Chromosomal abnormality referred to as in Edward syndrome is caused due to trisomy 18.It is manifested by abnormality is in different body parts and organs. trisomy 18 affect the foetus from the time of its development into the uterus ‚thus the growth of the child remain retarded.
New born babies have low birth weight and Often defect and deformities develop in several organs.
Most common abnormality appearing during intrauterine development are cardiac and nervous system abnormality is leading to defect in heart and shape of head (microcephaly).
Due to these reasons survival rate of affected individual often low many die either before birth or in an early infancy stages. only about 10% children are able to survive longer than a year,
That too with mental disabilities, malformations in body part and development delays. with the survival rate of the frequency of occurrence of trisomy 18 is about 1 in 5000 births since most features with the disorder are not able to complete the term. the chances of the genetic disorder also correlated with mother’s age.
Edward syndrome is characterized by the presence of an extra copy of chromosome 18 in addition to the two member of the homologus pair.
The additional chromosome 18 can be present in an individual is as complete copy or a some part of a genetic material.
The extra genetic material appears before fertilization that is it comes from either of the two gametes. usually it is a result of abnormal meiotic division that takes place during gamete formation.
The chromosome fail to segregate and nondisjunction of chromosome lead to appearance of 24 chromosome (additional chromosome 18) in the result in gammat instead of 23.participation of such gamete in fertilization result into a zygote with trisomy 18 and 47 chromosomes in all.
Mosaic Edward syndrome occurs due to presence of an extra copy of chromosome 18 in only some cells of the body and not all. this type of trisomy occurs during early stages of embryo development due to defect in cell division so that only some cell are trisomic and rest are diploid.
This condition is therefore not inheritable. the severity of this order depends upon the number and types of cells that are trisomic in the body.
There is another form of Edward syndrome called translocation trisomy 18, in which certain part of chromosome 18 get translocated and attached to another chromosome, so that the number of chromosome 18 remain as 2 but additional genetic material of chromosome 18 is present on another chromosome.
This causes the appearance of complexities of Edward syndrome in an individual. however the degree of severity ranges from low to high due to partial trisomy depending upon the extent of transfer of genetic material. classification of chromosome 18 also occur usually during gamete formation due to my meiotic defects.
In 1960 John Edward and his colleagues reported on an infant trisomic for a chromosome in the E group now known to be chromosome 18 designated as 47 ‚18 + .
Incidence is about 1 in 4000 live birth. 80% of trisomy 18 cases are due to mitotic nondisjunction 10% are due to translocation and 10% are the result of trisomy mosaics. This syndrome occurs three times more frequently in females than males. of the life born survivor beyond 1 year is rare.
Characteristics of trisomy 18 children are highly variable and pronounced.
The Demonstrated severe mental retardation odd shaped head Webbed-neck, short thumb , low set ears, small mouth, receding Chin, Index finger overlapping the middle and ring fingers, Rocker bottom feet and cleft in the lips and palate.
Heart defects are found in nearly all trisomy 18 birth as well
Survival time is less than four months on the average. this is usually caused by pneumonia or heart failure.
