Cytogenetics in the Practice of Pediatrics
Park S. Gerald MD1
1 Department of Pediatrics, Children's Hospital Medical Center and Harvard Medical School, Boston
The best known chromosomal abnormalities are alterations in the number of chromosomes. This type of chromosomal alteration is found in approximately 5% of all fetuses, most of whom are unable to survive pregnancy. Because of this selection process, only 0.6% of living newborns have a chromosomal abnormality. Approximately half of this group is grossly clinically abnormal. The remainder may not exhibit any clinical disturbance until later in childhood or at the time of reproduction.
The pediatrician must have sufficient knowledge of the overt cytogenetic syndromes to suspect the probable diagnosis. The pediatrician must also be sufficiently informed of the origins of chromosomal abnormalities to participate in the genetic counseling of the parents. Further, many of the recently recognized cytogenetic abnormalities are subtle and difficult to detect. Their demonstration will only be possible if the pediatrician is alert to the clinical situations in which they are likely to occur.
HUMAN CHROMOSOMES
The chromosomes which are seen through the microscope are chromosomes that have replicated. They are bipartite bodies, each half (or sister chromatid) is identical with the other half (Fig 1). The process of preparing the chromosomes for examination involves arresting cell division so that the chromosome does not undergo the normal process of division. The two chromatids touch each other at the constricted region known as the centromere.
