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Bases of Selection

(Selection for Single trait)

Introduction

Compute the mean phenotypic value of each family i.e. family mean
Now we have measurements of

Each individual and
Means of every family.

A choice of procedure for applying selection to these population is then open, we may utilize these measures

How to decide the appropriate basis of selection?

Only three things to know,

Kind of family: Full Sib or Half sib
Number of individuals in the family
Phenotypic correlation between family members

But before going to this detail we may see the general description of different basis (Information available) of selection

The genetic improvement through selection depends upon the ability to recognize genotypes with superior breeding values for further propagation.

The breeding value of an individual can be assessed on the basis of its own performance, or on the performance of its collateral relatives, progeny, ancestors, or on the basis of information combined from some or all of these sources.

Individual Selection

(selection on basis of individuality)

Animals are selected for breeding purpose on their own phenotypic performance

Preferred when the heritability of the trait under consideration is high.

can be selected on the basis of either single record or on the mean of several records.

Individual selection can be done directly for traits whose records are maintained, e.g. milk yield, growth rate etc.

Advantages of Individual Selection

easiest, quick and most commonly used method
most useful for traits expressed in both the sexes
only option in the absence of systematic record keeping
All animals can be evaluated simultaneously
gives an indication of high correlation between genotype and phenotype when heritability of a trait is high.

Disadvantages of Individual Selection

For traits , expressed only in females, individual selection is not possible for males.
characters that are expressed late in life,
not an effective method of selection for the traits having low heritability.

Selection based on pedigree

A pedigree is a record of an individual’s ancestors that are related through his parents, and the selection of individuals based on such records is known as pedigree selection.
More closely related ancestors should be given more emphasis
Pedigree selection more accurate for trait with high h²

Practical difficulties in pedigree selection

Ancestors records not always available
May be faulty records due to stray mating
Most characters have low h²

Advantages of pedigree selection

Less costly, only pedigree compilation is necessary
Allows selection at younger ages
good for multistage selection
good for sex limited and traits expressed in later life

Disadvantages of Pedigree Selection

Individuals from part. Pedigree can be culled inspite they have good genes, likewise some pedigree may get undue emphasis and favored
introduces nonrandom biase as records are of diff envt
not applicable for indivs. Having same pedigree

Family / Sib Selection

Collateral relatives are those that are not related to an individual directly either as ancestors or as their progeny.
full-sibs, half-sibs etc.
information on family means is used as a basis for identifying the superior genotypes.

When selection is based on the family mean where individual’s own record is included the selection is known as family selection

When the individual’s own record is not included the selection is referred to as sib selection.

Whole families are selected or rejected as units according to the mean phenotypic value of the family.üWhole families are selected or rejected as units according to the mean phenotypic value of the family.

What are the conditions for family selection?

low heritability, mean value of a number of relatives provides a more reliable guide to breeding value than the individual'’ own phenotypic value.
There is little variation due to common environment,
The family size is large.

Advantages of family / Sib selection

Selection at an early age for the traits,
traits that are expressed only in one sex
supplement individual selection for lowly heritable traits.

Disadvantages of family / Sib selection

Common environmental effect
small family size not suitable for family selection, only applicable in animals with high reproduction rate
costly of space and time

Within Family Selection

deviation of individual’s performance from the mean of the family to which it belongs.
The animals that exceed their family mean by greatest amount are considered as the most desirable.
this method is advantageous over the other is a large component of environmental variance common to members of a family,

for example pre-weaning growth in pigs. Here, a large part of the variation of individual’s weaning weight is attributable to the mother and is therefore common to members of a family. Selection within families would eliminate this large non-genetic component from the variation operated on by the selection.

Selection on the Basis of Information Combined from Different Relatives

The accuracy of selection for lowly heritable traits can be increased by paying attention to the phenotypes of close relatives in addition to the individual’s own phenotype.
Such a procedure of selection based on a combination of information from two or more sources is known as combined selection ( or index selection).
In the species with high prolificacy, It is more useful to base selection on an index in which information from relatives is combined with the individual’s own merit

Osborne Index ( Osborne, 1957)

This is a combination of individual cum family selection. It is a combined selection in which parents are selected based on an index taking into consideration individual production and the sire and the dam family average. The pullets and Cockerels with high score are selected and mated interse to produce the next generation avoiding full- and half-sib mating.

