Influence of genetics and Mendel's laws on the breeding of poultry

The significance of genetics for zootechnical science and the practice of poultry farming. In the theory of evolution of the plant and animal world, Ch. Darwin highlighted the special importance of variability and heredity, the laws of which were revealed by Gregor Mendel and subsequent research by scientists who created genetics as a new branch of biological science.

The study of the structure of deoxyribonucleic acid (DNA) as a carrier of material heredity, as well as cytogenetic and biochemical studies at the cellular level, led to the further development of genetics.

This was facilitated by the development of fundamental research at the Research Institute of General Genetics of the USSR Academy of Sciences under the leadership of Academician N. P. Dubinin and other academic research institutions. The development and implementation of a coordination plan for work in the field of genetics and the development of links between theoretical research and applied sciences laid the foundation for the increasingly expanding use of the results of genetic research in agriculture. By the end of the 19th century, it had been proven that Mendelian principles were applicable not only to plants but also to animals, and they served as a starting point for the development of the genetics of agricultural poultry.

In the first quarter of the 20th century and beyond, the efforts of a number of genetics specialists were directed at establishing changes in inherited qualities in accordance with Mendel's laws. These works were successful with regard to easily inherited traits.

Bateson and others demonstrated the dominant epistasis of white plumage in Leghorns, followed by the dominance of this plumage color in Cornish chickens and some other breeds. The results of these studies were used to produce white-plumaged hybrid chickens:

• When crossing Cornish with chicken breeds that have recessive colored plumage, white crossbreeds are obtained. Carcasses of such poultry are in higher demand than carcasses of chickens with dark-colored plumage, on which dark feather follicles—the buds of feathers—are visible, which impairs their appearance and reduces the marketability of the carcasses.
• Crossing Moscow chickens, whose plumage is generally black, with white Leghorns or the Russian White breed produces almost entirely white crossbreeds. However, incomplete dominance is observed, and individual black feathers can almost always be noted in the offspring's plumage. Nevertheless, this has no practical significance and does not affect the market quality of the meat products.

The dominant or recessive nature of the inheritance of certain exterior traits, established in genetic research, has been used in breeding lines with desirable properties, plumage characteristics, and comb shapes that distinguish one breed from another. The character of trait inheritance is presented in the table:

The use of sex-linked inheritance of traits in poultry has certain peculiarities due to the fact that hens are heterogametic; a number of traits in hens are inherited in a "criss-cross" manner. This has enabled breeders to develop breeds and obtain hybrid pullets and cockerels that differ in plumage color at day-old age. Such breeds and hybrid poultry are called autosexing or sex-determined. For example, when crossing black roosters with Barred Plymouth Rock hens, chicks are obtained among which day-old cockerels are easily identified by a white spot on their heads.

In our country, the study of the genetics of agricultural poultry began in the first years after the Great October Socialist Revolution under the leadership of Prof. N. K. Koltsov, and the results of the first scientific works were summarized in the monograph "Genetics of the Domestic Chicken." Prof. A. S. Serebrovsky and his associates conducted a number of studies on the genetic nature of traits in chickens. The sex-linked nature of alleles discovered by A. S. Serebrovsky, which determine the speed of feathering in chicks during the first weeks of life, was later used to develop a method for determining the sex of day-old chicks based on feathering speed. When mating hens possessing the dominant slow-feathering trait with roosters having the recessive rapid-feathering trait, differentiation is observed with rapid-feathering pullets and slow-feathering cockerels. The differences between rapidly feathering day-old chicks and slowly feathering ones lie in the fact that they have wing flight feathers with unfurling vanes and shorter coverts compared to the primary feathers (most often 1/2–3/4 of the length), which is immediately noticeable.

Using sex-linked inheritance of plumage color and feathering speed allows for faster and almost error-free determination of the sex of day-old chicks, which has great practical importance. For example, raising broilers separately by sex increases the live weight and meat quality of the poultry; to replenish the industrial stock of laying hens for rearing on a farm specialized in egg production, only day-old pullets can be accepted, as the economic efficiency of egg production increases as a result.

In our country, autosexing broiler crosses "Neva-2" and "Broiler-8" are used, and others are being developed; in foreign poultry farming, crosses differing in plumage color and speed of feathering of day-old chicks have become widespread.

It is important for a practical poultry farmer to understand how economically useful traits of poultry are inherited. Modern industrial poultry farming relies entirely on hybridization — the production of highly productive crosses and lines. At the heart of this work lies the management of the genetic structure of populations. By using targeted selection and matching, a breeder reshapes the genotype of the poultry to meet the needs of a specific production.

Any breed or line of poultry is a population possessing a certain heterogeneity in genotype (heterozygosity). Under natural conditions, diversity helps wild birds adapt to their environment. At a poultry farm, natural processes give way to artificial selection. If breeding work is stopped, the population will quickly return to its initial genotype structure, and the productivity of the poultry will fall.

Heritability coefficients and their significance for breeding

Each economically useful trait has its own degree of heritability. The higher the heritability coefficient, the faster and more effective breeding will be through the direct selection of individuals with the desired indicators. For traits with low heritability, more complex methods of evaluation and matching of parent pairs are required.

• Heritability of adult chicken weight — 60%
• Heritability of egg weight — 60%
• Heritability of egg production for the first year — 30%
• Heritability of egg hatchability — 15%

When conducting breeding work, one cannot be limited only to key productivity indicators. A breeder must also take into account the heritability of other economically useful qualities. This list includes:

• length and frequency of egg-laying cycles;
• shell thickness;
• plumage color.

Heritability coefficients fluctuate within wide limits depending on keeping conditions and the initial gene pool. A large range of values shows that a poultry farmer always has a reserve for improving the flock. The table below shows average benchmarks for the heritability of traits in chickens, obtained from the results of practical observations.