Poultry breeding has acquired pre-eminence over all other domesticated livestock enterprises. Production level of layer and broiler type of chicken touched spectular heights mainly because of sophisticated genetic approaches. However, tremendous potential for further improving the productivity not only of the high yielding stocks, but also the local breeds exists through modern breeding techniques. The poultry breeding programmes are based upon two primary components—one academic and another commercial. Academic poultry breeders have concentrated largely on research and monitoring the operation of the poultry industry whereas the commercial enterprises have dealt with development and sales.
The phenotypic expression of an individual is due to its genotype and the influence of the environment to which the genotype is subjected. The genotype refers to the genes it contains. The genes are the basic units of inheritance and are arranged linearly on the chromosomes. They constitute the link between parent and offspring, and are passed from one generation to the next through gametes. viz, ovum in female and sperm in male. Ovum and sperm contain only half the number of chromosomes characteristic of the species. The chromosome numbers are halved so also the genes during the germ cell division (meiosis). The chromosome number, however, is restored in the zygote due to union of sperm with the ovum at the time of fertilization.
Qualitative and quantitative characters
The qualitative characters are governed by one or a few pairs of genes, show discontinuous variation and least influenced by environment. In view of large enough effect on these genes, so as to permit their ready identification, they are sometimes referred to as major genes. Comb type, plumage pattern, plumage and shank colour are examples of qualitative characters in poultry. The characters determined by many pairs of genes or minor genes, each with a small effect and influenced by environment to a varying extent are known as quantitative or metric or polygenic characters. These traits show continuous variation, which can be measured or quantified. Most of the economic traits in poultry like egg production, egg weight, body weight, viability, fertility and hatchability are the examples of the quantitative traits.
Dominant and recessive characters
Qualitative characters are inherited in a simple Mendelian way. When a cross is made between a true breeding Rose Comb type and true breeding Single-comb type, all the resultant offsprings are Rose Comb type. Similar results are also obtained when cross is made between homozygous individuals possessing any other contrasting characters. The character which appears in the cross (F1) is called dominant and other recessive. The recessive genes although do not express themselves in F1 generation are not lost as they do appear in F2 generation when cross is made between two F1 individuals. In some cases, however, neither of the two characters entering into the cross but somewhat intermediate between the two appears. As for example when a cross is made between frizzle and normal plumage chicken, the resultant progeny are semi-frizzle. This condition is usually known as incomplete or lack of dominance.