Evidence for genetic control of nondisjunction in man.

Data on factors associated with the occurrence of Down syndrome in a highly inbred population were evaluated to investigate the presence of a genetic control of nondisjunction in man. In Kuwait, close consanguinity occurs in 40% of marriages. In its main obstetric hospital, 20 trisomic Down babies out of 11,614 singleton births were delivered over a 12-month period. Chi-square analyses indicate the occurrence of Down syndrome to be linked to two independent factors: consanquinity of parents and maternal age. The relative risk is approximately four times greater for closely related than for nonrelated parents (P less than .005); a possible explanation for this is the existence of a gene that induces mitotic nondisjunction in the homozygous fertilized ovum. An alternative explanation is the existence of an autosomal recessive gene which results in meiotic nondisjunction in the homozygous parents. Consanguinity is usually perpetuated in certain families, or sections of the population, and parents in highly inbred families have a higher probability to be homozygotes for that gene.     

Effects of caffeine on chromosomal loss and nondisjunction in Drosophila melanogaster

Three types of male larvae, normal X males and two types with structurally abnormal X chromosomes (ring X and short X sc ⁴ sc ³, y) were treated during the third instar with 0.5 per cent caffeine in nutrient medium. Upon eclosion, these males were mated to yellow and Oregon-R wild type females. The F₁ generation of each cross was scored for normal (XX and XY) and abnormal (XO and XXY) progeny. Statistical analyses of data demonstrate that caffeine increases chromosomal loss for all genotypes tested. The effect of caffeine on nondisjunction, however, is not clear. There are at least marginal increases in all cases when Oregon-R females are used. Slight increase and decreases noted for offspring of yellow females appear to be dependent upon the genotype of the inseminating male.     

XY chromosome nondisjunction in man is associated with diminished recombination in the pseudoautosomal region.

To assess the possible association between aberrant recombination and XY chromosome nondisjunction, we compared pseudoautosomal region recombination rates in male meiosis resulting in 47,XXY offspring with those resulting in 46,XY and 46,XX offspring. Forty-one paternally derived 47,XXYs and their parents were tested at six polymorphic loci spanning the pseudoautosomal region. We were able to detect crossing-over in only six of 39 cases informative for the telomeric DXYS14/DXYS20 locus. Subsequently, we used the data to generate a genetic linkage map of the pseudoautosomal region and found it to be significantly shorter than the normal male map of the region. From these analyses we conclude that most paternally derived 47,XXYs result from meiosis in which the X and Y chromosomes did not recombine.     

Evolution of Sceloporus grammicus complex (Sauria: Iguanidae) in central Mexico II. Studies on rates of nondisjunction and the occurrence of spontaneous chromosomal mutations

The iguanid lizard Sceloporus grammicus has a high level of karyotypic variability, and has often been cited as an example of chromosomal speciation. We examined a total of 2036 secondary spermatocytes from 30 S. grammicus males, and found that 16 of the 30 individuals (including a single lizard collected from a hybrid zone between two chromosome races) produced completely balanced spermatocytes. Fourteen of the 30 lizards (including both chromosomal heterozygotes and homozygotes) had relatively low (0.6% to 7.1%) levels of aneuploidy. Heterozygotes had a 1.1% increase over homozygotes in the number of aneuploid spermatocytes observed. The frequency of aneuploidy in S. grammicus may not be high enough to cause chromosomal speciation by any of the mechanisms that have been proposed for this complex. Most individuals showed balanced segregation of the autosomal trivalents, but nearly half of the lizards had a significant excess of spermatocytes with the X₁ and X₂ rather than the Y sex chromosomes. Five lizards had spermatocytes which had fission mutations not found in the somatic cells. As many as 5.9% of the spermatocytes in one individual had chromosomal mutations. This chromosomal mutation rate has important implications for chromosomal evolution in S. grammicus.     

Induced nondisjunction in drosophila oocytes.

Nondisjunction of the sex chromosomes and compound-second chromosomes was induced in Drosophila melanogaster by irradiation of immature oocytes and cold treatment of mature oocytes. These two treatments cause nondisjunction by different mechanisms. This results in a differential for stage sensitivity and the production of relatively different frequencies of the various exceptional progeny classes.     

Incidence of nondisjunction in mouse oocytes.

The chromosomes of more than 3000 ovulated mouse oocytes from strains C3H/Han, NMRI/Han, and (101 X C3H)F1 have been analyzed after spontaneous and hormonally induced ovulation. No significant difference in the incidence of nondisjunction was observed among the three strains with or without hormonal pretreatment. The incidence of nondisjunction was estimated to be 0.47% in NMRI/Han, 0.62% in C3H/Han, and 0.81% in (101 X C3H/F1. The incidence of chromosomal imbalance after the first meiotic division was slightly higher after adding the events following presegregation. Diploidy-spontaneous and hormonally induced-reached a significant leve in NMRI/Han. This may be interpreted as a consequence of hormonal interference with a genetically defined malfunction of gene product(s) during the late phase of oogenesis.     

Studies on dinoflagellate chromosomal basic protein

Routine cytochemical methods proved useless in demonstrating basic protein in dinoflagellate chromosomes because when the DNA was removed, these chromosomes dissolved away just as eubacterial nucleoids. However, with ammoniacal silver technique or alkaline Biebrich scarlet, DNA could be kept intact, all the dinoflagellate chromosomes examined gave positive reaction. The acid-soluble proteins were extracted from methanol-fixed Amphidinium carterae and methanol-fixed isolated nuclei of Noctiluca miliaris, and subjected to urea polyacrylamide gel electrophoresis. Only one band of basic protein co-migrating with histone H4 was found. The chromosomes of Oxyrrhis marina can retain their original forms after the removal of DNA. Their chromosomal basic protein can be demonstrated by various cytochemical methods. This protein co-migrates with histone H4 in urea gel too. Its amino acid composition has been determined. This protein can combine with DNA fibril to form a nucleosome-like structure which seems to be corresponding to two of the archaebacterial nucleosome-like structures and may represent the primitive nucleosome.