Genetics with the Basics of Breeding - M.P. Myhun - 2008

CHAPTER III. Inheritance of Chromosomal and Non-Chromosomal Genes

3.5. Non-Chromosomal Inheritance

METABOLISM/2.html">THE CONCEPT OF hereditary factors unrelated to nuclear chromosomal genes emerged in science in 1901, when C. Correns and E. Baur described cases of non-Mendelian inheritance of variegation in plants. Over time, it became clear that DNA and RNA can be located not only in nuclear Chromosomes but also in the nucleoplasm, Organelles (Cell/35.html">Mitochondria, Plastids) of Eukaryotic Cells, Plasmids of prokaryotes, and as part of infectious agents and endosymbionts. All this Genetic information can be inherited by offspring, but it is impossible to explain this from the standpoint of the chromosomal theory of inheritance.

The process of transmitting genetic elements to offspring via non-chromosomal cellular structures is called non-chromosomal inheritance.

Characteristic Features of non-chromosomal inheritance include: the absence of Mendelian segregation, the presence of maternal (sometimes paternal or mixed) inheritance patterns, the independence of trait inheritance from the presence of specific nuclear chromosomes, and discrepancies in the results of reciprocal crosses, among others.

When traits determined by the GENES OF THE Cytoplasm in Eukaryotic cells are inherited, it is referred to as CYTOPLASMIC INHERITANCE. Cytoplasmic inheritance is quite common in the living world and is characteristic of all eukaryotic taxonomic species known to science. Animal Eukaryotic cells contain two genetic systems, while plant cells contain three.

The first genetic system is represented by The Genome—the haploid set of chromosomes and the entirety of all genes localized within them that make up the nuclear genotype.

The second genetic system of cells is the plasmon—the set of extrachromosomal genes located in the cytoplasm, excluding plastids.

The third genetic system of cells is the plastome—the set of genes in plastid DNA.

Mitochondrial DNA genes constitute the so-called chondriome, while plastid DNA genes make up the plastome.

All these genes are transmitted to offspring along with the cytoplasm of Germ Cells, independently of nuclear genes, and the inheritance of these traits cannot be explained from the standpoint of the chromosomal theory of inheritance.

This is explained by the unequal contribution of female and male Gametes to The formation of hybrid material, which is due to the differing quantity or quality of cytoplasm in the egg cell and the sperm cell.

In most cases, the zygote receives cytoplasmic genes primarily from the egg cell. In such instances, the offspring replicate the phenotype of the maternal Organism, exhibiting a maternal inheritance pattern. A paternal pattern also occurs; in some species, male gametes serve as Donors of cytoplasm. There is also a mixed type of cytoplasmic inheritance, where cytoplasm is inherited from both parents.

It is impossible to speak of complete independence between nuclear genes and cytoplasmic genes (plasmogenes). Some traits are encoded simultaneously by nuclear DNA and cytoplasmic DNA, which indicates a close interaction between nuclear and cytoplasmic genes. An example of a trait determined by both nuclear and cytoplasmic genes is the so-called male sterility in plants (where pollen is either not formed or is incapable of Fertilization). However, cytoplasmic male sterility is also known, which is inherited exclusively through the maternal line.

The cytoplasm also influences the expression of chromosomal genetic information; this is known as the maternal effect. Its essence lies in the fact that The properties of the egg cell's cytoplasm are formed under the control of the maternal genome. These changes in cytoplasmic properties can affect the phenotype of the offspring at various stages of their development, and later, at all Stages of the organism's development. The phenomenon of restructuring the Properties of the egg cytoplasm (and consequently, the zygote) under the Influence of the mother's nuclear genes is called genetic predetermination of the cytoplasm.

Foreign elements can enter The Cell from the outside: genes of infectious agents and genes of symbionts. Existing within eukaryotic cells, they exhibit the properties of plasmogenes, determine certain phenotypic traits, and are inherited through the cytoplasm via the maternal line. In addition to infectious agents, PROKARYOTES AND EUKARYOTES are known to have their own extrachromosomal elements. In Bacteria, besides the main circular DNA called the chromosome, There are many small circular molecules known as plasmids. The inheritance pattern of plasmid genes resembles the inheritance of plasmogenes in eukaryotes. Extrachromosomal elements of eukaryotes are copies of certain genes and their fragments that exist as circular DNA molecules outside the chromosomes.

Thus, the most important problems of non-chromosomal inheritance include:

1) Mitochondria and Plastids as carriers of genetic information in cytoplasmic inheritance;

2) Inheritance of Traits controlled simultaneously by nuclear and cytoplasmic genes;

3) infectious agents, endosymbionts, and certain other extrachromosomal elements as carriers of genetic information;

4) predetermination of cytoplasmic properties by nuclear genes and the maternal effect in inheritance.