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SELECTION1
SELECTION
Selection
Selection is choosing or allowing some animals to be parents of next generation while
depriving others of the privilege.
There are two types of selection: 1) Artificial and 2) Natural selection
In artificial selection the breeder chooses the parents of the next generation. Castration is
one of the oldest forms of artificial selection. The strategies of genetic progress through
selection are 1) Selection between breeds or strains which can achieve dramatic and rapid
genetic change when there are large genetic differences between the breeds chosen for
the characteristics of economic importance. 2) Selection within breed/strain involves
comparing animals of the same breed and mating the preferred animals to produce the
next generation.
The natural selection operates through differences in fertility among the parents and
differences in viability among the offspring.
As discussed in the class, selection changes gene frequencies of the population. The gene
controlling the trait favored by the nature/breeder will tend to get fixed (accumulated) in
population.
Selection response (R)
Selection response is also called as genetic gain. The three factors that control genetic
gain (selection response) in a trait are:
1. Heritability
2. Selection differential
3. Generation interval
1. Heritability (h2)
This is a term used to describe the strength of inheritance of a character. A precise
definition would be – For a given trait Heritability is the amount of the superiority of
the parents above their contemporaries, which on average is passed onto the
offspring.
h2 is expressed on scale from 0 to 1 or 0 to100. The commonly used generalized
grading is:
0-0.1 (0-10%)
0.1-0.3 (10-30%)
0.3 < (above 30%)
-Low or weak
-Intermediate or medium
-High strong
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Heritability (h2) is the ratio of additive genetic variance to the total phenotypic variance.
h2 = VA/Vp
It expresses the proportion of the total phenotypic variance that is due to the average
effects of the genes.



h2 of quantitative traits is one the important properties of that trait.
h2 is not only the property of the trait but also of the population
If h2 is high, selection will be effective, and if it is low selection will be
ineffective (This will be clear to you when you see the equation of selection
response later on )
2. Selection differential (SD)
 Selection differential (SD) is a measure of how good the parents chosen to
produce the next generation will be.
 It is superiority of the selected parents over the mean of the population from
which they came.
Average of whole group (0.5 Kg)
Average of whole group (0.5 Kg)
Average of best 50% (0.54kg)
Average of best 10% (0.58kg)
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 If we have high variation, the SD will be high.
In the examples that we have done in the class, we have assumed that the mean of
selected males and females are same. There fore we have done it at one shot:
For example: Population mean of gain per day= 0.25 kg/day
Mean of the selected males = 1 kg/day
Mean of the selected females = 1 kg/day
Therefore the mean of the selected males and females = 1
So we have: SD = mean of the selected males and females – pop mean
SD = 1-0.25 = 0.75 kg/day
This can be done only if the mean of the selected females is equal to the mean of the
selected males.
So, generally we calculate the SD separately for males and females and average for the
two. For example:
To calculate selection differential for the males:
Gain/day
Mean of the selected males
2.00 kg/day
Overall herd mean (pop mean)
0.24 kg/day
Selection differential = 2 – 0.25 = 1.75 kg/day
To calculate SD for the females:
Mean of selected females
Overall herd mean (pop mean)
0.75kg/day
0.25 kg/day - This should be same as for the males because it
is the mean of the whole herd irrespective of whether it is male or female)
Selection diffrential = 0.75-0.25 =
0.50 kg/day
These SD are average to give :
1.75 (SD for males) + 0.5 (SD for females) = 2.25/2 = 1.13
2
kg/day
Selection Intensity (i)
Selection intensity also describes the superiority of the selected parents over the
population mean and is selection differential in terms of standard deviation unit.
I (i) = Selection Differential (SD)
Phenotypic standard deviation (p)
3. Generation Interval
This is the time interval between generations and is defined as the average age of
the parents when their offsprings are born. It varies between species and breeds.
The general average generation intervals for different livestock species are:
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Horse
Dairy cow
Pig
9-13
4-5
2-2.5
Beef cow
Sheep
Chicken
4.5-5
3-4
1-1.5
The generation interval severely restricts the genetic progress. The generation
interval adds onto the length of time the breeder has to wait until the sufficient
data are available from an animal on which to make a decision. An example of
this is – Waiting for completion of 1 st lactation milk yields of daughters before a
bull is widely used in a herd or for AI.
The way in which the three factors (Heritability, selection differential and
generation interval) are put together to give an estimate of genetic gain is as
follows:
Genetic gain or selection response (R ) per generation = h2 X SD
Genetic gain or selection response (R) per year = (h2 X SD) / GI
i = (SD)
(p)
, Therefore R = (h2 * p * i)
R=Response
h2 = Heritability
SD = Selection Differential
GI = generation interval
i = Selection intensity
From the above equation you can see that:
1. Higher the heretability, higher will be the genetic gain.
2. Higher the Selection Differential, higher will be the genetic gain.
3. Higher the Generation Interval, lower will be the genetic gain.
Selection Limits
In a closed population, there will be good response to selection for few or several
generations and will slow down and eventually stop. This point is referred to as
selection limits. This phenomenon occurs because; the population is running out
of usable genetic variation. However, if some new variation (animal from
different herd or breed) is introduced, progress from selection continues until the
population reaches another plateau and so on.
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Aids to selection
These are the source of information based on which we can decide whether to
choose a particular or group of animal(s) or not:
1. Individual or mass selection
This aid to selection is used for traits of high heritability where the animal’s
own performance is an accurate guide as to how it’s progeny will breed.
2. Lifetime performance records
Here the breeder has more than one record of animal’s performance, such as
series of lactation yields in cow. A good animal will generally perform well
each season and this will be seen in the above average yield despite the ups
and downs in the herd due to environment. If a breeder looked at one of the
cows past records he could make a fairly safe predictions of her future
productions.
3. Pedigree information
- Based on the performance of the parents and grand parents and great
grand parents and so on…
4. Progeny performance
- Based on the production performance of the progeny (sons and
daughters)
5. Performance of other relatives (family selection)
- Based on the production performance of the cousins, brothers,
sisters, uncles, aunties, nieces, nephews…
Selection methods
1. Tandem selection
Selecting one trait and improving it for several generations till it satisfies the
breeder’s goal. Then going for another trait for improvement. The traits
should not be negative correlated. Or else the improvement in the 1st phase of
selection will be cancelled by 2nd phase of selection.
2. Independent culling level
Under this method, the animal has to reach a set standard for all the criteria set
by the breeder. Or else the animal gets culled.
This is similar to examination system where in you have to pass all the
subjects to get to the next class level.
3. Index selection
Under this selection method the animal is assessed on the overall score.
This is similar to examination system where in you can fail in any number of
subjects and you can still get to next class if you pass in the overall average.
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