It has long been observed that parents transmit their characteristics to their offspring. Gregor Mendel discovered the rules through research carried out on peas, from which he formulated the classic rules of inheritance. Some characteristics, for example the winding direction of the shell of paludal snails, apparently follow Mendel's inheritance rules but are one generation late. Such characteristics are not caused by the species' own genes, but by those of the mother. This is called maternal effect. Maternal effect is responsible for preparing the ovules before conception, therefore it is important for the healthy development of the offspring. The embryo protection programmes target the successful functioning of the maternal effect by accustoming future mothers to a healthy life style before conception. Another non-Mendelian inheritance type is maternal inheritance. Contrary to Mendel's observations, recessive characteristics do not appear in later generations either. Which character becomes dominant depends on the mother. The genes of characteristics that show maternal inheritance are not found in the nucleus but in the mithocondria of the cytoplasm, which come with the ovule, so all the genes they carry are maternal origin too.

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