Principles of Animal Breeding Theory and Methods of Selection
ANIMAL GENETICS & BREEDING
UNIT – III
Principles of Animal Breeding
Theory
Methods of
Selection
Dr K G Mandal
Department of Animal Genetics & Breeding
Bihar Veterinary College, Patna
Bihar Animal Sciences University, Patna
Methods of Selection
Introduction:
•
The
net economic value
of an animal depends on
performance
of several characters
.
•
For example,
a dairy cow will be more economical
to maintain
if she produces
more milk
(more LMY) with higher
fat
content in milk
( fat %) for a longer period of time (longer
lactation length
), remain dry for a shorter time between two
successive calvings (
shorter Calving Interval
) etc.
•
It is therefore , essential
to estimate
the total breeding
worth i.e.,
net merit of an animal.
Multi trait selection:
•
Selection is practiced for
several traits simultaneously
to
improve the overall merit of the individuals.
Requirement and efficiency of multi trait selection
:
i.
Estimation of
economic value
of the traits.
ii.
Genetic significance of the animal
in terms of h
2
of the
traits and
genetic correlations
among the traits.
iii.
The
methods of selection
and
number of traits
to be
included in the selection criteria.
Methods of Selection:
1.
Tandem method
2.
Independent Culling Level method
3.
Selection Index or total score method
1. Tandem Method
•
This is a
multi trait selection
.
•
Tandem method is practiced for improvement of several
traits but at different times
i.e.,
one trait at a time
and
selection process will continue
for several generations
till the
improvement is made in that trait
up to desired level.
•
Thereafter,
selection is practiced for improvement in the
second trait
and the selection process will also continue for
several generations
till the goal is achieved to the desired
level
.
•
In this way
selection will continue for all the traits one by
one.
•
If number of traits considered for selection is ‘ n ‘, then the
average genetic improvement in each of n independent and
equally important traits per generation would be only 1/n
times.
•
The efficiency of this method depends on the
genetic
correlation
among the traits under selection.
Merit of Tandem method:
•
This method is
easy to understand
and
easy to conduct
.
•
If
genetic correlation
between the traits
is
positive
then
improvement in any one trait
would lead
improvement in the
correlated traits.
Demerit of tandem method
:
•
This method is less efficient
than other methods.
•
The
genetic progress per unit of time is less.
•
It
requires more time
for improvement in
all the traits.
•
An
undesirable genetic correlation between two traits
would
neutralize the genetic progress made in any one trait.
2.
Independent Culling Level Method of Selection
•
Two or more traits
are taken at a time
for selection of an
individual.
•
A
minimum standard (level) is fixed
for
each and every trait
.
•
Each and every animal
has to
achieve the target fixed for
each character
for its selection.
•
If
an animal fails to meet the minimum standard
fixed for
any one character
will be rejected
even though it is
exceptionally good for all other traits.
•
Example: IC L method of selection in buffalo
•
Buffalo no. 2 is rejected though it is excellent in LMY.
•
Buffalo no. 1 is selected as it has achieved the minimum
target fixed for all the three traits.
Advantages of IC L:
i.
It is
superior to tandem method
because selection is
practiced for
more than one trait at a time
.
ii.
Culling process
may be started
at the early age
for the traits
expressed at early age.
Disadvantages:
i.
It is
inferior to Selection index
or total score method.
ii.
No compensation for superiority
of an excellent trait.
iii.
More emphasis is given
to the traits
expressed at early life
.
iv.
Intensity of selection is reduced
with the increase in number
of traits.
v.
Only
mediocre animals are selected.
Efficiency of the method:
•
It depends on standard (level) fixed for each character.
•
If level fixed is low, very few animals are to be rejected.
•
If standard kept at very high level, only few animals will
have the opportunity to be selected.
•
It is
superior than the tandem method
of selection.
•
It is
inferior to Selection index
or total score method.
3. Selection Index
Introduction:
•
Selection index is the numerical score assigned to an individual to
estimate its breeding value on the basis of economic weight of
the traits considered for selection.
