Genetics - A. V. Sivolob 2008

Genetics of Multicellular Eukaryotes
Cytoplasmic Inheritance: Mitochondrial and Chloroplast Genetics
Main Characteristics of Organelle Gene Inheritance

In diploid eukaryotic species, nuclear genes determine the Inheritance of Traits according to Mendelian laws. This occurs because, with the exception of sex-linked traits, offspring receive one copy of each Gene from both parents. Cell/35.html">Mitochondria and Plastids are not distributed among Gametes and daughter Cells in the same manner as nuclear Chromosomes; therefore, the inheritance patterns of organelle genes and the traits they determine deviate significantly from Mendelian inheritance. Since mitochondria and plastids reside within the Cell Cytoplasm, this type of inheritance is referred to as extranuclear (extrachromosomal) or CYTOPLASMIC INHERITANCE.

The distinct features of extranuclear inheritance stem primarily from the fact that gametes of different sexes contribute vastly unequal numbers of mitochondria and plastids. Typically, female gametes contain thousands of mitochondria (and plastids in plants), whereas male gametes contain only a few. As a consequence, the zygote is dominated by maternal Mitochondria and METABOLISM/14.html">Chloroplasts. In some species, particularly humans, male mitochondria fail to enter the zygote altogether. Consequently, the inheritance of organelle genes is predominantly maternal (for details on extranuclear inheritance in humans, see Chapter 7). However, this general rule is not absolute—in certain rodent species, mitochondrial transmission to the zygote from both parents has been observed.

The characteristics of chloroplast gene inheritance were first investigated by studying leaf variegation in the four-o'clock plant. When flowers from an aclorophyllous (colorless) plant were used as the maternal parent and pollinated with pollen from a green plant, only aclorophyllous offspring (which soon die) were observed in F1. In the reciprocal cross, all F1 plants were normal. When flowers of variegated plants were pollinated with pollen from a green form, F1 yielded aclorophyllous, variegated, and green plants. The reciprocal cross produced exclusively green plants. Thus, this example demonstrates the maternal Inheritance of the Chloroplast Genome in plants, which is characterized by a stark contrast between the results of reciprocal crosses. Quite similar discrepancies in such crosses are observed when green and variegated forms are crossed in other plant species. However, everything depends on The amount of cytoplasm contributed to the zygote by the egg cell and the sperm. For instance, if flowers of variegated geraniums are pollinated with pollen from a green plant, up to 30% of the offspring will be variegated and 70% green. In the reciprocal cross, 70% of the offspring are variegated and 30% are green. In other words, biparental inheritance of chloroplast genes also occurs in plants.