I_♀♀ = (P-Pbar) + b₂ (F_d – Pbar) + b₃ (F_s – Pbar) index for pullet selection

I_♂♂ = b₂ (F_d – Pbar) + b₃ (F_s – Pbar) index for cockerel selection

Where,

P = Individual’s production

Pbar = Flock average

Fd = Dam family average

Fs = Sire family average, and

b₂ and b₃ are weighting factors whose values are available from readymade table for varying number of progenies under different dams and sires.

Abplanalp Index (Abplanalp, 1974)

Selection on the Basis of Information on progeny performance

Individual selection tells us what an animal seems to be.
pedigree is an indication of what an animal ought to be
progeny tells what an animal actually is.

Progeny testing means judging the genetic worth (breeding value) of an animal on the basis of performance of its progeny.

Progeny of an individual is the index of parents genetic worth.
Each progeny inherits one half of the genes from each parent due to halving nature of inheritance, called as transmitting ability of parent and hence BV of parent is twice the mean deviation of progenies from population mean
Sampling nature of inheritance creates chance at segregation that may results in any one or few progenies receiving a good or poor than average sample of genes from its parent.
Therefore, estimation of BV of parents based on one or few progenies may be misleading
Chance at segregation may result in any one or a few offspring receiving a better or poorer than average sample of genes from the parents.
Progeny testing technique is generally used for males because they are responsible for more progenies in their lifetime than any one female.
Use of progeny test is not a very practical preposition to establish the breeding value of females, since the number of offspring per female is small. When the individual produces sufficiently large number of offspring, the individual has already completed its productive life and the need for selection will be already over.
It is very important that all of the progeny and not just a selected sample of the progeny be included in the progeny test appraisal. Omitting the poor progeny is unfair and misleading because, similar poor progenies are just as likely to be produced among the next group of progeny.
Progeny testing may be used in selection of traits expressed in both traits. When heritability is low, fewer progenies are required to make the progeny test.
However the accuracy of progeny test is reduced when there is an environmental correlation among progenies due to non-genetic factors. This situation arises when several progeny tested sires are being compared, but their progeny had been tested at different locations.
Feeding and management also influences the progeny group differences. These will reduce the accuracy of progeny testing. Progeny testing are conducted to compare the performance of progeny of two or more parents.
Usually sires rather than dams are progeny tested because generally sires produce more progeny in a given season or year.

Superiority of progeny testing over other bases of selection

Individual Selection : not possible for traits with low h², Sex limited traits,, traits expressed late in life etc.

Pedigree selection – not practical as only one sample is tested and uncertain due to random halving of genes

Family or sib selection – Accuracy is very low due to sampling nature of inheritance, also every individual receives different sets of genes from its parent and the probability to get same set of genes by individual and its sibs is very less

Precaution while doing Progeny testing

Dams mated to all sires should be selected randomly.
Feed, same ration and in same manner to avoid bias.
Compare diff. parental groups raised in as nearly the same environment as possible.
Compare the parent groups born in same year/season when possible.
Include all progeny of a particular parent in the test, if possible whether they are inferior or superior.
Larger the number of progeny tested per parent, within limits, the more accurate the estimate of that parent’s probable breeding value.
Errors like effects of year, season and location should be eliminated as far as possible.

Advantages and Limitations

Advantages

For selecting sex limited traits.
For selecting traits require sacrifice of the animal (carcass traits)
For selecting traits expressed late in life
For traits having low heritability value.
For selection of animals that nick or combine well.
For testing animals for recessive traits.

Limitations

More number of animals must be progeny tested.
It prolongs the GI, hence time consuming and expensive
Sires can be selected only when progenies come for production and by the time sire may become old and useless. Therefore, annual genetic gain is lowered.
Hence it is time consuming and expensive

Types of Progeny testing programmes

Farm / Station Progeny testing programme
Associated herd Progeny testing programme
Field Progeny testing programme

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