We have discussed in this section, how Mendelian ratios can be variously modified due to lethal genes, genic interactions, modifiers, suppressors and pleiotropy. There are other cases where certain gametes may be either lethal or may fail to take part in fertilization. This may also lead to modified ratios. One such example is killer gene in wheat which causes death of pollen. Another example is found in Drosophila, where a particular class of gametes do not take part in fertilization according to expected meiotic segregation pattern. This phenomenon was called meiotic drive by Sandier and Novitski (1957). For instance certain male flies with genotype en bw/cn+bw+ (en = cinnabar; bw = brown; cnbw/cnbw = white due to interaction) when mated with white eyed cnbw/cnbw females give rise to a ratio of 25 : 1 (wild = cnbw/cn+bw+ : white eyed = cnbw/cnbw) instead of expected 1 : 1 ratio.

It has been proved that this extreme modification of ratio is not due to mortality at zygotic level but is due to the location of a segregation distorter (SD) gene carried on cn⁺bw⁺ chromosome. It is also established that half the sperms carrying SD preferentially fertilize the eggs, so that the genotype cn+bw⁺/cnbw predominates. It has also been shown that several loci (at least five) are involved in segregation distortion, located on SD chromosome and that there should be a complex mechanism involved (Hartl, 1980).

The SD chromosome is found at a frequency of few per cent in most natural populations. The heterozygotes for SD, in extreme cases, can produce upto 99% progeny with SD due to dysfunction (nonfunctioning) of gametes carrying the homologue of SD. These dysfunctional sperms carrying homologue of SD exhi-bit abnormalities in chromatin condensation (Hartl, 1975, 1980).

Another important system causing meiotic drive has been studied in mice, where a transmission ratio distorter, t haplotype causes partial lethality in male gametes (sperms). In a heterozygote for t (i.e. t⁺/t), the sperms carrying t function normally and the sperms carrying t⁺ do not function. Consequently, heterozygotes for this allele (t) transmit this allele (like SD in Drosophila) in frequencies as high as 99%. Other systems causing meiotic drive include SR (sex ratio) in Drosophila, MD (male drive) in mosquito and SK (spore killer) in Neurospora.

The above two examples i.e. SD complex in Drosophila and Mocus complex of mice were considered to be the two earliest examples of what were described as selfish genes. These selfish genes were defined as those genes, which propagate themselves, despite being detrimental to the organisms that carry them. The natural selection seems to favour them. Other examples of such selfish genes include B chromosomes, replicative transposons, the psr chromosome of wasps and mitochondria that cause male sterility in plants. Another recently discovered class oi selfish genes includes Medea (M) in common flour beetle. While, in SD and t complexes, the allele is transmitted in excess through male (prior to fertilization), in case of M, in the progeny of heterozygous mothers (M +), the zygotes which do not carry M (homozygotes, + +) die before pupation (post fertilization). Such homozygotes will appear in crosses M + x + + or M + x M +, but not in crosses when either of the parent is MM. This gene is located on a chromosome.

The term 'selfish DNA' has also been used sometimes for repeated DNA sequences in a different sense.