Cytoplasmic inheritance

1. DR. ASHISH PATEL
Assistant professor
Dept. AGB, Veterinary College, AAU,
Anand

2. In mendelian inheritance
 The contribution of both male and female is equal, so,
the reciprocal crosses are identical.
 The segregation produces 3:1 ratio in F1 monohybrid and
9:3:3:1 in F2 dihybrid.
 The gene showing the mendelian inheritance are located
on chromosomes of nuclei.

3. In non mendelian inheritance
 The reciprocal cross gives different results.
 The trait onlyfrom the female parent is transmitted in to
next generation (Because the cytoplasm is usually
contributed entirely by one parent).
 Nuclear genes can be easily mapped on chromosomes,
but it is difficult to map cytoplasmic genes or prepare
linkage map for such genes.
 There is no segregation F2 generation. So, the non
mendelian inheritance are divided in to two categories:
Maternal Effects
Extra nuclear inheritance / Cytoplasmic inheritance

4.  The evidence of cytoplasmic inheritance was first
presented by Correns in mirablis jalapa.
 In 1943, Sonnenborn discovered kappa particles in
paramecium and they are inherited through cytoplasm.
 In extra nuclear inheritance the trait or character of
female parent is only transmitted to the progeny. The
reciprocal crosses exhibit difference in phenotypes of
progeny and there is no segregation of gene in F2
generation. Such type of inheritance is called as
cytoplasmic inheritance, extra chromosomal
inheritance and maternal inheritance.

5.  The genes governing the characters showing non
mendelian inheritance are located outside of nucleus and
found in cytoplasm, these genes are called as plasma
genes or cytoplasmic genes or cytogenes or
extranuclear genes or extrachromosomal genes.
 The total gene present in the cytoplasm of a cell or an
individual is known as Plasmon, while all genes in plastid
called as plastome.
 The genes present in the mitochondria called as
chondriome.
 The mitochondrial genes are abbreviated as mt DNA and
the chloroplast genes are abbreviated as cp DNA.

6. Sr. Mendelian inheritance Non mendelian inheritance
1 Governed by nuclear genes. Governed by plasma genes.
2 Distinct segregation pattern. No distinct segregation.
3 Reciprocal differences not
observed.
Reciprocal differences
observed.
4 Does not show maternal
effects.
Shows maternal effects.
5 Genes can be easily mapped on
chromosomes.
Mapping of plasma genes is
very difficult.

7. Examples for Non mendelian inheritance
 Plastid inheritance in Mirabilis
 Shell coiling in snail
 Kappa partcles in Paramecium
 Cytoplasmic male sterility in maize
 Sigma virus in Drosophila melanogaster
 Milk factor in mice
Classes of Non mendelian Inheritance
There are three different classes:
 Maternal Effects
 Inheritance Involving Infective Particles
 Cytoplasmic Inheritance

8.  When the expression of a character is influenced by the
genotype of female parent, it is referred to as maternal
effect.
 Such characters exhibit clear cut differences in F1 for
reciprocal crosses.
 Example: Coiling Pattern of Shell in Snail (Lymnea
Peregra)
 Two types of coiling pattern - right handed (dextral)
(clockwise) and left handed (sinistral) (anticlockwise).

9.  The dextral coiling: dominant allele D and
sinistral by recessive allele d
Dextral coiling female X Sinistral coiling male
All F1 and F2 progenies are of dextral type coiling.
However, in F3 generation 3 dextral and 1 sinistral types
of coiling observed.
 In reciprocal cross,
Sinistral coiling female X Dextral coiling male,
All F1 progenies have sinistral coiling pattern but
in F2 generation all progenies have dextral coiling pattern.
However, in F3 3 dextral and 1 sinistral types of coiling
observed.
 This indicates that the inheritance of coiling direction in
water snail depends on the genotype of female parent
and not on its own genotype.

11.  The F1 progeny from both the crosses had the same
genotype, Dd but they showed the different phenotypes.
The phenotype of offspring by the mother’s genotype for
coiling.
 When F1 undergo for self fertilization the F2 offspring
from both the crosses, irrespective of their own
genotypes, showed the same phenotype (Dextral).
 The true nature of inheritance of coiling is indicated when
each F2 individual undergoes for self fertilization to
produce F3. In F3, 3 dextral and 1 sinistral types of
coiling observed and therefore this pattern of inheritance
is also called as delayed mendelian inheritance.

12.  “Phenotype of progeny exclusively depends on the
genotype of mother irrespective of the offspring’s own
genotype. Thus the ‘DD’ and ‘Dd’ mothers produce
dextral progeny while, ‘dd’ mother always produce
sinistral progeny”.

13.  In some cases, Non mendelian inheritance is associated
with infective particles like parasite, symbiont or viruses
which are present in the cytoplasm of an organism.
However, such cases are not considered as true
examples of cytoplasmic inheritance.
 Examples: Kappa Particles in Paramecium and Sigma
Particle in Drosophila
 T. M. Sonneborn described the inheritance of some
cytoplasmic particles known as kappa in Paramecium
aurelia.
 There are two strains of Paramecium: killer and sensitive.

14.  Killer strain produces a toxic substance called paramecin
that is lethal to other individuals called "sensitives" . The
production of paramecin in killer type is controlled by
certain cytoplasmic particles known as kappa particles.
The sensitive strains lack these particles.
 The kappa particles are transmitted through the
cytoplasm. The existence, production and maintenance
of kappa particles are controlled by a dominant gene ‘K’
present in the nucleus. However, ‘K’ cannot initiate the
production of kappa in the total absence of kappa in the
cytoplasm.

15.  When a Paramecium of killer strain is having the
genotype “KK” or (K+) conjugates with the Paramecium
of non-killer strain having the genotype “kk”, the
exconjugants are all heterozygous for “Kk” genes.
 The development of a particular type depends upon the
duration of cytoplasmic exchange If conjugation is
normal, i.e., lasts only for a short time, and no exchange
of cytoplasm takes place between the two, both killers
and non-killers (sensitive) are produced.
 However in rare or prolonged conjugation in addition to
the nuclear material, the cytoplasmic materials are also
exchanged.

16.  During this cytoplasmic exchange, the kappa particles
present in the cytoplasm of the killer type enter the non-
killer type and convert it into a killer type. So all the
offspring produced by the exconjugants are killer type.

17.  There are two types of cytoplasmic inheritance:
(1) plastid inheritance
(2) mitochondrial inheritance.
 The first example of cytoplasmic inheritance was
reported by Correns (1909) in a variegated variety of the
four-o'clock plant Mirabilis jalapa.
 Variegated plants have some branches which carry
normal green leaves, some branches with variegated
leaves (mosaic of green and white patches) and some
branches which have all white leaves.

18.  Flowers on wholly green branches produce seeds that
grow into normal plants.
 Flowers on variegated branches yield offspring of three
kinds- green, white and variegated in variable
proportions.
 Flowers from branches wholly white produce seeds that
grow into white plants that is without chlorophyll.
 The phenotype of the progeny always resembled the
female parent and the male made no contribution at all to
the character. So cytoplasm of the egg influences the
type of leaf in Mirabilis.

19.  The explanation for this unusual pattern of inheritance is
that the genes concerned are located in the plastids
within the cytoplasm, not in the nucleus and are therefore
transmitted only through the female parent.
 Plastids are of two types, namely green chloroplasts and
colourless leucoplasts.
 Green branches contain Green plastids in their leaves .
 Variegated branches contain Green plastids and
Colourless plastids.
 Colourless branches are due to the presence of
Colourless plastids.