<P> The grasshopper Melanoplus femur - rubrum was exposed to an acute dose of X-rays during each individual stage of meiosis, and chiasma frequency was measured . Irradiation during the leptotene - zygotene stages of meiosis (that is, prior to the pachytene period in which crossover recombination occurs) was found to increase subsequent chiasma frequency . Similarly, in the grasshopper Chorthippus brunneus, exposure to X-irradiation during the zygotene - early pachytene stages caused a significant increase in mean cell chiasma frequency . Chiasma frequency was scored at the later diplotene - diakinesis stages of meiosis . These results suggest that X-rays induce DNA damages that are repaired by a crossover pathway leading to chiasma formation . </P> <P> In most eukaryotes, a cell carries two versions of each gene, each referred to as an allele . Each parent passes on one allele to each offspring . An individual gamete inherits a complete haploid complement of alleles on chromosomes that are independently selected from each pair of chromatids lined up on the metaphase plate . Without recombination, all alleles for those genes linked together on the same chromosome would be inherited together . Meiotic recombination allows a more independent segregation between the two alleles that occupy the positions of single genes, as recombination shuffles the allele content between homologous chromosomes . </P> <P> Recombination results in a new arrangement of maternal and paternal alleles on the same chromosome . Although the same genes appear in the same order, some alleles are different . In this way, it is theoretically possible to have any combination of parental alleles in an offspring, and the fact that two alleles appear together in one offspring does not have any influence on the statistical probability that another offspring will have the same combination . This principle of "independent assortment" of genes is fundamental to genetic inheritance . However, the frequency of recombination is actually not the same for all gene combinations . This leads to the notion of "genetic distance", which is a measure of recombination frequency averaged over a (suitably large) sample of pedigrees . Loosely speaking, one may say that this is because recombination is greatly influenced by the proximity of one gene to another . If two genes are located close together on a chromosome, the likelihood that a recombination event will separate these two genes is less than if they were farther apart . Genetic linkage describes the tendency of genes to be inherited together as a result of their location on the same chromosome . Linkage disequilibrium describes a situation in which some combinations of genes or genetic markers occur more or less frequently in a population than would be expected from their distances apart . This concept is applied when searching for a gene that may cause a particular disease . This is done by comparing the occurrence of a specific DNA sequence with the appearance of a disease . When a high correlation between the two is found, it is likely that the appropriate gene sequence is really closer . </P> <P> Crossovers typically occur between homologous regions of matching chromosomes, but similarities in sequence and other factors can result in mismatched alignments . Most DNA is composed of base pair sequences repeated very large numbers of times . These repetitious segments, often referred to as satellites, are fairly homogenous among a species . During DNA replication, each strand of DNA is used as a template for the creation of new strands using a partially - conserved mechanism; proper functioning of this process results in two identical, paired chromosomes, often called sisters . Sister chromatid crossover events are known to occur at a rate of several crossover events per cell per division in eukaryotes . Most of these events involve an exchange of equal amounts of genetic information, but unequal exchanges may occur due to sequence mismatch . These are referred to by a variety of names, including non-homologous crossover, unequal crossover, and unbalanced recombination, and result in an insertion or deletion of genetic information into the chromosome . While rare compared to homologous crossover events, these mutations are drastic, affecting many loci at the same time . They are considered the main driver behind the generation of gene duplications and are a general source of mutation within the genome . </P>

When is the most likely time for homologous chromosomes to cross-over resulting in recombination