Uniparental inheritance of organelle genes

the remarkable and varied molecular and cellular mechanisms that produce these patterns. I will also consider the evolutionary consequences of uniparental inheritance and some of the proposed evolutionary explanations for this remarkable difference between the inheritance of organelle and nuclear genes. Patterns and mechanisms of uniparental inheritance vary widely Correns studied the four-o'clock (Mirabilis jalapa) in which the chloroplasts are inherited exclusively from the female parent, pollen having no plastids. Although this maternal inheritance is often considered a general phenomenon of organelle gene inheritance, it is not. In Pelargonium zonale, Baur found that many progeny received chloroplasts from the female parent only, but a number received plastids from both parents, producing variegated progeny in which the maternal and paternal chloroplasts segregated into different cells. A few progeny showed chloroplasts of only the paternal type. Baur correctly guessed that this is a consequence of the random segregation of plastids between embryonic and extraembryonic cells, so that embryos and adult plants sometimes received only green or only white plastids. Studies of chloroplast gene inheritance were quickly extended to other plant species. More than 80% of angiosperm genera show strictly maternal inheritance of chloroplast DNA (cpDNA), while the remainder produce varying numbers of offspring with biparental or even paternal transmission. Thus far, all angiosperms have been found to show strictly maternal inheritance of mitochondrial DNA (mtDNA). In striking contrast, most gymnosperms show maternal inheritance of mtDNA but paternal inheritance of cpDNA. Both mitochondrial and chloroplast genomes are inherited paternally in the coast redwood Sequoia sempervirens. The inheritance of the poky mitochondrial mutations in Neurospora showed maternal inheritance, where the maternal and paternal parents are defined as those providing the stationary and cytoplasm-rich protoperithecia versus those providing the much smaller conidia, respectively. The conidia do contain mitochondria, however, so the mechanism of maternal inheritance is obscure in this case. A key part of Mendel's model of heredity is commonly referred to as his first law; in modern language, it says that alleles of a gene segregate during gametogenesis (and thus not during vegetative cell divisions). Another key element of his model is the idea that, during sexual reproduction, genes are inherited equally from the two parents; hence reciprocal crosses give the same results. The cellular mechanisms were clear within a few years of the rediscovery of Mendel's laws: during vegetative divisions, each chromosome replicates exactly once and mitosis ensures that each daughter cell gets …