The identification of the chromosomes of the E group (16-18 Denver): an autoradiographic and measurement study.

Abstract

An attempt has been made to classify the E chromosomes of four subjects with an abnormal chromosome complement using two criteria: the autoradiographic grain counts in cells labelled towards the end of the DNA synthetic period and a ‘morphological index’. This was found to be a better discriminant than the total length and was obtained by dividing the arm ratio of each chromosome into its total length. The first patient, a girl heterozygous for a presumptive deletion of the long arm of the X chromosome, had only 15 chromosomes in the C group and seven in the E group. One of these, which had a very high morphological index, was by far the most densely labelled and was therefore identified as the abnormal X chromosome. The other six chromosomes could be sorted into a pair with high index (No. 16) and a group of four which could be further subdivided into a pair with low grain counts and intermediate index (No. 17) and a pair of high grain counts and low index (No. 18). The second patient had a ring chromosome for one of the E chromosomes. The patient’s five normal E chromosomes could be divided into a pair with high and a pair with low morphological index, while the fifth chromosome had an intermediate value. This chromosome had a mean grain count similar to that of the ring chromosome and lower than that of any other E chromosome. It was therefore concluded that this patient was heterozygous for a No. 17 ring chromosome. The results obtained in this patient clearly showed that, in this group, morphologically similar chromosomes (presumptive homologues) have similar grain counts. The last two patients had similar abnormal phenotypes, and both were heterozygous for a deletion of the short arm of one of the E chromosomes. The five normal E chromosomes of these patients could not be sorted into pairs by the separate analysis of the morphological index and grain counts. However, they could be divided into a sub set of three chromosomes with grain counts as high or higher than that of the deleted chromosome, which should have included the homologue of the abnormal E chromosome, and a sub set of two chromosomes with lower grain counts. The study of the morphological index of the first group clearly showed that two chromosomes with a high morphological index formed a pair, while the third chromosome had a low index and was unpaired. An analysis of the grain counts over the normal E chromosomes confirmed that the chromosome with the lowest index was unpaired in both patients. It was therefore concluded that they were heterozygous for a deletion of the short arm of chromosome No. 18.