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Variation.doc
VARIATION
Animal breeding is based on variation. Geneticists consider variation as the raw material for
improvement of livestock. In the absence of variation, all animals would be exactly the same and
there would be no possibility of selecting better animals to improve a breed or herd. There are
two basic types of variation: genetic variation (invisible) and phenotypic variation (visible).
Genetic variation refers to the amount of variation that is controlled by genes. Where as
phenotypic variation refers to the actual variation we observe among animals for each trait of
interest.
Source of variation
Mutation contributes to genetic variation in a population.
Chromosomal mutation: Changes in number or structure of chromosomes; major cause of
embryonic death
Gene/point mutation:
A copy of DNA do not match the original at single or more bases because of mistake in coding
Usually not harmful
Positive effect: Variation in color, shape and function, which arose from mutation, allows the
wild animals to exploit new food sources and habitats.
1/100,000 – 1/1000,000 sperm or eggs carry a newly mutated gene
The breeders’ tool to work on variation is selection. So basically population genetics is all about
variation and selection.
How to express variation?
The best way to express variation in a herd or flock (population) is by drawing a distribution
diagram of it for a particular trait (e.g. egg weight - exercise you have done)
The other examples of such quantitative traits would be: live weight at certain age, fleece weight,
milk yield, milk fat content, litter size.
1. A reasonable number of observations have to be made.
2. First group sizes (range) are chosen.
3. Then the frequency or number of observations (e.g. egg weight) that fell into each class is
counted.
4. From the above, histogram is constructed.
5. A free hand curve can be drawn through the tops of the columns to describe the
distribution.
With a large enough observation or population, the curve is bell shaped for most of the
quantitative traits. This curve is called as normal distribution curve and it has its features such as:
1. The curve describes mean and variation spread around it
2. In the area bounded by 1σ above and 1σ below the mean, there will be about 68% of all
the observations (+1σ).
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CNR/AGB
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Variation.doc
In the area bounded by 2σ above and 2σ below the mean, there will be about 95% of all
the observations (+2σ).
In the area bounded by 3σ above and 3σ below the mean, there will be about 99% of all
the observations (+3σ).
(σ - sigma, this Greek letter is a mathematical notation for standard deviation)
No of
students
Thus the mean and standard deviation can quickly describe the variation of a group of animals.
This also introduces the concept of judging or describing individual animals performance by
comparing with the mean of the population (In other words – taking the deviation of
performance of that particular animal from the mean of the population). From this comparison
you get the idea whether the animal is above or below average – this idea you get is the basis of
your decision in animal improvement.
Practical cases:
1. Now you have a particular animal (e.g. cow) which you think has superb production
performance. Now, you have to quantify that excellence by comparing with the others
(population).
2. As described above make a distribution table/curve and take the deviation (comparison
with the population mean) of your animal’s performance from the mean performance of
the population.
3. Still, you found that the deviation of the animal’s performance is very much higher than
the mean of the population.
4. Then the question you should ask is – why this animal is producing more than the others?
5. The answer could be – 1) its genetic merit 2) environmental influence – management,
good feeding, good microclimate. The other environmental influences could be – age of
the dam (mother) when giving birth, suckling capacity of the mother, incidence of
external parasites and so on.
6. This implies that, if the superiority of an animal in a population is due mainly to a good
environment, then the improvement of mean performance through breeding (genetics)
will be harder.
7. If the superiority is due to good genes, then the breeder can proceed to make decisions
with much more confidence that what he sees will be passed on to future generations.
8. To avoid this dilemma, we have to compare animals in a same flock/herd kept under
similar environment (nutrition, management…) and go for selection then.
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