Geneticists originally assumed that it makes no difference, from the standpoint of the characteristics of an individual, which of its genes is inherited from the mother and which from the father. However, over time, a substantial amount of information has been accumulated demonstrating that this simple concept is not valid. It was found that some genes are expressed only from the sections of chromosomes derived from the father and others from sections derived from the mother. Genome imprinting is responsible for this phenomenon. Some sections of genes are labelled in the developing sex cells of the parent organism, e.g. by methylation, where this labelling differs in the male and in the female. The given gene is then expressed in the zygote according to how it is labelled. During the differentiation of sex cells, the sex-specific labelling of genes derived from the mother and from the father is “erased” and is replaced on all the chromosomes by sexually specific labelling corresponding to the sex of the individual in whose body the differentiation of sex cells occurs. In some cases, it seems that erasing of the methylation labels on the chromosome set derived from the father already occurs in the fertilized oocyte (Ferguson-Smith & Surani 2001). Genomic imprinting can, of course, take place in hermaphrodites; however, then the DNA of the sex cells will be labelled according to whether differentiation of the gamete occurs in the tissue of male or female sex organs.
Genome imprinting is most obvious in cases where there is a conflict of the biological “interests” of the father and mother in relation to the amount of resources that they are to invest in their future progeny (Burt & Trivers 1998). This situation frequently occurs in species where the embryos are formed inside the female organism, i.e. primarily in angiosperm plants (Hardling & Nilsson 2001) and in live-bearing animals, such as mammals (Moore & Haig 1991). As a female can reproduce with various males in the population during her lifetime, it can frequently happen that individual, simultaneously developing embryos can have different fathers. Consequently, the biological interests of genes derived from the father and from the mother can differ very substantially. It is in the interests of the gene derived from the father that a maximum of maternal resources be invested in the development of the embryo in which it is located. These investments can even occur at the expense of damage to the other developing embryos, which can have a different father and even at the cost of exhaustion or destruction of the maternal organism. The next offspring of the particular female could have a different father, so the genes of paternal origin need not take into account the future reproductive potential of the female. Thus, for example, imprinted genes of an embryo derived from the father can program the formation of a large placenta, from which the embryo would derive nutrients from the body of the mother, or can cause the production of hormones controlling the level of nutrients in the maternal organism, which can be manifested, e.g., in maternal diabetes (Haig 1993b).