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Response to selection

The change produced by selection is the change of the population mean in the offspring. This is called as the response to selection, symbolized by “R”. The response to selection is the difference of mean phenotypic value between the offspring of the selected parents and the whole of the parental generation before selection. The response to selection is also called as the expected genetic gain, symbolized by Δ G.

R or Δ G = h² S

R or ΔG/ year = h² S / GI

where, h² = heritability, S = selection differential and GI = generation interval

Factors affecting Response to selection

These are heritability, selection differential and generation interval.

Heritability: The genetic gain depends on the h² of the trait in the generation from which parents are selected and if the h² is high, the genetic gain will also be more, because the environmental variation will be less.

Additive genetic variability of traits

Intensity of selection

Accuracy of selection (r_AP)

Population size

Generation interval

Selection differential: The average superiority of the selected parents is called as selection differential, symbolized by “S”. It is defined as the difference between the mean phenotypic value of the individuals selected as parents and the mean phenotypic value of all the individuals in the parental generation before selection.

SD = (Ps - P), can also be expressed as,

S = i s_p

The intensity of the selection symbolized by “i” is also called as selection pressure and it is the mean deviation of the selected individuals in units of standard deviation. It depends on the proportion of the individuals selected and it can be determined from the tables of properties of normal distribution.

Factors affecting selection differential

Proportions of the animal selected for breeding; smaller the number larger the selection differential,
Hherd size; larger herd size, smaller proportions of animals selected,
Reproductive rate; in cattle selection differential will be less whereas in pigs, it will be more because of more litter size and
Use of AI and frozen semen increases selection differential or selection intensity in case of males and in females, super ovulation and embryo transfer increases the selection differential or selection intensity.
Generation interval: It is the time interval between generations and is defined as the average age of the parents when the offspring is born. This varies between species and selection procedure. Management practices for early breeding in females reduces GI and breeding practices like progeny testing increases the GI.

The average generation intervals for different species are:

Cattle- 4-5 yrs
Sheep- 3-4 yrs
Swine- 1.5-2 yrs
Chicken- 1- 1.5 yrs
Horse- 8-12 yrs.

Accuracy of selection

The accuracy for selection is directly related to the heritability of the trait. The heritability is high, the selection on phenotype will permit an average estimation of breeding value. If heritability is low, many errors will be made. Increased accuracy in selection can be obtained by comparing the animals in controlled environmental conditions. Correlation may be made for the age of the individual, age of the dam and sex to remove non-genetic variations. The techniques may increase the heritability of the trait by reducing the environmental variation. When the accuracy of selection on individual is low, accuracy can be increased by

using additional measurements for the trait from the same individual,
using measurements of correlated traits and
using measurements of relatives.

Selection limit

When the selection is carried out continuously, the response to selection will be more for a few generations, and then it slows down and finally stops. When the response to selection has stopped, the population is said to be at “plateau” or “selection limit”. The main cause for this is fixation of favorable genes. This causes reduction or absence of genetic variation. Therefore further improvement depends on introduction of new genetic variation. The new genetic variation can be introduced by cross breeding, , mutation and genetic engineering.

Indirect Selection

Indirect selection is the selection applied to some character other than the one to which it is desired to improve.
If we want to improve character x, we might select for another character y and achieve progress through the correlated response of character x.
The character to which selection is applied is called as secondary character
CR_x is correlated response of character x resulting from selection applied to the secondary character Y

CR_x = i_y h_y r_A σ_A(x)

Advantages or application of indirect selection

Applicable when the two characters have high genetic correlation
Heritability of character under selection (h²_x) is sufficiently high than h²_y
Difficult to measure character Y than X
Information on character X is early in life
Information on character X is costly to measure
Desired character is measurable in only one sex
Used for reducing the generation interval

Introduction to Breeding for Disease Resistance in Farm Animals

Disease: Pathological condition,

from infection,

genetic defect and/or

environmental stress

Resistance:

An ecological consideration of the interaction between the host and the pathogen species.

