Variation Dr. Ripon Chandra Paul - PowerPoint Presentation

1. VariationDr. Ripon Chandra PaulAssociate Professor

2. Variation Importance of variation Types of variation Phenotypic variationPartitioning of phenotypic variationCauses of phenotypic variation Genetic variation Non genetic variation/ Environmental variation Genotype Environment interaction Gene actions

3. Variation Variation is defined as the difference we observe in a sample of individuals in a population. Variation is measured in terms of variance or standard deviation.Dog

4. Importance of variation Variation is the raw materials with which the animal breeder works for herd or flock improve. If there were no variation among individuals, there would be no need to select or cull animals for breeding purposes. Because from the genetic standpoint they would all look or perform alike. The variation which is observed arises from both genetic and environmental influence. Genetic variation results from differences in the gene conditioned by genetic recombination and less frequency (frequently?) by gene mutation and chromosomal changes.

5. Types of variation There are mainly three types of variation which are as follows: Phenotypic variation Genetic variation Environmental deviation or Non-genetic variation.

6. Phenotypic variation Phenotypic variation refers to the observable or measurable differences among individuals within a population for a particular trait. This is the raw materials with which the animal breeder must work. The phenotypic variation that we can see or measures in animals in a herd are seldom, if ever, completely due to difference in genes.

7. The phenotypic variation among individuals is the raw material with which the animal breeder must work because where there is variation between individuals there is a chance to produce superior offspring than the parents. If there are no variations among individuals there would be no need to select or cull the animals for breeding purposes. Because from the genetic standpoint they would all look or perform alike. If no opportunity for selection or culling exists, improvement through the application of breeding principles would become impossible. So, it is said that, “No variation no sire can develop.” “Phenotypic variation is called the raw material for breeding principles”, Justify.

8. continuous and discrete variation in farm animalContinuous variation: The traits which expresses their phenotypic values into decimal figures and exhibit a kind of continuous variation. It usually exhibits the continuous scale of measurement. Example: Milk yield/ day of different individuals of a population = 1.2 kg, 1.0 kg, 3.2 kg, 3.9 kg, 2.2 kg, etc.Discrete variation: There are some quantitative traits which do not exhibit continuous scale of measurement. Rather their observations are seen to be discontinuous in nature.Example: Litter size of different goat:Goat 01: 1Goat 02: 3Goat 03: 2

9. Partitioning of phenotypic variationThe total phenotypic variation for traits in farm animals is due to heredity, environment or the joint effect between heredity and environment.Phenotypic variation is denoted by σ2pσ2p = σ2H + σ2E + σ2HE = σ2A + σ2D + σ2I + σ2E + σ2HE = σ2A + σ2D + σ2I + σ2PE + σ2TE + σ2HEWhereσ2H= Variation due to heredity/ genotypeσ2E= Variation due to environmentσ2HE= Variation due to interaction between heredity and environmentσ2A= Variation due to additive gene actionσ2D= Variation due to dominant gene actionσ2I= Variation due to epistatic gene actionσ2PE= Variation due to permanent environmentσ2TE= Variation due to temporary environment When σ2HE is used?Phenotypic Variationσ2p = σ2H + σ2EExtension of Phenotypic Variationσ2p = σ2H + σ2E + σ2HEThe extended equation becomes applicable when at least one genotype but more than one environment is under consideration.Example: When a HF cow is kept in intensive farm management system first and then at extensive farm management system, then the extended equation in applicable.

10. Causes of phenotypic variationThe total phenotypic variation for traits in farm animals is due to heredity, environment or the join effect between heredity and environment. The importance and influence of each of these will be discussed separately.

11. Variation due to heredity or Genetic variation (σ2H )  The genotype of an individual is fixed at conception and except mutation it remains the same for the remainder of its life. The genetic make up of an individual is determined by the gene that it received from both parents. The individual as well as its parents possess thousands of genes, the exact number has not been determined. Probably the only two animals resulting from sexual reproduction that are alike genetically are identical twins produced by a single fertilized egg. Members of an inbred line are more likely to be alike genetically. Parents are not homozygous for all of the genes they posses and since they may differ genetically, no two of their offspring are genetically alike, except identical twins. Within same families the children have a closer resemblance to each other than in other families. This is because the children in those families are more alike genetically than in other.

12. Gene action Gene action can be defined as the way how genes can express themselves phenotypically. Gene action is basically of two types-1. Additive gene action2. Non-additive gene action.Additive gene action: The kind of gene action where the qualitative capability of individual genes are added together to form the final genetic merit of the individual is called additive gene action. Most of the quantitative traits are controlled by additive type of gene action.Example: suppose that, milk yield of a cow is controlled by 40 additive gene action and this cow give 2 kg milk/ day. So, 40 additive genes are responsible for 2 kg milk. Therefore, 1 additive gene is responsible for 0.05 kg milk/ day, but it is not equal for all genes. Gene no. 1may give 0.50 kg milk/ day, gene no 2 may give 0.0052 kg milk/ day.

13. Non-additive gene action: In non-additive gene action, the phenotypic expression of one gene doesn’t necessary (necessarily?) add to the phenotypic expression of the other, but members of allelic pairs may interact to give a certain phenotypic effect or two entirely separate pairs of genes may interact with each other to produce a particular phenotype.Non additive gene action may be several different types: Dominant gene action Recessive gene action Over dominant gene action Co-dominant gene action Double dominant gene action Epistatic gene actionGene action

14. Components of genetic variation (σ2H). Which component is most important and why?Components of genetic variation: Variation due to additive gene action (σ2A)Variation due to dominant gene action (σ2D)Variation due to epistatic gene action (σ2I) The additive gene action for a certain trait is most important because 50% of it is transmitting to the next generation with guarantee. For other kinds of gene, such as, dominant, recessive, epistatic, etc. such guarantee can not be provided. The additive gene action part of the genetic variation for certain trait is called its breeding value which is very important and central to the concept of animal breeding.

