Ontogenes and Chromosome Nondisjunction in the D. melanogaster Meiosis

A mutation in an ontogene acts as a conditional dominant lethal: it is lethal in a certain genotype but not lethal in another. In total, 30 mutations of this type residing in the Drosophila melanogaster X chromosome have been assayed for their ability to cause meiotic nondisjunction. The level of X nondisjunction in the females heterozygous for the mutation in ontogene appears to be very high. The share of matroclinous daughters reaches 24.7% of the overall offspring and of patroclinous males, 24.9%. Neither inversion in the opposite X chromosome nor additional Y chromosome has any effect on the X nondisjunction. The balance of the XX and X0 egg cells is disturbed: exceptional daughters are prevalent in the offspring of the females with a normal opposite X chromosome and exceptional sons, in the offspring of the females with an inverted X chromosome. In addition, 12% of the matroclinous daughters of the females with a normal opposite X chromosome are homozygous for the marker of one of the maternal X chromosomes (“equational” nondisjunction). A “fading” parental effect of the mutation in ontogene on the X chromosome nondisjunction is also observed. Under experimental conditions, the mutant ontogenes reside in meiotic densely compacted X chromosomes. We infer that the ontogenes are DNA regions with controlled compaction. It is postulated that the genetic activity of ontogenes is determined by this compaction and has a biophysical (electromagnetic) nature. In a meiotic cell, ontogenes induce physical fields providing the operation of meiotic proteins. The structure of these fields is distorted in the mutants for ontogenes, thereby decreasing the efficiency of proteins and, as a consequence, causing meiotic defects.

Sporophytic nondisjunction of the maize B chromosome at high copy numbers

It has been known for decades that the maize B chromosome undergoes nondisjunction at the second pollen mitosis.Fluorescence in-situ hybridization （FISH） was used to undertake a quantitative study of maize plants with differing numbers of B chromosomes to observe if instability increases by increasing B dosage in root tip tissue.B chromosome nondisjunction was basically absent at low copy number,but increased at higher B numbers.Thus,B nondisjunction rates are dependent on the dosage of B＇s in the sporophyte.Differences in nondisjunction were also documented between odd and even doses of the B.In plants that have inherited odd numbered doses of the B chromosome,B loss is nearly twice as likely as B gain in a somatic division.When comparing plants with even doses of B＇s to plants with odd doses of B＇s,plants with even numbers had a significantly higher chance to increase in number.Therefore,the B＇s nondisjunctive capacity,previously thought to be primarily restricted to the gametophyte,is present in sporophytic cells.

The Study of Apolipoprotein E4 Allele Distribution in Parents of Down’s Syndrome Children as a Risk Factor in Khorasan Razavi Province, Iran

Backgrounds: Down syndrome (DS) is the most common chromosomal abnormality. The most important factor in DS is increased maternal age so after the age of 35, the risk of Down syndrome in pregnancy increases. Down syndrome can be diagnosed during pregnancy by prenatal screening. Nondisjunction in cell divisions is the main cause of the DS. Apo lipoprotein E is a 317 amino acid glycoprotein that plays an essential role in metabolism and cholesterol transport. Alzheimer’s disease (AD) is one of the symptoms of adults with DS. The apoE allele e4 has been identified as a risk factor for AD and also, played a main role in nondisjunction. An increased risk of AD in mothers of adults with DS has been reported. We hypothesized that young mothers of DS children (Methods: In this case-control study, 33 DS children and their parents were compared in case of age with 90 families without any history of DS. Genotyping was performed by ARMS-PCR technique. Statistical analysis was performed by SPSS v.21 software. Results: It indicated that there is a significant difference in allele distribution between case and control groups. The C allele for 112 codon of APOE gene and the C allele for 158 codon of APOE gene may associate with nondisjunction. In 112 codon of APOE gene, it seems having T allele reduces the risk of nondisjunction and in contrast C allele may be a risk factor in happening of nondisjunction. (p-value = 0.000006, OR = 2.66, 95% CI = 1.74 - 4.06). In 158 codon of APOE gene, it seems having T allele reduces the risk of nondisjunction and in contrast C allele may be a risk factor in happening of nondisjunction. (p-value = 0.0000, OR = 3.89, 95% CI = 2.38 - 6.34). E4 allele frequency in mothers of DS is about 14% more than those in control group. According to results of this study the C allele in 158 codon of APOE gene and the C allele in 112 codon of APOE gene could be considered as susceptibility genetic factors for nondisjunction in Northeast of Iran.

Effects of age on segregation of the X and Y chromosomes in cultured lymphocytes from Chinese men

Chromosome malsegregation in binucleated lymphocytes is a useful endpoint to evaluate age effect on genetic stability. However, the investigations on chromosome malsegregation in binucleated lymphocytes from Chinese are scarce. In this study, peripheral blood lym- phocytes were collected from 14 old （60-70 years） and 10 young （22-26 years） healthy Chinese men. To detect malsegregation of the sex chromosomes, multi-color fluorescence in situ hybridization （FISH） was performed on binucleated lymphocytes, cytokinesis-blocked by cytochalasin B at the first mitosis after phytohaemagglutinin stimulation. Compared with that in young men, a significant increase in frequencies of loss of chromosome X （9.2± 3.2‰ vs. 1.1 ± 0.9‰, P 〈 0.001） and Y （2.5 ± 1.9‰ vs. 0.2± 0.3‰, P 〈 0.001） was found in old men. Similarly, nondisjunction of chromosome X （16.5± 3.4‰ vs. 3.5 ± 1.1‰, P 〈 0.001） and Y （7.2 ± 2.6‰ vs. 2.4 ± 1.3‰, P 〈 0.001） occurred more frequently in old men than in young men. Regardless of donor＇s age, nondisjunction is more prevalent than loss for both chromosome X and Y. The frequencies of observed simultaneous malsegregation were relatively higher than the expected, suggest- ing an association between malsegregation. These results indicated that in Chinese men, malsegregation of the sex chromosomes increases with age in an associated fashion, and nondisjunction accounts for the majority of spontaneous chromosome malsegregation.

Maize centromeres:where sequence meets epigenetics

The centromere is a highly organized structure mainly composed of repeat sequences,which make this region extremely difficult for sequencing and other analyses.It plays a conserved role in equal division of chromosomes into daughter cells in both mitosis and meiosis.However,centromere sequences show notable plasticity.In a dicentric chromosome,one of the centromeres can become inactivated with the underlying DNA unchanged.Furthermore,formerly inactive centromeres can regain activity under certain conditions.In addition,neocentromeres without centromeric repeats have been found in a wide spectrum of species.This evidence indicates that epigenetic mechanisms together with centromeric sequences are associated with centromere specification.