Table 9. Examples for regulation of gene distribution and variation

Genetically controlled process


Some aspects of mitosis and amitosis

The ciliate protozoan Paramecium contains, in addition to a diploid micronucleus, a highly polyploid macronucleus, which divides amitotically.

In the snail Limnaea, a maternal gene determines orientation of the spindle, which controls handedness of the coil (left-handedness is determined by a homozygous, recessive allel).

The rate of mitosis, and polyploidy in certain organs (eg human liver) is under genetic control

Differences in gene distribution between somatic and germinal cells

Germ and somatic cell lines can differ in nuclear genetic composition due to genetic factors acting on the spindle, centromeres, and the linear arrangement of chromosome segments. Mutant genes in Drosophila and maize give rise to divergent spindles, leading to chromosome loss. In Sciara, the fungus gnat, whole chromosomes are eliminated during early cleavage stages

Crossing over and synapsis are under genetic control

In Drosophila, only females undergo meiotic crossing over.

Gene distribution during meiosis and meiotic drive

Meiotic drive in Drosophila, segregation distortion

Ploidy changes in a sexual cycle

In eukaryotes, ploidy changes from diploid to haploid to diploid. In Hymenoptera, ploidy determines sex, and is under behavioural control of the diploid queen, which can produce haploid males

Somatic endoreplicaton

Results in somatic polyploidy or polynemy, and is under genetic control

Mitosis and mutation rates

The number of mutations increases with mitotic activity, which is under genetic control. Note that many cancer cells are mutants whose mitotic rate has increased

Point mutations

Point mutations in prokaryotes are controlled by gene-coded mutagens and antimutagens, as well as mutator and antimutator DNA polymerases. In eukaryotes, strains of Drosophila melanogaster from various regions have different spontaneous point mutation frequencies, increased up to tenfold by some mutator genes increase. The regulation of mutability involves cell susceptibility to viruses and episomes as well as the relationship between nuclear and extranuclear genes: Rous sarcoma virus increase mutation rates in Drosophila; simian virus SV40 induces chromosomal mutants in human cell lines.