•
Several number of traits are considered simultaneously for
selection of an individual.
•
Selection index is an index of the net merit of an animal for
many traits.
•
Deficiency in some traits is taken into account by superiority of
other traits.
•
Certain value (economic weight) is assigned to each trait
depending its h2 and economic value as well as genetic correlation
among the traits.
•
Values of all the traits are added together to get the total score
for an animal. For this reason selection index is also known as
total score method.
•
Credits and penalties are given to each animal according to the
degree of superiority or inferiority in each trait.
•
The animal with the highest score is then selected for breeding.
•
Some weightage is given to each trait due to their unequal
economic importance and thus allows equal attention to each
trait.
•
The amount of weightage given to each trait in relation to other
traits determines the influence of each trait on the final index.
•
The amount of weightage depends upon the economic weight and
genetic significance of the traits.
•
Selection index is the sum of the product of phenotypic
value with the respective economic weight of different
characters incorporated into the index.
•
Thus, selection index,
I = ∑bixi
= b1.x1 + b2.x2 + b3.x3 + ………….. + bn.xn
Where,
xi
(x1, x2, x3 ……..xn) represents the phenotypic values for
different traits and
‘bi’
are the weighting factors (partial
regression coefficient) given to each trait.
•
Selection index was developed by Prof. R A Fisher in 1936
and first used for genetic improvement in animals by Hazel
in 1943.
•
Merit of selection index:
i.
It is
superior than the tandem and ICL methods.
ii.
It
overcomes the disadvantages shown by tandem and ICL
.
iii.
Selection on the basis of an index is a more balanced
approach since
it combines information of various traits
on the basis of their economic importance.
iv.
It allows individuals which are superior in some traits to
be selected regardless of their inferiority in other traits.
v.
Efficiency of S.I. increases
with the
increase in the
number of traits
to be selected but the
response in the
individual traits become less.
vi.
Hence, only those traits considered more important should
be included into the index.
Requirements in the construction of S.I.:
i.
Economic value
or relative economic importance of
each of
the trait.
ii.
Genetic and phenotypic variances
of all the traits.
iii.
Genetic and phenotypic covariance
among all the traits.
THANK
YOU
Animal breeding involves selecting for desirable traits to improve the overall merit of animals. Methods such as tandem selection and multi-trait selection are used to enhance genetic progress. Economic value, genetic significance, and selection criteria play important roles in the breeding process. Tandem selection focuses on improving one trait at a time over several generations, while multi-trait selection aims to enhance multiple traits simultaneously. Understanding genetic correlations is crucial for successful breeding programs.
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Presentation Transcript
ANIMAL GENETICS & BREEDING UNIT III Principles of Animal Breeding Theory Methods ofSelection Dr K G Mandal Department of Animal Genetics & Breeding Bihar Veterinary College, Patna Bihar Animal Sciences University, Patna
Methods of Selection Introduction: The net economic value of an animal depends on performance of several characters. For example, a dairy cow will be more economical to maintain if she produces more milk (more LMY) with higher fat content in milk ( fat %) for a longer period of time (longer lactation length), remain dry for a shorter time between two successive calvings (shorter Calving Interval) etc. It is therefore , essential to estimate the total breeding worth i.e., net merit of an animal.
Multi trait selection: Selection is practiced for several traits simultaneously to improve the overall merit of the individuals. Requirement and efficiency of multi trait selection: i. Estimation of economic value of the traits. ii. Genetic significance of the animal in terms of h2 of the traits and genetic correlations among the traits. iii. The methods of selection and number of traits to be included in the selection criteria. Methods of Selection: 1. Tandem method 2. Independent Culling Level method 3. Selection Index or total score method
1. Tandem Method This is a multi trait selection. Tandem method is practiced for improvement of several traits but at different times i.e., one trait at a time and selection process will continue for several generations till the improvement is made in that trait up to desired level. Thereafter, selection is practiced for improvement in the second trait and the selection process will also continue for several generations till the goal is achieved to the desired level. In this way selection will continue for all the traits one by one.
