O Chromosome Lethal Frequencies in Serbian and Montenegrin

1364 From late 1930, lethal genes have been used as ge netic markers to detect concealed variation in Droso phila populations [1–4]. In order to study lethal genes a lethal balanced strain is required. This strain is a ge netic construction involving a pair of homologous chromosomes with the following characteristics: they have to contain natural or induced chromosomal in versions to block recombination, dominant markers to detect each balanced chromosome and recessive lethal genes to prevent the homokaryotypic combination of these balanced chromosomes. Usually, the dominant genes are also recessive lethals to combine the last two genetic characteristics in only one gene. There are a limited number of Drosophila species in which lethal balanced strains are available (for instance, D. melan ogaster, D. willistoni, D. prosaltans, D. pseudoobscura, D. persimilis and D. subobscura) [5]. To obtain lethal frequencies in a natural population, appropriate cross es have to be carried out with the lethal balanced strain (for instance see [5, 6]). In a population, the frequency of lethal genes is a good measure of its genetic variabil ity. With this procedure it has been possible to study the spatial patterns of these frequencies [7–11], the annual variation within the same population [12–14] and even seasonal effects [14–16].