Sheep - AgRIS

the exercise these animals did not drink as much water as the HK type animals. Physiologically
a rise in body temperature above normal limit is detrimental to an animal and natural situation
may be in favour of animals which show a lower rise in body temperature during stress. The
authors have further conjectured that in chronically water deficient areas the evolution and
physiological adaptive mechanism might have been directed primarily towards saving of body
water but the situation seems to be of very complex nature , as Purohit et. al.(1973) working on
effects of varying degree of water restriction on the distribution of body water in high and low
potassium type in Marwari sheep have reported that there was no difference in the voluntary
water intake of the HK and LK animals. In both types of animals the total body water, total
blood and plasma volume and extracellular, intracellular and interstital fluid volume started
decreasing as the water intake was reduced below 25% of the daily requirement. In both the
phenotypes 75 reduction was similar in its effects to complete deprivation.
Summarizing there are two types, viz. HK (High Potassium) and LK (Low Potassium)
distinguishable on the basis of whole blood potassium. These types are controlled by a single
autosmal locus with two alleles, the allele for HK being recessive. In addition to the locus , a
large number of other loci with minor effects also control the potassium inheritance. Most of
the Indian breeds of sheep show a higher frequency of HK. The proportion of HK type varies
from 62.91% . Although no specific reasons for the predominance of HK Indian sheep can be
assigned but this is suggestive of adaptive advantage in hot arid climate. Genes controlling the
potassium types are not completely neutral and contribute towards the variability in some
production and adaptation characters and fitnes need to be carriedout.Potassium types are
reported to be associated with fleece quality, the LK producing fleece with lower percentage of
medulated fibres.
5.4 DNA Polymorphism
Polymorphism is the availability at ajl)Lhenotypes with respect to the genotypes present in
the population. If for reason there is selection coeficient, but selection (S) depends on gene
frequency. When gene frequency is low, the selection will be positive but if the gene frequency
is very high, selection (S) will become negative. this is for keeping all the individuals in a
population.
If selection coeficient changes along with gene frequency change, it will try to maintain all
individuals in the population. If the environment is highly hetero genous i.e. fluctuating, the
population will tend to maintain almost most of the genotypes and almost all the genes
available.
It is the existence in a population of indiliduals with readily visible differences caused by
genes at intermediate frequencis.
In a inbreed population we will find more & more homozygous.
If we take two population. Some may be undesirable for one and others for other
undesirable recessive. It is not over dominance. It is only covering of recessives by dominants.
When these population are mated, the crossbred will have the recessive.
When we cross inbred lines or unrelated population lines the heterozygote are better. This
is not overdominance. But only cover ng effect. Linkage is not required to explain. This
operational overdominance. But the person who put f.zurth this hypothesis sounds that there
may be some linkage. Polymorphism can be alternative explanation to the overdominance at a
single loeus.
When h = 0.5; q = .75
We find all the genotypes in the population.
5.5 Inheritance of Qualitative Traits
Each chromosome carries hundreds and possibly even thousands of different genes. Some
of these genes affect only one trait, whereas others affect several. Further the same trait may be
affected by many different pairs of genes Therefore thorough knowledge of the different types
of gene action is necessary to understand and apply genetics to devise satisfactory mating and
selection systems for the improvement of sheep.
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