Download Selection Purpose change over a period of several generations the

Selection
Purpose change over a period of several generations the gene frequency in a population so that the genotypes produced
are those closely producing the phenotype the producer/breeder requires. Selection itself is defined as differential
reproduction among individuals of different genotypes.
Directional selection
Selection by the producer of individuals to be parents of the next generation whose phenotypes more nearly approach a
maximum or minimum for some trait is the directional selection. If the trait is heritable of the phenotypic superiority of
the individuals will be due to their genetic merit for that trait. These individuals selected will be above the population
mean in their breeding value. The alleles (at some loci) with favourable effects on the trait will be at a higher frequency
in the selected individuals than in the population at large. These favourable alleles will contribute to the next generation.
Alleles with favourable effects are increased in frequency at the expense of those with unfavourable effects at the same
loci. There is need to establish a selection programme in order to do directional selection and to define objectives of the
selection programme.
The selection objectives
Statement of what is to be achieved clearly stating the time frame and levels to be attained. Define the measurement to
be recorded that is the selection criterion. The criterion to be
 Closely related to objectives
 Easy to measurement
 taken at early in the life of the animal
 cheap and accurately measured
All the animals in the population are then measured to the criterion. Those closely meeting the breeder's objectives are
selected.
Types of selection
1. Truncation selection
Individuals are ranked for the phenotypic trait of interest and replacement chosen for the high ranking individuals. If the
entire population above a specific rank is selected while the population below the rank is rejected it is called truncation
selection. If the families (i.e. of the full-sibs and paternal half sibs) are ranked with all members of the better families
selected and all members of the poorer families rejected the system is called Family selection. If better individuals
within each family are selected (regardless of rank of the entire family) it is called within family selection.
2. Stabilization selection
Type of selection when the optimum phenotype is the intermediate rather than extreme. It does not cause any directional
change in the phenotype. Results in a fairly constant mean possibly somewhat reduced variance for a trait from one
generation to the next. It is done by selecting those individuals for the trait that are near the average for the trait. It may
occur with or without knowledge or intent of the breeder. The best example is birth weight.
3. Disruptive selection
Favours both phenotypic extremes less common in livestock. These three could be artificial or natural selection.
Methods of selection
1. Tandem selection
Selection of one trait at a time for several generations before taking on another for several generations. It’s a single file
selection. The simplest method. The least efficient. If the traits are negatively correlated may nullify or negate the gains
made in the earlier traits. It lengthens the generation interval. For that it is rarely used today.
2. Independent Culling Level (ICL)
It involves setting independent minima and maxima for two or more traits. Cull all those that fail in any one trait. At
least selecting for more than one trait. Can use adjustments for culling levels to reflect relative importance of each trait.
This flexibility is determined by the proportion to be saved. It does not involve complicated calculations and reduces the
generation interval. However, it may involve culling of some superior animals, as it does not allow for compensation of
poor performance in some trait by good performance in others.
To give example
3. Selection index
Involves selecting for more than one trait simultaneously. It uses an index of net merit. The index is constructed by
adding into one figure the credits and penalties given to each animal according to its superiority/inferiority in each trait.
Calculated as;
I = w1a1+w2a2+.....+wnan
where wi is the weighting factor which shows the relative economic importance of the ith trait. It is calculated using
relative economic importance, heritability, dp and rg.
Advantages
 selection for more than one trait
 Most accurate method
 facilitated by the availability of computers
Disadvantages
 involves complex calculations to come up with the index that become more so with increasing number of traits
included in the index.