Botany · Ch 2 — Classical Genetics
Extra Chromosomal Inheritance (Cytoplasmic Inheritance)
Extra Chromosomal Inheritance (Cytoplasmic Inheritance)
DNA is the universal genetic material, and genes located in the nuclear chromosomes follow ordinary Mendelian inheritance. Certain traits, however, are governed by genes located not in the nucleus but in the cytoplasmic organelles - the mitochondria or the chloroplasts - a phenomenon called extra-nuclear or extra-chromosomal inheritance, and, because it works through the cytoplasm rather than the nucleus, also called cytoplasmic inheritance. It is a form of non-Mendelian inheritance, based on an independent, self-replicating extra-chromosomal genetic unit called a plasmogene, carried inside the chloroplast and the mitochondrion. Because the egg cell contributes the bulk of the zygote's cytoplasm while the sperm or pollen contributes essentially only its nucleus, cytoplasmically inherited traits are typically passed on maternally - offspring resemble the maternal parent for the trait in question, regardless of which direction a cross is made, which is the key diagnostic difference from a nucl …
Chloroplast Inheritance
Chloroplast inheritance is demonstrated in the four o'clock plant (Mirabilis jalapa), which has variegated leaves occurring in two forms - plants with dark-green leaves and plants with pale-green leaves. Ordinary Mendelian inheritance predicts that a cross should give the same F1 result regardless of which parent supplies the pollen and which supplies the egg. But when dark-green pollen is used to fertilise a pale-green plant, and, in the reciprocal cross, pale-green pollen is used to fertilise a dark-green plant, the two F1 generations are not identical - in each case, the F1 offspring instead resemble whichever parent served as the female (egg-bearing) parent, not the male pollen parent. This maternal pattern arises because the trait is carried by chloroplast genes present in the cytoplasm of the egg cell; the male gamete (pollen) contributes only its nucleus at fertilisation and essentially no cytoplasm, so it cannot pass on cytoplasmic chloroplast genes to the offspring at all. At the cellular level, a variegated leaf is a patchwork of cells carrying different proportions of green (chlorophyll-containing) and white (chlorophyll-lackin …
What this figure shows. Two reciprocal crosses in Mirabilis jalapa shown side by side: dark-green-leaved plant (female) x pale-green-leaved plant (male) gives an F1 of dark-green leaved plants; pale-green-leaved plant (female) x dark-green-leaved plant (male) gives an F1 of pale-green leaved plants - in both crosses the F1 matches the female (egg-donating) pare …
What this figure shows. A diagram of a variegated leaf's cells showing a nucleus surrounded by a mixture of green chloroplasts and white (chlorophyll-lacking) chloroplasts, with arrows showing that a cell/ovule inheriting only green chloroplasts gives rise to all-green offspring tissue and one inheriting only white chloroplasts gives rise to all-white offspring tissue. …
Mitochondrial Inheritance
Mitochondrial inheritance is best illustrated by male sterility in pearl millet (Sorghum vulgare/Pennisetum), a condition called cytoplasmic male sterility because it is inherited maternally through genes carried in the mitochondrial DNA rather than through the nucleus. Plants of this species occur in two cytoplasmic types - normal cytoplasm (N), which is male-fertile, and aberrant cytoplasm (S), which is male-sterile - and, exactly as with chloroplast inheritance in Mirabilis, these types show reciprocal differences: an S-cytoplasm plant used as the female parent cannot itself be used as a pollen (male) source, because its pollen is sterile. The picture is complicated further by nuclear restorer-of-fertility (Rf) genes, which are ordinary nuclear genes but have no effect of their own unless the sterile (S) cytoplasm is present - Rf genes exist purely to counteract cytoplasmic male sterility when it occurs. Consequently, fertile pollen is produced either when N cytoplasm is combined with the recessive rfrf genotype, or when S cytoplasm is combined with the dominant RfRf genotype (the restorer allele switching fertility back on); S cytoplasm combined with rfrf, lacking any restorer allele, produces only ste …
What this figure shows. Two reciprocal crosses in pearl millet: a normal-cytoplasm (N) male crossed with an aberrant-cytoplasm (S) female gives male-sterile F1 offspring, so the cross cannot be repeated in reverse (an S-cytoplasm plant cannot serve as the male, since its pollen is sterile), illustrating that male sterility here is inherited strictly through the …
What this figure shows. Two crosses contrasting fertility outcomes: an S-cytoplasm female with the restorer genotype RfRf, crossed with a fertile N-cytoplasm rfrf male, produces fertile male offspring because the dominant restorer allele is present; an S-cytoplasm female with genotype rfrf, lacking any restorer allele, produces sterile male offspring even when crossed with a fertile male, showing that fertility restoration in S cytoplasm strictly requ …
Atavism
Atavism is the reappearance of an ancestral trait that had apparently been lost over the course of evolutionary generations - a kind of evolutionary throwback. Traits that disappear at the level of phenotype do not necessarily disappear from the organism's underlying DNA sequence; the gene itself can persist, intact but switched off (inactive), for many generations. As long as the sequence remains intact somewhere in the genome, a fault in whatever genetic control normally keeps that gene suppressed can allow the old character to reappear, sometimes after a very long evolutionary gap. The flowering plant Hieracium pilosella (mouse-ear hawkweed), in which se …