If number of traits considered for selection is n , then the average genetic improvement in each of n independent and equally important traits per generation would be only 1/n times. The efficiency of this method depends on the genetic correlation among the traits under selection. Merit of Tandem method: This method is easy to understand and easy to conduct. If genetic correlation between the traits is positive then improvement in any one trait would lead improvement in the correlated traits.
Demerit of tandem method: This method is less efficient than other methods. The genetic progress per unit of time is less. It requires more time for improvement in all the traits. An undesirable genetic correlation between two traits would neutralize the genetic progress made in any one trait.
2. Independent Culling Level Method of Selection Two or more traits are taken at a time for selection of an individual. A minimum standard (level) is fixed for each and every trait. Each and every animal has to achieve the target fixed for each character for its selection. If an animal fails to meet the minimum standard fixed for any one character will be rejected even though it is exceptionally good for all other traits.
Example: IC L method of selection in buffalo Traits Standard Set Buffalo No. 1 40 1850 7 2 44 2200 6.5 AFC (months) LMY (Kg) Fat % 42 1800 7 Buffalo no. 2 is rejected though it is excellent in LMY. Buffalo no. 1 is selected as it has achieved the minimum target fixed for all the three traits.
Advantages of IC L: i. It is superior to tandem method because selection is practiced for more than one trait at a time. ii. Culling process may be started at the early age for the traits expressed at early age. Disadvantages: i. It is inferior to Selection index or total score method. ii. No compensation for superiority of an excellent trait. iii. More emphasis is given to the traits expressed at early life. iv. Intensity of selection is reduced with the increase in number of traits. v. Only mediocre animals are selected.
Efficiency of the method: It depends on standard (level) fixed for each character. If level fixed is low, very few animals are to be rejected. If standard kept at very high level, only few animals will have the opportunity to be selected. It is superior than the tandem method of selection. It is inferior to Selection index or total score method.
3. Selection Index Introduction: Selection index is the numerical score assigned to an individual to estimate its breeding value on the basis of economic weight of the traits considered for selection. Several number of traits are considered simultaneously for selection of an individual. Selection index is an index of the net merit of an animal for many traits. Deficiency in some traits is taken into account by superiority of other traits. Certain value (economic weight) is assigned to each trait depending its h2 and economic value as well as genetic correlation among the traits.
Values of all the traits are added together to get the total score for an animal. For this reason selection index is also known as total score method. Credits and penalties are given to each animal according to the degree of superiority or inferiority in each trait. The animal with the highest score is then selected for breeding. Some weightage is given to each trait due to their unequal economic importance and thus allows equal attention to each trait. The amount of weightage given to each trait in relation to other traits determines the influence of each trait on the final index. The amount of weightage depends upon the economic weight and genetic significance of the traits.
Selection index is the sum of the product of phenotypic value with the respective economic weight of different characters incorporated into the index. Thus, selection index, I = bixi = b1.x1 + b2.x2 + b3.x3 + .. + bn.xn Where, xi (x1, x2, x3 ..xn) represents the phenotypic values for different traits and bi are the weighting factors (partial regression coefficient) given to each trait. Selection index was developed by Prof. R A Fisher in 1936 and first used for genetic improvement in animals by Hazel in 1943.
Merit of selection index: i. It is superior than the tandem and ICL methods. ii. It overcomes the disadvantages shown by tandem and ICL. iii. Selection on the basis of an index is a more balanced approach since it combines information of various traits on the basis of their economic importance. iv. It allows individuals which are superior in some traits to be selected regardless of their inferiority in other traits. v. Efficiency of S.I. increases with the increase in the number of traits to be selected but the response in the individual traits become less. vi. Hence, only those traits considered more important should be included into the index.
Requirements in the construction of S.I.: i. Economic value or relative economic importance of each of the trait. ii. Genetic and phenotypic variances of all the traits. iii. Genetic and phenotypic covariance among all the traits.