15. Variation due to Environment or Non-genetic variance (σ2E) Phenotypic variations in economic traits due to environment are also of great importance. The environmental portion of the phenotypic variation may be denoted by the symbol σ2E. Environment includes all such factors as disease, nutrition supply, temperature effects, accidents and others which the individuals may encounter from the time of conception until its death. Phenotypic variations due to environment are important because-They are not transmitted from parents to their offspringThey overshadow variations due to heredityThe proper environment is necessary for an individual to reach its genetic potential: Rapid improvements can be made in the efficiency of animal production by supplying uniform and superior environmental conditions to breeding animals and those used for commercial purposes.

16. Types of environmental effect Basis 1:σ2E= σ2ET+ σ2EPWhere, σ2ET = variation due to temporary environment σ2EP= variation due to permanent environmentBasis-2: σ2E= σ2EG+ σ2ESWhere, σ2EG= variation due to environment general σ2ES= variation due to environment special

17. A. Permanent environmental effect:Permanent environmental effects are those which influence all observation made on the individuals or any item that creates a permanent barrier to the animal performance.Example: when a cow in her early stage of life due to accident lose a teat and for this she suffer in her whole life (in production)The calf which born with more weight its growth rate will be more.B. Temporary environmental effect:A temporary environmental effect is one which influences only a single observation on the individual. It may involve either higher or lower performance.i) when a milking cow is affected by diseases and subsequently reduced milk yield.ii) Milk yield variation due to temperature.  

18.  C. Special environmental effect: By special environment we mean the within individual variance arising from temporary or localized circumstances. e.g. A cow will not give the same amount of milk every day.D. General environmental effect:By general environment we mean the within individual variance contributing to the between individual component and arising from permanent or non localized circumstances.e.g. cow A and B, when they are of same breed, same location, same nutrition and giving the same feed, labor, etc. the cow will differ in their production. Such as Cow A Milk yield X=1.7 kg/dayCow B Milk yield X=1.9 kg/day 

19. Characteristics of environmental variance: The nature of environmental variance very much depends on the character and organism studied.Error of measurement is another very important source of non-genetic or environmental variation. It can be reduced by careful management and proper experimental design Nutritional and climatic factors are the commonest external causes of environmental variation and they are at least partly under experimental control. Maternal effects from another source of environmental variation that is sometimes important, particularly in mammals but is less susceptible to control. Some variation arises from developmental variation that is which cannot be attributed to external circumstances but to accidents or to errors of development as a general cause. Characters more susceptible to the influences of the external environment, in contrast, are those connected with metabolic processes, such as growth, fertility and lactation. Tick, lice, parasite are the cause of productivity hamper.

20. Genotype Environment Interaction (H×E or HE):The interaction between heredity and environment means that animals of a certain genotype may perform more satisfactorily in one environment than in another. In other words, one environment permits the expression of genetic characters in a breed or stain, while another does not. This can be illustrated from the following figure-Three different genotypes, including Brahman×Hereford, Friesian×Hereford and Simmental× Hereford are used to illustrate this principle. No interaction between heredity and environment is illustrated between Brahman×Hereford and Simmental× Hereford because they gain relatively the sane amount of body weight at weaning in medium level of nutrition and in high level of nutrition, although they differ in their performance but their graphical representation is in the same manner and Brahman×Hereford performed better performance than Simmental× Hereford in both environment (level of nutrition). An interaction is demonstrated between Friesian×Hereford and other two genotypes Brahman×Hereford and Simmental× Hereford. Friesian×Hereford was superior in performance to either genotypes Brahman×Hereford or Simmental× Hereford in high level of nutrition and was medium to either in both low and medium of nutrition. This illustrates the importance of having the right strain or breed for a particular environment in order to obtain optimum performance.

21. Heredity or Environment- which one is more important for getting optimum production and why? Heredity and environment, both are of very great importance. The best possible environment will not develop a superior herd or flock unless the proper inheritance is also present in the animals. On the contrary, the best possible inheritance will not result in a superior herd or flock unless the proper environment is also supplied so that the animals can attain the limit set by their inheritance,This is why the dairy men do not use beef breeds for milk production and the racing enthusiasts do not include draft in their racing strings. In these extreme examples it can readily be seen that, the limit to performance set by the animals own heredity and the best possible environment will not cause that individual to exceed its genetic potential. To make the most possible use of good inheritance one must select breeding animals that are superior because they possess more desirable genes or combination of genes.

22. Heredity or Environment- which one is more important for getting optimum production and why?To make the most possible use of good inheritance one must select breeding animals that are superior because they possess more desirable genes or combination of genes. Superiority due to environment is not transmitted by the parents. The proper environment is of great importance from the economic standpoint. In addition, it is becoming more and more obvious that animals must be kept in the kind of environment that allows them to show that they possess desirable inheritance for a particular trait. For example, the selection of individuals for increased disease resistance. Sometimes confusion arises if breeders believe that effects upon the animal’s phenotype also influence the genotype. In animal breeding, we are looking for sustainable improvement of the livestock and genetics is the only avenue through which it can be brought up. Because after the formation of zygote there is no chance of changing the genetic make-up and the genotype remain same through the whole life of individual. Therefore, it requires more attention than others. On the other hand, environment is not permanent and may vary from time to time depending upon the climatic condition of the region and managemental practices.From the above discussion it is very much clear that genetics is more important than environment.