“Poultry Genetic Principles: Basics of Breeding”
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 case of female and sperm in case of male. Ovum and sperm contain only half the number of chromosomes, characteristic of the species. As the chromosome numbers are halved so also the genes during the process of 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 influence of environment on genotype varies greatly almost from nothing to a very great extent depending upon the character. The characters which are least influenced by environment, are governed by one or a few pairs of genes, show discontinuous variation and are known as qualitative characters. Comb type, plumage pattern, plumage and shank colour are examples of qualitative characters in poultry.
The characters determined by many pairs of genes, each with a small effect and influenced by environment to a varying extent, are known as polygenic traits. These traits show continuous variation, which can be measured or quantified and hence known as metric or quantitative traits.
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 offspring 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 enter 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 is semi frizzle. This condition is usually known as incomplete or lack of dominance.
Chromosomes
Chromosomes occur in pairs and their number varies from species to species. There are 39 pairs of chromosomes in chicken, 41 pairs in turkeys and 39 pairs in Japanese quails. Only one of the several pairs of chromosomes, characteristic to a species, are sex chromosomes and the rest are autosomes. In avian species, sex chromosomes are designated as Z and W corresponding to X and Y chromosomes in the mammals. In avian species the male is homogametic (ZZ) and the female is heterogametic (ZW).
Any character whose genes are located on sex chromosome is called a sex-linked character, c.g. rapid and slow feathering, low barring and unbarring in chicken. A hen can transmit her sex-linked characters to her son but not to her daughter but the cock transmits the character to both sons and daughters. This behaviour of sex-linked genes are utilized for autosexing at day-old stage. As for example when homozygous males for rapid feathering (kk) are mated to hens with slow feathering (K-) all the female chicks will have rapid feathering and male chicks slow feathering.
One of the common methods of estimating the importance of sex-linked genes in the inheritance of quantitative traits is to make reciprocal crosses between two strains or lines. When sex linkage is important the mean of cross approaches more towards the sire line than the dam line.
Sex-limited traits like egg production need to be differentiated from sex-linked traits. Sex-limited traits are those which are expressed in one sex but not in the other.
Inheritance of Quantitative Traits
The quantitative characters are determined by many pairs of genes. Although the inheritance pattern remains Mendelian with respect to individual pairs of genes, it becomes difficult to identify one phenotype from the other. This is because the phenotypic ratios become numerous, and the differences among phenotypic classes reduce considerably and overlap. The number of phenotypes in such cases becomes 2n where n is the number of gene pairs involved.
The multiplicity of gene pairs and the environmental effects complicate the study of inheritance of quantitative traits. Environment here includes all the non-genetic effects such as feeding, management and disease exposure. An approximate method of separating the genetic effects from the environmental influences is to estimate the heritability of the trait. When the heritability is high it is presumed that the environmental influences are less important than the genetic effects and vice versa. Heritability is an important attribute of a quantitative trait and is necessary for almost all breeding work connected with improvement of the species. The heritability of a trait ranges from 0 to 1. Traits connected with fitness of the organism such as egg production, viability, fertility and hatchability are lowly heritable and those with luxuriance or size are highly heritable. Examples of highly heritable traits in poultry are body weight, egg weight and egg quality traits.
When several traits are considered together in a breeding programme it is essential to know the relationship among them. The relationship among two biological traits is best expressed in terms of their genetic and phenotypic correlations. Genetic correlation between two traits is due to the same genes affecting the two traits (pleiotropy). Phenotypic correlation is due to simultaneous action of genes as well as environment. The magnitude of genetic and phenotypic correlations ranges from -1 to +1. Knowledge of genetic correlation enables us to predict how selection for one trait will bring about simultaneous changes in other traits.
Objective of a Breeding Programme
The primary objective of a poultry breeding programme is to improve product output per individual bird, to increase the efficiency of production and to improve the quality of existing product. Improvement in egg yield, fertility, hatchability, growth rate, viability etc. are different aspects of these three broad goals. Both choice of individuals to reproduce the next generation (which constitutes selection) and the manner in which they will mate (mating system) have been successfully used to bring about improvement in desirable direction.