Breeding & Production of Animal, Poultry and Fish (PDF)

Animal Breeding Science Principals of Population Genetics  Trait Genetics Environment Interaction Selection Genetic improvement of livestock Mating system...

Animal Breeding Science Principals of Population Genetics  Trait Genetics Environment Interaction Selection Genetic improvement of livestock Mating system I. Animal Breeding: I. Genetic Constitution of Populations. II. Factors Affecting Gene and Genotype Frequencies. III. Qualitative and Quantitative Traits. IV. Variations in Economic Traits In Farm Animals. V. System of Mating: I. Relationships and Inbreeding. II. Outbreeding and Hybrid Vigour. VII. Some Genetic Parameters of Populations: I. Heritability. II. Repeatability. III. Correlations. VIII. Principles of Selection. IX. Genetic Improvement of Livestock. II. Poultry Production:  Genetic Constitution of Populations It is that science which uses the principles of population genetics for the improvement of efficiency of livestock production. Genetics: is a science that studies heredity and variation. Heredity: is the transmission of traits from the parents to the offspring via genetic material. Variation: (refers to genetic variation) is the occurrence of differences among individuals of the same species. Therefore, genetics in general concerns with: - The genetic constitution of individuals, and - The laws governing the transmission of this hereditary information from one generation to the next. The science of genetics can be broadly divided into four major sub disciplines: 1.Transmission genetics (classical genetics). 2. Molecular genetics. 3. Population genetics. 4. Quantative genetics. Transmission genetics: is primarily concerned with genetics processes that occur within individuals, and how genes are passed from one individual to another. Thus, the unit of study for transmission genetics is the individual. In molecular genetics: we are largely interested in the molecular nature of heredity (DNA) and how DNA is transcribed to mRNA and how mRNA is translated to polypeptide chance (gene expression or phenotype). - Consequently, in molecular genetics we focus on the cell. Population genetics: studies the heredity of traits in groups of individuals that are determined by one, or only a few genes. Quantitative genetics: studies the heredity of traits in groups of individuals, that are determined by many genes. The main purpose of animal breeder is not to genetically improve individual animals, but to improve animal populations and to improve future generations of animals. A population may be defined as: Set of individuals or a group of individuals of the same species in a defined location. A population, in the genetic sense, is not just a group of individuals, but a breeding group. Populations are always dynamic; So, the gene pool is the sum total of all of the genes and combinations of genes that occur in a population. A trait: is any observable or measurable characteristic of an individual. a. Observable traits: Traits we would normally mention in describing the appearance of an animal. Example: Coat colour Size Muscling Leg set Head shape b. Measurable traits: traits we would likely refer to in describing how an animal has performed. Example: Weaning weight Lactation yield Time to run a mile The phenotype is the value taken by a trait. in other words, it is what can be observed or measured.  Examples of traits and phenotypes Trait Possible phenotypes Presence of horns Horned , polled , dehorned Yearling weight 250, 300 kg Shell colour White, brown Calving ease Assisted, unassisted Litter size 5,11,14 The genotype of an animal represents the gene or the set of genes responsible for a particular trait. The genetic makeup of an individual. Fixed  Qualitative traits (affected by characteristic of one or few genes): Remains the organism. It unchanged throughout life remains constant (e.g. hair colour). throughout life.  Quantitative trait (affected by many genes): Changes continually throughout the life (e.g. milk yield). Unchanged by  Qualitative traits: Not environmental affected by the environment. factors  Quantitative trait: affected by environmental factors.  