EXTRA CHROMOSOMAL INHERITANCE

CYTOPLASMIC INHERITANCE

DNA in the nucleus is the universal genetic material. However, not all the genetic material of a cell is found in the nucleus. Some traits are governed by genes present in the cytopslasm. Those traits which are transmitted by DNA in cytoplasmic organelles are called as cytoplasmic inheritance.

They are also referred to as extra-nuclear inheritance, non-Mendelian inheritance and maternal inheritance. As the genes for cytoplasmic inheritance are located in the cytoplasm, they are referred to as plasmagenes, cytoplasmic genes, extra-nuclear genes or extra-chromosomal genes. Plasmagenes are located in the DNA present in mitochondria (mt -DNA) and chloroplasts (cp-DNA); together these DNA’s are termed as organelle DNA.

Cytoplasmic inheritance was first reported by Carl Correns (1909) while studying the inheritance of leaf variegation in Mirabilis jalapa.

FEATURES OF CYTOPLASMIC INHERITANCE

1. In most organisms, the cytoplasm of the offspring is inherited from the mother. Hence, the cytoplasmically inherited characters are passed only from mother to offspring and never from the father.

2. As a result, reciprocal crosses (AxB; BxA) exhibit differences and deviate from Mendelian pattern.

3. Cytoplasmic genes are not uniformly distributed during cell division. Hence, there is an extensive phenotypic variation as the cells of an individual will contain cytoplasmic genes in various proportions. Usually the females has more influence on the trait as they carry more cytoplasm than the male.

4. Unlike nuclear genes which show linkage, the extra-nuclear genes fail to show linkage.

MITOCHONDRIA AND CYTOPLASMIC INHERITANCE

Organelles like mitochondria and chloroplasts contain DNA. Mitochondria are presently considered as living organisms. The mitochondrial DNA (mtDNA) is a circular molecule ranging from 16 kb to several hundred kilobase pairs. The mtDNAs of humans, mice and cattle exhibit the same basic organization. Each mtDNA has 2 rRNA genes, 22 tRNA genes and 13 structural genes. The entire mtDNA is equivalent to one operon in bacteria. The human mitochondrion contain about 15,000 nucleotides and enclodes 37 genes while the nuclear DNA contains 3 billion nucleotides and encodes 35,000 genes.

MATERNAL EFFECTS

Maternal effect is a genetic phenomenon often confused with cytoplasmic inheritance. In maternal effect, the phenotype of the offspring is determined by the genotype of the mother. Eg. Shell coiling in Snail.

In Limnaea (a snail), the dominant allele D produces right-handed coiling, while the recessive allele d produces left-handed coiling. The direction of shell coiling in an individual is governed by the genotype of its female parent and not by its own genotype. As a result, reciprocal crosses show difference in coiling in F1 and there is no phenotypic segregation in F2. The phenotypic effect of segregation is observable in F3 crosses between female with left handed coil (dd) and males having rans Q 00 x a Of Qu x oof right handed coil (DD) produce Fi oextma, | SiMIBTRAL SmURTRAL | DEXTRAL progeny (Dd) with left handed coil |

since the genotype of the female Fi, core ween, <

parent is dd. In Fo, segregation of )))

Dd produces three genotypes in @

the ratio of 1:2:1. But snails with Fe too 200: 168 106 1 10 5 aa

DD, Dd as well as dd genotype Bee

exhibit right handed coiling since avogermas ‘AML Coacrman

genotype Dd_ producing right tx oo

handed coiling in all it progeny

(irrespective of the genotype of the

OD 100:2 O0:1ed oa

mh DEMTRAL 1 OO:200; 100 siwisTRAL

‘SINFSTRAL

ALL OEXTIUL

progeny). The F3 progeny from F2 individuals with the genotype DD and Dd will show

right handed coiling while those from dd

of their shells this produces the typical 3:

F2 individuals will exhibit left handed coiling

1 ratio (right handed : left handed) in F3.

The reciprocal crosses (female right handed DD x male left handed dd) on the other hand, yields right handed coiling in F; (Dd) as well in the three genotypes, 1 DD :2Dd : 1dd, obtained in Fy. But in F3 2/3 of the progenies show right handed coiling since they are derived from F2 individuals with the genotypes DD and Dd. The remaining 1/3 of the F3 progenies exhibit left handed coiling since the female parents had the genotype dd; this yields the typical 3:1 ratio in F3.

PLASTID INHERITANCE

Inheritance of characters due to

genes located in plastids is known as

plastid inheritance. This was the first

case of cytoplasmic inheritance to be

discovered by Correns and Baur in

1908.

Leaves of Mirabilis jalapa, the

four 0’ clock plant, may be green, white

or variegated. Correns made reciprocal

crosses in all combinations among the

flowers produced on these three types

of branches. When flowers from a green

branch are used as female, all the

progeny are green irrespective of the

phenotype (green, white or variegated)

*' Cuttings taken tom A, B & C produce

A= white plants

8 - variegated plants

C— green plants

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B.Sc.Hons.Agri/GPB 214 Principles of Genetics and Cytogenetics (2+1)/Dr. T. Sabesan et al./ 141

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of the male parent. Similarly, progeny from the crosses involving flowers from white

branches as the female parent were all white irrespective of the male flowers being

from green, white or variegated branches. But in the progeny from all the crosses

involving flowers from variegated branches as the female parent, green, white and

variegated plants were recovered in variable proportions.

The maternal transmission of plasmagenes in higher plants and animals is

explained on the basis of unequal contribution by male and female gametes to the

cytoplasm of the zygote. During fertilization, ordinarily only the nucleus enters the egg

cell and its cytoplasm is left outside the egg cell. As a result, the mitochondria and

proplastids present in the zygote are derived from the female parent only. Therefore,

plasmagenes located in these organelles will also be derived from the female parent

only.

NUCLEAR CYTOPLASMIC INTERACTION

An unusual case of interaction between nuclear and cytoplasmic genomes is reported in maize. A type of variegation, called iojap, is produced by a recessive gene ij; plants homozygous (ij ij ) for this gene develop the typical iojap variegation. But once this variegation is produced by the nuclear gene ij, it shows a typical cytoplasmic inheritance. Clearly, the nuclear genotype ij ij has a mutagenic effect on the plastid genome. Once, this mutation is induced in some cp-DNA molecules the variegation is inherited cytoplasmically.

Female Female Male A cross between normal (Ij Ij ) plants as female and iojap (ij ij) plants as males produces all green plants in Fi with the nuclear genotype Jj ij. In Fo generation of this cross Y4 progeny are ij ij and develop the iojap variegation, % of the progeny are normal green. When the iojap F2 plants are mated with normal green plants, a marked-reciprocal

Pigmented

chloroplast Colarless

chloroplast

Nuchear —» (Iplj) (yap) cy) lp

ee ‘© Co

- 0000

iif diij Tig aif {a) All white leaves

F, (b) All striped leaves ayig

Fig. Maternal inheritance of iojap gene (ij) in maize. The wild type

gene is Ij. The appearance of leaf colour is solely determined by the

phenotype of the female parent i.e. maternally inherited difference is observed in the progeny. When iojap plants are used as males and green

plants as females, all the progeny are normal green. But in the reciprocal cross green,

white and iojap progeny are recovered, the ratio between the three types of progeny is

quite variable.

EXTRA CHROMOSOMAL INHERITANCE | CYTOPLASMIC INHERITANCE , PLASTID INHERITANCE , NUCLEAR CYTOPLASMIC INTERACTION