The most common trisomy among viable births is that of chromosome 21, which corresponds to Down Syndrome. One can only admire and respect the creative imagination of these investigators in their ability to construct often elaborate albeit sometimes incorrect models about meiotic chromosome behavior based exclusively on the phenotypes and frequencies of offspring they recovered from genetic crosses.
Calvin Bridges and Thomas Hunt Morgan are credited with discovering nondisjunction in Drosophila melanogaster sex chromosomes in the spring ofwhile working in the Zoological Laboratory of Columbia University. Of all of the chromosomal disorders, abnormalities in chromosome number are the most obviously identifiable from nondisjunction sex chromosome in Wisconsin karyogram.
The only known survivable monosomy in humans is Turner syndromewhere the affected individual is monosomic for the X chromosome see below. Triploid intersexes in Drosophila melanogaster. This nondisjunction sex chromosome in Wisconsin is not present in our closest genetic relatives, the chimpanzees.
Behavior of aberrant chromosome configurations in Drosophila melanogaster female meiosis I. New York: McGraw-Hill. As a consequence, the organism evolves as a mixture of cell lines with differing ploidy number of chromosomes. Nelson Textbook of Pediatrics, 19th Edition 19th ed.
Diploid and haploid cells are involved in sexual reproduction of higher eukaryotic organisms. Nondisjunction is known to occur more frequently in the cells of older individuals. Two daughter cells genetically distinct from the mother cell are formed with half the number of chromosomes.
The majority of these are a culmination of nondisjunction in maternal meiosis. Hum Genet. This is a unique combination nondisjunction sex chromosome in Wisconsin nondisjunction leading to autosomal trisomy and subsequent loss of the unpaired chromosome, leading to the existence of 2 copies of a chromosome of uniparental origin.
Although his lifestyle never impeded his scientific accomplishments, it did contribute to an unfortunately early death at age Equally important to his intellectual contributions, Bridges largely shaped Drosophila into the facile experimental system it is today.
Figure 1. When females are the exposed parent, only chromosome gain can be detected. Multiple aneuploidy of one or several chromosomes is very uncommon, except for the sex chromosomes.