Previous Year Questions(Solved) | Animal Husbandry & Veterinary Science Optional for UPSC PDF Download

Q1: Write short notes on Non-additive genetic variance.   (1993 Paper 11) 
Ans:
If the genotype at different loci show epistatic interaction, the interactions give rise to a component of variance V₁, which is the variance of the interaction deviations. Theoretical description of the properties of interaction variance rests on its further subdivision into components. It is first subdivided according to the number of loci involved: two-factor interaction arises from the interaction of two loci, three loci, etc. Interactions involving a larger number of loci contribute so little variance that they can be ignored. The next subdivision of the interaction variance is according to whether the interaction involves breeding values or dominance deviations. There are thus three sorts of two-factor interactions. Interaction between the two breeding values gives rise to additive x additive variances, V_AA. Interaction between the breeding value of one locus and the dominance deviation of the other gives rise to additive x dominance variance, V_AD, and the interaction between the dominance deviations gives rise to dominance x dominance variance, V_DD. So the interaction variance is broken down into components thus:

V₁ = V_AA + V_AD + V_DD + etc.,

The terms designated 'etc.,' being similar components arising from interactions between more than two loci.
Interaction between loci controlling quantitative characters is of frequent occurrence. It has been demonstrated in many studies of Drosophila. It is not easy, however, to estimate the amount of variance that it generates, and little is known about its relative importance as a source of variation. In the partitioning of the variance by relatively simple experiments, such as the one considered above, most of the interaction variance is included with the dominance component, which is then referred to as non-additive genetic variance.

Q2: Write a short note on: Lethal factors and genetic abnormalities. (CSE 1994)
Ans:
Lethal factors have been defined as genes which, when present in the homozygous condition, cause the death of the embryo, and when present in the heterozygous condition, cause a serious impairment in the individual, often leading to non-survival.
Lethals may be dominant or incompletely dominant, but many are certainly recessive. They are not always recognized since they may cause the death of the embryo early in development, and the mating may be regarded as having been infertile. Several lethals are met with in cattle such as parrot-mouthed, in which calves die a few hours after birth; amputed, in which calves are born dead with legs and lower jaw absent. Semi-lethals include over-shot jaws in calves.
All breeds of livestock harbor some genetic defects. But their incidence is usually low. From time to time, however, specific defects become more frequent in certain breeds and give cause for concern. The expression of several abnormalities in laboratory animals and plants varies within the range of normal environments. Bar-eye condition in Drosophila is one such example. Scrotal hernia in swine is another. Several defects in farm animals are conditioned partially by hereditary variations of a quantitative nature in resistance and partially by environmental factors. The best-known of these is cancer eye of cattle; vaginal and uterine prolapse in cattle appears to be similar. Most lethal and abnormalities are recessive in inheritance, whether due to one or several genes. A few cases are known where recessiveness is not complete and the heterozygotes are 'carriers' and have characteristics which make them more desirable to breeders than homozygous individuals. Bulldog calves (lethal achondroplasia) in Dexter cattle is an example. Cattle of this type are always heterozygous for a semi-dominant gene which, when homozygous, produces a lethal achondroplasia (bulldog calves). Dexters themselves show the effect of this gene by the shortness of the leg. Gonadal hypoplasia in Swedish cattle is another example.

Q3: Variation in animal population plays an important role in animal improvement. Describe sources of variation and its implications in breeding program. (CSE, 1994)
Ans: Variation arises from two sources: 

• Genotype - The particular combination of genes which directs the development of the animal. 
• Environment - The condition of nutrition, climate, competing animals, stress, exposure to disease organisms, and other non-genetic conditions which influence the expression of the trait.

Genetic variation is transmitted to future generations while environmental differences are not transmitted. The relative importance of these two sources of variation determines, to a large extent, the speed and ultimate chance of achieving a desired goal through selection. The breeder tries to select the animal with the best genotypes but often environmental effects cover up the true genotypic values.

Genetic variation: The genotype is the collection of genes which comprises the hereditary material of each cell of the animal. In a population of animals, a wide variety of genotypes are present. This variation in genotypes is the result of differences among the paired genes at the same locus from one animal to the next. The existence of heterozygous animals at a particular locus is proof of at least two alleles in the population at that locus. When two heterozygotes (Aa) are mated, they produce three progeny genotypes: heterozygotes (Aa) and two different homozygotes (AA and aa), providing a demonstration of genetic variation. The exact number of gene loci in any species of farm livestock is not known but is considered to be at least a few thousand. Some of these loci contain only one allele but some are found to contain more than one at most loci. Multiple alleles contribute to genetic variation.

Differences among genotypes constitute the genetic variation present in a species at any one time. However, the transmission of genetic variation from one generation to the next is of considerable importance. A parent which is homozygous at a particular locus can produce progeny homozygous at this locus only if the other parent contributes the same allele. Genetic variation during the transmission of genes can be attributed to several sources: (1) Recombination of genes. (2) Gene interactions. (3) Mutations. (4) Chromosome aberrations. (5) Change in chromosome numbers.

Implications of genetic variation: Recombination of genes during transmission from one generation to the next introduces the elements of chance into breeding programs. The chance recombination of genes produces a characteristic distribution of progeny when large numbers of progeny are obtained from the same parents but the results of individual matings can be predicted with certainty in only a few simple cases. Recombination of genes for quantitative traits produces some progeny which are far superior to their parents and others which are distinctly inferior, but the majority of the progeny are not greatly different from the average of their parents for quantitative traits.

If dominance, epistasis, or random environmental influences have affected the phenotype, the progeny will tend to average nearer to the group average than the parents. This is very important because it means that only a portion of the superiority of outstanding individuals will be transmitted to the progeny. Outstanding individuals should be used for parents because their progeny will average better than the progeny of any other parents.

Environmental variation: Variation due to non-genetic effects is not transmitted to the offspring. Two types of environmental variations are involved: (1) Non-random, (2) Random. Non-random variations are those which are constant from one animal to the other of the same group like those due to climate, level of feeding, husbandry practices, or such other influences which affect all animals in the same group. The presence of these differences is often known or noticed but not their effect. The problem of known non-genetic effects is not limited to between-herd comparisons as records in the same herd but by cows calving at different ages are also difficult to evaluate because of non-random environmental effects.

Random environmental effects are those which influence records of animals but are not constant for all individuals being compared like lameness, minor infections, and chance accidents, etc. Random environmental influences cause a large share of the total variation for many quantitative traits and often their causes are not recorded or even known.

Errors in choosing the best genotypes result when breeding animals are affected by different environments. If all environmental effects are random, then mistakes reduce the efficiency of selection but the selected group is genetically superior to that rejected. However, if one group receives consistently better environment than another, the selected animals will always be from the group with better environment unless (1) adjustment is made for the effect of the environment, (2) the genetic differences between animals are greater than the difference in environmental effects.