Ability of the host to exert control over the pathogen life cycle up to certain degree

Tolerance:

Net impact on performance at a given level of infection, i.e. the regression of performance on pathogen load.

Host having greater tolerance maintains high level of fitness

Resilience:

Ability to continue a normal production life for an animal which has been infected

Where all animals are infected, resilience becomes a useful concept.

Why Selection/Breeding for Resistant?

Breeding for resistance or tolerance?

Enhance animal health but have potentially different effect on the prevalence of infection.

Breeding for tolerance would not necessarily affect the pathogen.
Breeding for resistance would reduce disease transmission possibly by imposing selection pressure upon the pathogen.
Therefore, breeding animals tolerant to a disease would be disadvantageous when the aim is to reduce the transmission of infection.

Hereditary architecture of host resistance

Host resistance to certain pathogen or disease mainly manifested through the variability in host immune responses to infection
Therefore, there is possibility to improve genetic resistance to most diseases however, ascertaining resistance phenotypes under field conditions is still challenging

Genetic resistance is permanent and cumulative resistance to a particular disease.

Disease resistance – multi-factorial (polygenic) – No absolute resistance.
Selection for genetic resistance - in conjunction with other disease control measures to reduce the size of challenges or to assist in treating existing infections / infestations.
In cases where the gene(s) controlling expression of an affected phenotype has been identified, reduction in the frequency of affected animals can be done through selection to increase the frequency of the desirable phenotype in the population.
In cases where genetic cause has not been identified, culling affected animals and their relatives may be effective, but the success depends on the disease incidence, and progress will become slower as the incidence falls.

Appropriate Selection Strategies

Selective breeding (response) - heritability of the trait associated with disease resistance + variation among the animals.
The direct selection - Individual or family selection in normal as well as in challenged environment is ideal selection strategy for detection of genetic variation.
This is one of the simplest method, applied to various diseases like Marek’s disease, New Castel disease etc.
However, this classical selection approach incurs high cost, large infrastructure as well as time.
Finding direct markers for disease resistance is tough and thereby alternate way is to look for the markers that are non-pathological, easy to detect and are associated with disease.

Such markers are,

Observables or Phenotypic markers
Genetic markers
Transgenesis

Phenotypic markers

Skin, hair, mucous membranes, secretions like tears, urine, ruminal content, saliva, mucous, skin secretions, etc.
Variation between the hosts in such secretions may be associated with certain pathological conditions.
Other such markers are faecal egg count of GI tract parasites, Tick count on body à tick infection, somatic cell count of milk à mastitis, Ig response in case of certain poultry diseases etc.

Genetic markers

Sequencing of genomes have led to discovery of several genetic markers and even genes related to the immune system.
Immune responsiveness would be an useful indicator of disease resistance in cattle. (Hernandez et al. 2003)
Various genetic markers; microsatellites, SSCP, RFLPs, RAPDs, and AFLPs have been utilised to select animals for resistance against specific infectious diseases.
The resistant animals are selected based on the marker allele linked to the trait.

Breeding for resistance to infectious diseases

Disease resistance - detecting variance in the speed of epidemic development among groups - requires huge data and “replicated” epidemics for computation.
Measuring of resistance phenotypes, is costly and difficult.
Treatment with antibiotics is choice but carry the risk for emergence of resistant pathogens.
Ideal experimental design for,

detecting genetic variation in resistance and

identifying associated SNP

This would be to exploit continuously varying phenotypes measured on animals subjected to identical environmental and challenged conditions.

Three possible ways for individual selection for disease resistance

- - 1. Selection under normal environmental condition
    2. Selection under challenging environment
    3. Use of indicators of resistance is an indirect approach where genetic

In short, Selection for disease resistance/tolerance require,

Estimates of genetic parameters (require large amount of data and pedigree records).
Recording disease incidence in the progeny (selecting parents having less incidence in their progenies)
Analysis of available genetic variation to get the heritability estimates for a set of immune functions related traits essential for resistance to disease.

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