Loose ends

T wo is the fundamental number for genetics; all of us are the products of single zygotic events but the zygote is formed from two gametes, and two different sets of chromosomes make up our genomes. When I first learnt genetics, it was about diploids, and meiosis and the generation of haploid gametes was central to its understanding. Incidentally, lacking a classical education, I thought haplo came from some Greek word meaning half, and, of course, half of two is one, so I thought they could have called it monoploid. Later I discovered that haplo does come from a Greek word, but it means single or simple. Those of us who entered bacteriophage genetics with a biological background had to unlearn all the genetics we knew and adapt to the genetics of organisms with single (haploid) genomes. The physicists, knowing no biology, had a much easier time. At that time, there were strong doubts that viruses could be said to have any genetics at all, and Delbriick even ascribed genetic exchange between phages to some kind of directed mutation. Hershey got it right and called it recombination. Much the same confusion bedevilled early bacterial genetics, but with the discovery of Hfr strains and the elucidation of the mechanisms of genetic exchange by Hayes and Wollman and Jacob, E. coli not only had genetics but sex as well. We had males, females and mating; there were zygotes and segregants.There were in fact diploids, but these were incomplete and temporary. Later, with episomes, true diploids could be constructed and E. coli genetics did begin to look more and more like the old genetics we had all forgotten. There were geneticists working with yeast and Neurospora with well studied sexual cycles, but the diploid phase was only a brief stage in the life histories of these organisms and could be safely ignored. Generations of students came to learn genetics through haploid organisms, and terms such as leptotene, meiosis, gametes and polar bodies disappeared. Then we all turned to higher organisms and everybody had to learn diploid genetics and come to terms with heterozygotes and the difficulties of finding recessive mutants. Fortunately, I alighted on Caenorhabditis elegans, which is a self-fertilizing hermaphrodite with rare males, essentially giving all the benefits of haploid genetics in a diploid organism. The fact that it is driven to hom0zygosity makes it sensible to talk about a single genome …