BASIC PRINIPLES OF INHERITANCE

Dr. RAMESWAR PANDA

MVSC

BOMBAY VETERINARY COLLEGE

One of the major functions of all living things is reproduction. The reproductive

process of higher vertebrates, particularly of farm animals, is sexual in nature. In

them the body cells are of two types, Somatoplasm (or somatic cells) and

Germplasm (or sex or germinal cells). The germinal cells are concerned with the

reproduction or breeding of the animals. These cells are present in the testes in the

male and ovary in the female. The germinal cells give rise to spermatozoa or ova,

the gametes.

The cell, either germinal or somatic, is held in bounds by a cell wall. The cell

consists of a nucleus, nucleolus and the cytoplasm. There is certain darkly staining

material inside the nucleus known as chromatin. This chromatin will be in different

forms depending on the stage of cell activity. It may appear as scattered particles,

interwoven threads or rod shaped. Chromatin threads are known as chromosomes.

The chromosomes occur in pairs (diploid number) and they carry a large number of

genes, which are the basic units of heredity. Each species has a specified number of

chromosome pairs. For example cattle have 30 pairs, sheep 27, swine 19, and horse

30 and so on.

Gene, the factor, the determiner, is the unit of inheritance, which is

transmitted in the gametes or reproductive cells, and which by interaction with other

genes and with the environment, controls the development of a characteristic. Genes

are linearly arranged on chromosomes. Biochemically, a gene is a fraction of a

molecule of DNA. Genes are the physical or material basis of heredity.

The somatic cells grow and divide to bring about growth in body tissues. In

this process, known as mitosis, the chromosomes and accompanying genes

duplicate themselves, and each new cell is a carbon copy of the parent cell. A

specialized type of cell division takes in the formation of gametes (sperms and ova).

This is called meiosis. In meiosis, two cell division take place, one immediately

followed by the other. In one of these divisions, the chromosomes do not duplicate

themselves. Thus, the resulting cells will have only half the number of chromosomes

with one member from each pair of chromosome. Sometimes, there is process called

crossing over, in which intertwining and exchange of pieces between chromosomes

in a pair takes place at the time of cell division. This further adds to the possibility of

new genetic combinations. The process of meiosis during spermatogenesis and

oogenesis is crucial for production of individual sperm cells in the male and individual

ova in the female animals, comprising one half of inheritance from the male and

female parent, respectively.

In meiosis, the cell division takes place in the ovary in females and in the

testes in the males for the production of gametes. As the cell develops, at one stage

of development, two cell division take place in rapid succession. One of these is

similar to mitosis or the usual cell division when two identical daughter cells form with

diploid number of chromosomes. In the succeeding meiotic cell division, the

chromosomes do not duplicate themselves, thus producing haploid cells, which are

the gametes.

Fertilization takes place when the haploid male gamete joins the haploid

female gamete to form the diploid zygote. The zygote undergoes multiple divisions to

develop into a new individual.

Thus, an individual gets half of its inheritance or genes from male parent and

the other half from its female parent. A parent genetically superior in production is

most likely to transmit some percent (upper limit being 50%) superior genotype to its

offspring. To what extent a parent will transmit its genotype to its offspring, however,

purely depends on chance, i.e., which of the possible many combinations of ‘half of

its chromosomes’ are inherited by the offspring.

References

Livestock Production and Management by Thomas and Sashtry