Genetic progress - what controls it?

Written by Dr Clive Dalton

There are three parts to making genetic gain, and you have to consider them all at the same time.  They are:

  • Heritability (h²)
  • Selection Differential (SD)
  • Generation Interval (GI)
  • These three components go together in a formula that is the basis of all animal improvement.  It says the genetic gain made per year is controlled by the Heritability multiplied by the Generation Interval, all divided by the Generation Interval.  So multiplication speeds things up, and division slows things down.
Genetic gain/year = (Heritability x Selection Differential)/Generation Interval
  • So the way to get maximum gain/year is to breed for traits that have a high heritability, then maximize the Selection Differential through choosing top performing stock, and keep a low Generation Interval.

Heritability

  • This is the strength of inheritance of the trait.  It’s how much of the trait in the parents you will see in the next generation.
  • Some traits are strongly inherited and others are not.
  • A scale from 0 to 100% is used, but generally we talk generally about low (0-10%), medium (10-30%) or high (above 30%) heritability.
  • Where heritability is low, it shows that the environment has a much greater influence than genetics.

Examples:

Dairy cattle Trait Heritability %
  Milk yield 25-40
  Milk flow rate 67
  Fat yield 27-43
  Udder size and shape 20-40
  Fat % & protein % 32-87
  Calving interval 0-15
Beef cattle  Birth weight 20-60
  Weaning weight 20-55
  18month weight (pasture) 30-55
  Tenderness 60
  No. calves born/weaned 0-15
Sheep Weaning weight 10-40
  Weight of lamb weaned 30-40
  Greasy fleece weight 30-40
  Mean fibre diameter 40-70
  No. lambs born/weaned 0-15
Pigs  Daily weight gain 21-40
  Feed efficiency 20-48
  Mean backfat thickness 43-74
  Eye muscle area 35-49
  Litter size at birth 15
   Litter size at weaning 7
Poultry   Feed efficiency 20-50
  Body weight 25-65
  Breast width 15-35
  Age at sexual maturity 15-30
  Egg size 40-50
  Fertility 0-5
  Shell colour 30-90

 
The table shows a few of the wide range of traits commercial breeders are concerned with today – all of them aimed at making a profit, by increasing production, cutting costs, or both.

  • Traits associated with growth or size have high heritability, and fertility and reproduction traits are weakly inherited.
  • It would be easy to conclude not to breed for traits of low heritability as progress will be slow, but some like fertility are basic to profit.
  • We just have to take a different approach to these weakly inherited traits and use different breeding techniques (see later).  We can’t change heritability so have to accept it.