Download Extrachromosomal Inheritance



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.


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

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 (80120Å 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.

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

Species
cpDNA (in
kb pairs)
No.
genomes
per
organelle
No.
Organelles
per cell
cp DNA as
% total
DNA
Chlamydomo
nas
reinhardii
195
70-110
1
11%
Euglena
gracilis
135
40
15
3%
Higher Plants
120-210
variable
variable
variable
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
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 nongreen, 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