenital heart disease. This disease is also called Patio’s syndrome. This syndrome can be also due to isochromosme of chromosome-13.

18-Trisomy: They show low set ears, simian crease, deformities of fingders, toes and feet, congenital heart disease and receding chain. Head will be flattened 90% of people die within one year.it is also called Edward’s syndrome.

Abnormalities of Chromosome Structure

Chromosomal abnormalities like deletions, translocations, ring formation etc. Are lethal even as heterozygotes, resulting in zygotic rules, still births or infant deaths. Sometimes infants with small chromosome deficiencies survive. However these infants die at an early age.

Cri-du-chat syndrome: It is due to the chromosome deficiency in the short arm of chromosome 5. This deficiency is designated as 5P. Infants with this syndrome cry like a cat mewing. Other features are small forehead, broad face with saddle nose, widely spaced eyes with epicanthic folds and physical and mental retardation.

Chronic Myelocytic Leukemia: This disease is caused when there is a deletion of chromosome 22. Exposure to X-rays can be a cause of this disease.

GENETIC DISEASES DUE TO DEFECTS IN THE CATABOLISM OF PHENYLALANINE

1. Phenylketonuria (PKU): The conversion of phenylalanine into tyrosine is mediated by the enzyme phenylalanine hydroxylase. The absence of this enzyme causes a disorder in a man.It is called phenylketonuria. In people affected with this disorder,phenyalanine will not be converted tyrosine.Instead,phenyl-acetic acid(PAA) are formed.Phenylpyruvic acid causes damage to nervous system which leads to mental disorder.it is a serious disease and can be detected by simple blood and urine test.Affected infants can be given a diet that is low in phenylalanine.
2. Alkaptonuria: This disease is caused due to the accumulation of homogentisic acid. Homogentisic acid is responsible for the oxidation of phenylalanine during catabolism. Absence of enzyme responsible for the oxidation of homogentisic acid, it gets accumulated and passes into the urine. When such urine is exposed to air, alcoptone gets oxidized and forms pigments. The absence of this oxidase enzyme is due to to air, alcaptone gets oxidase enzyme is due to and a recessive genes.Alkaptouria is not a serious gene.
3. Albinism: The color of the skin and the hairs is due to the presence of a dark pigment called melanin. Individuals who do not have melanin are pale in colour.Such individuals are called albinos. This is due to a metabolic block in the conversation of tyrosine into melanin. This block is due to the absence of the enzyme catalyzing the conversion of tyrosine into DOPA (3, 4-dihydroxy phenylalanine).This enzyme is called tyrosine, Albinism may be due to deficiency in the processes that takes place after the formation of DOPA. 

Sickle-cell Anaemia

The anaemic condition of the person due to change in the blood cells is called sickle-cell anaemia.It is a chronic, haemolytic disease.In the past, patients died early due to infections or cardiac failure. The sickled red cells become trapped in the small blood vessels. This causes impaired blood circulation resulting in the damage of vital organs. The sickle cells are more fragile and hence haemolyse readily, so have a shorter life than the normal cells. They will suffer from severe anaemia.They are generally genetically transmitted. When two individuals who have sickle – cell trait marry 25% of the progeny will have sickle-cell anaemia, 50% will have sickle –cell trait (carriers) and 25% will be normal individuals.

AMINOCENTESIS

It is prenatal detection of inherited diseases. Human fetus lies in the amniotic cavity in the uterus of the mother. A fluid, amniotic fluid, surrounds the fetus and fills the amniotic cavity. Cells from the fetus are shed into this fluid. The amniotic fluid along with the fetal cells can be collected by introducing a syringe through the abdominal wall of the mother after the third month of pregnancy. This procedure is called amniocentesis. This amniotic fluid is further analyzed.

The fetal cells separate from the amniotic fluid are called amniotic cells. Identification of sex of the fetus helps to terminate pregnancy in people who have a history of sex-linked diseases such as hemophilia, color blindness. Turner’s syndrome and klinefelter’s syndrome can be detected by observing the number of Barr bodies.

Amniotic cells can be cultured in an appropriate medium for 2-6 weeks and can be used for chromosome analysis and biochemical studies. Chromosome analysis helps to determine the karyotype of the foetus .When any kind of an abnormality is found the mother is asked for abortion which is legal.

Disadvantage of Amniocentesis

It can be done only after 12-16 weeks after conception. Analysis of the culture will take another 2-6 weeks.Therefore, if any kind of abnormalities are found and the mother is asked for abortion it will be too late.Aminocentesis will give a mild shock to the fetus affecting its growth.