Inheritance due to genes located in cytoplasm is called cytoplasmic inheritance. Since genes governing traits showing cytoplasmic inheritance are located outside the nucleus and in the cytoplasm, they are referred to as plasmagenes.

1. CYTOPLASMIC
INHERITANCE

2.  Inheritance due to genes located in cytoplasm
(plasmagenes) is called cytoplasmic inheritance.
 Since genes governing traits showing cytoplasmic
inheritance are located outside the nucleus and in the
cytoplasm, they are referred to as plasmagenes.
 The plasmagenes are located in DNA present in
mitochondria (mt DNA) and in chloroplasts (cp DNA).
 Together both the DNAs are called organelle DNA.
Therefore, this type of inheritance is often referred to as
organellar inheritance, plastid inheritance or
mitochondrial inheritance.

3.  In this, generally, the character of only one of the two
parents (usually female) is transmitted to the progeny.
 Hence such inheritance is usually referred to as extra -
nuclear or extra-chromosomal or maternal or uniparental
inheritance.
 The cytoplasmic inheritance is of two types: 1) Plastid
inheritance and 2) mitochondrial inheritance.

4. 1. Plastidial or chloroplast inheritance
 Plastids are self duplicated and have some
amount of DNA and plays an important role in
cytoplasmic inheritance.
 Plastids have green pigments called chloroplasts.
Chloroplasts contain a unique circular DNA (cp
DNA) in the stroma that is completely different
from the nuclear genome.

5. Leaf variegation in Mirabilis Jalapa
 The conclusive evidence for cytoplasmic inheritance was first
presented by C. Correns in Mirabilis jalapa (Four ‘O’ clock
plant) in 1909.
 He studied inheritance of leaf variegation in M. jalapa.
 Variegation refers to the presence of white or yellow spots of
variable size on the green background of leaves.
 In M. jalapa, leaves may be green, white or variegated. Some
branches may have only green, only white or only variegated
leaves.
 Correns made crosses in all possible combinations among the
flowers produced on these three types of branches.

6.  When flowers from green branch were used as
female parent, all the progeny were green
irrespective of the phenotype (green, white or
variegated) of male parent.
 Similarly, progeny from crosses involving flowers
bloomed on white branches as female parent
were all white irrespective of the phenotype of
male parent.
 But in progeny from all crosses involving flowers
born on variegated branches as female
parent, all the three types i.e. green, white and
variegated individuals were recovered in
variable proportions.

8.  The green leaf branches have normal chloroplasts,
white branches have mutant chloroplasts and
variegated have a mixture of both normal and
mutant chloroplasts.
 The above results indicated that the inheritance is
governed by chloroplasts.
 Since the cytoplasm is contributed to the zygote
mainly by female parent, the plastids are
transmitted to the zygote from the female parent.
 Thus the plastids are responsible for variation in the
crosses of green, white and variegated leaves.

9. Inheritance of faulty mitochondrial genes
(maternal inheritance)
 The number of mitochondria in every cell of a
person’s body varies from a few, to hundreds.
 All of these mitochondria, and therefore the DNA
within the mitochondria, descend from the small
number of mitochondria present in the original
egg cell at the time of that person’s conception.
 The sperm contributes very few mitochondria to
the baby.

10.  An individual’s mitochondria are generally only
inherited from his or her mother. A change
(mutation) in one of the mitochondrial genes that
makes it faulty, can therefore be passed by the
mother to a child in her egg cells.
 This pattern of inheritance is therefore often referred
to as maternal inheritance.

11.  The egg cell contains many mitochondria, each
having on average one to several copies of the
mitochondrial genes.
 If a particular gene in every mitochondrion in an
egg cell is faulty, the disruption to energy
production would be so severe that the early
embryo would probably not survive.
 The fact that a person survives to birth and is
affected with a mitochondrial condition means
that they must have inherited two types of
mitochondria from his or her mother: some
containing the working copy of the gene, and
some containing the faulty gene.

12.  The working copy of the mitochondrial gene will
still be able to send the right instructions, but the
amount of energy produced may be impacted
and may result in a mitochondrial condition.
 On the other hand, having some mitochondria
with a faulty gene may cause no problem at all.