Dr. ANJU BATTA SEHGAL
Mendel’s principles of segregation and independent assortment are based on the assumption that genes are located on chromosomes inside the nucleus of a cell. For most genetic characteristics this assumption is valid and Mendel’s principles allow us to predict the type of offspring that will be produced in a genetic cross. However, not all genetic material of a cell is found in the nucleus some characteristics are encoded by genes located in cytoplasm. These characteristics exhibit cytoplasmic inheritance. Newly conceived foetus contains 50+50 genome of both the parents, but the cytoplasm entirely comes from mother. Sperm only acts as a delivery capsule of paternal genome. The first cell of foetus is donated by mother. Few organelles namely chloroplast and mitochondria contain DNA.
Maternal inheritance of cytoplasm continues in the progeny by girl child where as male even after inheriting from mother are not capable to transfer the cytoplasm to the next generation. Because sperms are only made up of nuclear portion no cytoplasm. If we draw a family tree the lineage of cytoplasmic inheritance stops from male child. The human mitochondrial genome contains around 15000 nucleotides of DNA, encoding 37 genes. Compared with that of Nuclear DNA which contains around 3 billion nucleotides encoding for some 20000 to 25000 genes. Size of Mitochondrial DNA is too small nevertheless mitochondrial and chloroplast genes encode some important characteristics.
Cytoplasmic inheritance differs from Nuclear inheritance. A zygote inherits nuclear genes from both the parents, but typically, all its cytoplasmic organelles and thus all it’s cytoplasmic genes come from only one of the gametes, usually egg. A sperm only contributes a set of nuclear genes from the male parent. Meaning thereby all the cytoplasmic genes are inherited from the mother generally. Cytoplasmic inheritance was recognised by Carl Correns in 1909 as an exception to Mendel’s Principle. Biologist Correns studied the inheritance of leaf variegation in four o’clock plant, Mirabilis jalapa. Correns found that leaves and shoots of one variety of Mirabilis jalapa were variegated, displaying a mixture of green and white splotches. He observed that some branches of variegated strain had all green leaves, other branches had all white leaves. Each branch produced flowers, so cross breeding could be done between flowers from variegated, green, and white branches in all combinations. Seeds from green branches will always give rise to green progeny, no matter whether the pollen was from a green, white or variegated branch. Same was true for flowers.
Correns crosses demonstrated cytoplasmic inheritance of variegation in Four o’clock plant. Phenotype of the progeny was determined entirely by maternal parent, never by the parental parent (The source of Pollen). Variegation in these plants is caused by defective Cytoplasmic genecpDNA, which results in a failure to produce the green pigment chlorophyll. Cells from green branches contain normal chloroplasts only, cells from white branches contain abnormal chloroplasts only, and cells from variegated branches contain a mixture of normal and abnormal chloroplasts.
A number of human diseases that exhibit cytoplasmic inheritance have been identified. These disorders arise from mutations in mtDNA (Mitochondrial DNA), most of which occur in genes encoding components of electron transport chain, which generates most of the ATP (Adenosine Triphosphate) the energy bonds, in aerobic respiration. One such disease is Leber Hereditary Optic Neuropathy. Patients who have this disorder experience rapid loss of vision in both eyes, resulting from death of cells in optic nerves. This loss of vision occur in early childhood (between the age of 20-24). But it can occur anytime after adolescence. There is much clinical variability in the severity of the disease, even within the same family. Leber Hereditary Optic Neuropathy exhibits cytoplasmic inheritance; the trait is passed from mother to children, sons and daughters alike.
(Author is ex Principal NSCBM Govt. College Hamirpur, Himachal Pradesh and former Professor of Botany)
