Extrachromosomal Inheritance

Introduction The existence of genes, located in chromosomes and controlling phenotypes is known and predictable. But the firm establishment of such a chromosomal mechanism of inheritance does not necessarily preclude a role for other extra nuclear cell parts. Certain inheritance patterns in mutants like abnormal segregation pattern showing inheritance from a single parent, mostly the maternal inheritance demonstrated the possibility of some extra nuclear factors that play role in inheritance.

Based on the pattern of hereditary transmission, there are four observations that point to the existence of cytoplasmic genetic system: Because the female gamete contributes almost all of the cytoplasm to the zygote and the male gamete contributes only a nucleus, an inheritance pattern that differs between reciprocal crosses suggest a cytoplasmic involvement. This is clearly the basis for uniparental or maternal inheritance where the progeny always resemble one parent, most commonly the female parent. Whenever traits fail to demonstrate classical segregation patterns and deviate from standard ratios, the conclusion is again a cytoplasm-based type of inheritance.

When the traits failed to show linkage to any known nuclear linkage groups, and assorts independently from nuclear genes, a cytoplasmic mode of inheritance is suggested. Many types of mutants that fit the above criteria will show segregation during mitotic division. This is very common in variegated plants that carry more than one type of plastid (chloroplast) per cell. This leads to variegation, suggesting somatic or vegetative segregation of the plastid types

Chloroplast Chloroplasts are specialized organelles found in all higher plant cells. These organelles contain the chlorophyll hence provide green color. They are very important for the plant, because they constitute the sites of photosynthesis. The chloroplast is bounded by two membranes, an outer and an inner membrane with an intermembrane space between them.

Each granum consists of disc-shaped membranous sacs called thylakoids (80-120Å across) piled on top of each other. Regions of the thylakoid membranes in contact with each other are known as the stacked regions, while regions not in contact are known as the unstacked regions. The number of thylakoids in a granum may vary. The grana are interconnected by a network of anastamosing tubules called intergrana or stroma lamellae. Single thylakoids, called stroma thylakoids are found in chloroplast. Electron dense bodies called globuli are found in the stroma.

Genomic Structure of Chloroplast Chloroplast genomes resemble large bacterial plasmids or small chromosomes. Their size ranges from 110,000 bp to 160,000 base pairs, depending on the species. cpDNA is a closed circular double stranded molecule. In maize, each molecule of cpDNA is about 139,00 base pairs long and there are about 50 copies of this DNA per chloroplast. Each molecule is long enough to encode approximately 140 proteins

Features of chloroplast DNA (cp DNA) in lower eukaryotes and higher plants Species cpDNA (in kb pairs) No. genomes per organelle No. Organelles per cell cp DNA as % total DNA Chlamydomonas reinhardii 195 70-110 1 11% Euglena gracilis 135 40 15 3% Higher Plants 120-210 variable

Examples of Chloroplast Inheritance The Mirabilis jalapa plant may exist in three forms. Normal green variegated (patches of green and or non-Green tissue), and white (no chlorophyll). Egg cells from green plants carry normal green plastids, those from white plants may have both plastid types or just one type. If the plastids are defective with regard to chlorophyll synthesis, the F1 plant will be non-green, if they are not defective, the F1 plant will be green. Variegated parents produce eggs that contain both normal and defective plastids. These eggs give rise to plants, whose cell receive a majority of green plastids or whose cells receive larger number of white 7 plastids, which results in variegated plants.

Iojap trait is characterized by contrasting stripes of green and white colors on leaves. This character is controlled by gene (ij) located on chromosome 7 in corn. The dominant allele Ij is responsible for normal green plastids and its recessives allele ij for iojap (striped) character, so that IjIj is green and ijij is iojap. Female gametes from ijij can therefore, carry either only green plastids or only abnormal white plastids or both. Consequently if ijij is used as female parent and IjIj as male parent the F1 individuals (Ijij) will be green, white or stripped

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