P=G+E Environmental effects Its genotype An individual’s phenotype Phenotypic milk yield = G + E where: G: Genotype E: Environment Goodfrey Hardy, English mathematician, and Wilhelm Weinberg, a German physician developed in 1908 mathematical models and equations to describe what happens to gene pool of a population under various conditions. The Hardy–Weinberg law establishes a set of ideal conditions that allows us to estimate allele frequencies and genotype frequencies in populations in which initial assumptions about random mating, absence of selection and mutation, and equal viability and fertility hold true. Obviously, it is difficult to find natural populations in which all these conditions are met. In nature, populations are dynamic and changes in size and structure are part of their life cycles. Moreover, the Hardy–Weinberg equation describes a mathematical relationship that allows the prediction of the frequency of offspring genotypes based on parental allele frequencies. The Hardy–Weinberg law states that: In a large random–mating population with no selection, mutation, migration or chance, the gene frequencies and the genotype frequencies are constant from generation to generation and furthermore, there is a simple relationship between the gene frequencies and the genotype frequencies. 1. The population is sufficiently large - sampling errors and random effects are negligible. 2. Mating within the population occurs at random. 3. There is no selective advantage for any genotype i.e. no differential mortality and no differential reproduction. 4. The population is closed i.e. no immigration nor emigration 5. There is no mutation from one allelic state to another. 6. Meiosis is normal so that chance is the only factor operative in gametogensis. If the conditions of the Hardy–Weinberg law are met, the population will be in genetic equilibrium and two results are expected: First: If population is at equilibrium, the allelic frequencies do not change from one generation to the next. Moreover, allelic frequencies predict genotypic frequencies.  The Hardy – Weinberg law for two alleles: Male gametic Female gametic frequencies frequencies p(A) q (a) p (A) p2 (AA) pq (Aa) q (a) pq (Aa) q2 (aa) Moreover, the genotypic frequncies will be in the proportions p2, 2pq and q2 after one generation of random mating. Genotype AA Aa aa Frequencies p2 2pq q2 The frequency of the A allele among the offspring in above table = Freq. of AA + 1/2 Freq. of Aa = p2 + pq = p (p + q) =p Second, the equilibrium genotypic frequecies are attained in one single generation, of random mating. Whatever, the genotypic frequencies among the parents, if the allelic frequencies are p and q in males as well as in females, the genotypic frequencies among the offspring will be p2, 2pq and q2. 0.7 Max. heterozygosity p = q = 0.5 0.3 Several aspects of this relationship should be noted: 1. The maximum frequency of the heterozygote is 0.5, and this maximum value occurs only when the frequencies of A and a are both 0.5; 2. When the frequency of one allele is low, the homozygote for that allele is the rarest of the genotypes. - This point is also illustrated by the distribution of genetic diseases in humans such as albinism, for example, is a rare recessive condition in humans. 3. It is important to note how fast heterozygous increase in a population as the value of p and q move away from zero. 1. To determine the frequencies of alleles of a particular gene in a given population. 2. To track and predict how gene frequencies will be transmitted from generation to generation given a specific set of assumptions. 3. It represents an idealized situation (which may never happen). Real situation can be compared to the ideal. The divergence from the equilibrium tells you how the population is changing. 4. The Hardy–Weinberg principle can be also used to estimate allele frequencies of recessive genetic disorders (diseases). A practical use of the Hardy–Weinberg equation can be seen in analysis of genetic diseases example: cystic fiberosis, phenylketonurea (PKU) and albinism. Many genetic diseases are recessive so their expression is only exhibited by individuals who are homozygous. Genotype Phenotype AA Normal Aa Normal aa Disease expressed Note that in the case of a recessive allele, the heterozygote is indistinguishable from the dominant homozygote. 5. Estimation of frequency of heterozygotes or carriers of recessive in a population: The frequency of a recessive phenotype usually can be determined by counting such individual in a sample of the population. Their frequency in a population is represented by q2, provided that mating has been at random and all Hardy–Weinberg conditions have been met in the previous generation. If Hardy–Weinberg equilibrium can be assumed, the frequency of heterozygote is given by 2pq. The frequency of heterozygote among normal individuals, denoted by H, is the ratio of genotype frequencies Aa ----------- where, a is the recessive allele. AA + Aa So that, when q is the frequency of a,  2pq In general, the frequencies H = ------------- of all three genotypes can p2 + 2pq be estimated once the 2pq frequency of either is = -------------- known and Hardy-Weinberg p (p + 2q) conditions are assumed. 2q = ----------- q = 0.007 p + 2q 2q H  2q 1 q = ------------- 2 (0. 0 0 7 ) p+q+q = 1  0.0 0 7 2q 0.1 4 = ---------- =  0.0 1 4 1+q 1.0 0 7