Breeding and Selection Flashcards
Function of a breeding programme
- increase production and product quality;
- increase productivity and cost efficiency;
- maintain genetic diversity;
- support the conservation and use of specific breeds
- consider animal welfare and sustainable systems
Animal Breeding
The process of selective mating of animals with desirable traits to maintain or enhance these traits in future generations
QUALITATIVE traits
Single gene (locus) traits
• Phenotypes fall into discrete classes
• Controlled by a single locus or a small
number of loci
• Minimal effect of the environment on phenotype
• Inherited in a simple Mendelian manner
QUANTITATIVE traits
Polygenic traits
- Phenotypes measured on a continuous scale
- Controlled by many (thousands) of loci with small additive effects
- Often a large effect of the environment on phenotype
- Inheritance described by Fisher’s infinitesimal model
Single locus traits - examples
• Coat colour • Polled v Horned • Numerous inherited health conditions • Scrapie resistance in sheep May see epistatic interactions with a limited number of other loci
Single locus traits – inherited disorders
Usually associated with a mutation in a locus coding for a protein essential for development or metabolism
Usually (but not always) recessive Usually rare unless it has been
inadvertently selected for
Often DNA tests are available to identify genotype
Inheritance follows Mendel’s Laws
Collie Eye Anomaly Mutation in NHEJ1- Influences differentiation of tissues in the eye
Mendel’s Law’s of segregation
Members of a gene pair separate into the gametes, such that half carry one member of the gene pair and half carry the other
Mendel’s Law’s of independent assortment
Genes controlling separate traits segregate independently
Polygenic traits
Complex traits involving MANY metabolic pathways • Lots of enzymes • Lots of receptors • Lots of transporters • Lots of proteins • Lots of loci!!
The Infinitesimal model
– developed by Ronald Fisher in 1918
• Variation in a quantitative trait is influenced by an infinitely large number of genes, each of which makes an infinitely small (infinitesimal) contribution to the phenotype, as well as by environmental factors
Phenotype =
Genotype + Environment
BREEDING VALUE
Additive value of genes
Genotype
Additive value of genes
(BREEDING VALUE)
+
Non-additive genetic effects at each locus
Each parent passes HALF their alleles at each locus to the progeny
SO
1⁄2 Breeding value of Sire \+ 1⁄2 Breeding value of Dam = Breeding Value of progeny (Average performance of progeny)
HERITABILITY (h2)
The proportion of the
superiority in performance of the parents in a trait that is
passed on to their offspring
Breeding Strategies
- Between breed selection – using the best breed for the job
- Crossbreeding
- Within breed/line selection
Crossbreeding
Crossbreeding
Mating animals of different breeds
• Exploits complementarity
• Exploits heterosis (hybrid vigour)
Crossbreeding – UK Sheep stratification
Exploits complementarity
• Exploits heterosis (hybrid vigour)
The stratified system is divided into three tiers: hill, upland and lowland. Some sheep will stay on the same farm, or at least in the same tier, for their whole lives, while others are moved down the system. This system is crucial in keeping the UK sheep industry productive and efficient, and a collapse of any area would change the entire face of the industry
hybrid vigour
Crossbred animal often outperform the purebred parents
Heterosis is:
The difference between the average performance of the parent breeds and the performance of the crossbred.
Three-way rotational cross
In this cross females of breed A are crossed to males of breed B. Their female offspring (F1: AB) are crossed with a male of breed C. Their female offspring (F2: 25 % A, 25 % B and 50% C) are crossed then with a male of breed A. Their female offspring (F3: 62.5 % A, 12.5 % B and 25 % C) are crossed with a male of breed B etc
If the breeding goal is To maintain or increase production outputs from prolific ewes that are capable of rearing
twins off grass
The selection criteria might be…
• Number of lambs reared per ewe • Weight of lambs reared per ewe • Carcase quality of lambs
• Longevity of ewes
In a grass based production system
Recording Performance
We can only identify genetically superior animals IF we have accurate and objective measures of their performance
Record and evaluate animals
What we are really interested in is the animals with the best genes – the best breeding values
• Animal’s own performance
• Performance of relatives
• Compared to animals in the same environment
• Direct measures of genetic differences between animals at DNA level
Clues to an animal’s breeding value
• Own performance • Repeated measures of performance • Performance of relatives • Performance in related traits • Genomic information
Select and mate animals
- Select the animals with the highest breeding values to be parents of the next generation
- Generation on generation improvement – cumulative and permanent
- Also consider maintenance of genetic variation
What influences the rate of improvement?
- Heritability of the traits
- Selection pressure actually achieved
- Accuracy of selection
- Generation interval
- Genetic correlations with other traits
Selection pressure
Selection Differential (S) – the difference between the average of animal selected as parents and the average of the population Influenced by: Variation in the population Proportion of animals selected Population size (if small)
How accurately can we identify the animal’s with the best genes
Influenced by:
- Heritability of the trait
- The quantity of information used to calculate the EBV
- The quality of information used to calculate the EBV
Generation Interval
The average age of the parents when the progeny are born
If selection is effective younger animals are always better than older animals (on average)
The shorter the generation interval the better
Genetic correlation
The relationship between the breeding values for two traits
Some genes influence more than one trait (pleiotropy)
Antagonistic correlations can slow down genetic progress
