AGB 121: Principles of Animal Genetics and Population Genetics (2+1)

Definition

REPEATABILITY

Repeatability is defined as the correlation between measurements on the same animal for traits, which are measured more than once.

• It is represented by the symbol ‘r’.
• It ranges from 0 to 1 or 0 to 100 per cent.
• Example: Lactation milk yield, Fleece weight in sheep, and Litter size in swine.

Repeatability Vs Heritability

• The meaning of repeatability becomes clearer when it is compared with heritability.
• Repeatability is an indicator of the extent to which an animal’s superiority in one measurement will be seen in subsequent measurements of the same animal.
• But heritability indicates the extent to which the superiority of parents will be seen in their offspring.

Repeatability Vs Environmental Variance

• To understand repeatability and to calculate it, a different division of environmental variance is required.
• Some environmental effects are permanent and therefore influence permanently in all periods and affect all measurements.
• Other environmental effects are temporary and vary from one period to the next. Since the temporary effects are independent from period to period, they are likely to be positive and negative and tend to be zero over several periods.
• Permanent environmental factors
  • Repeatability is concerned with the extent to which differences between individuals are permanent. i.e remain throughout their lives. Those permanent differences must be due to factors that remain permanent throughout the individuals’ lifetime. These include the individuals genotype (G=A+D+I) and a deviation due to any permanent environmental factors E_p. i.e. a deviation due to environmental factors that exert the same effect on all measure of performances throughout the individuals life time.
    • Example: permanent defects like udder damage in cows.
  • E_P is also referred to as general environmental effect.
• Temporary environmental factors
  • This is due to temporary environmental factors E_t, which affect one measurement but not the rest and vary from one period to next.
    • Example, quality / quantity of feed, climate etc.
  • E_t is also referred to as special environmental effect.
  • Using the distinction between E_p and E_t we can say that permanent differences between the performances of individuals are due to differences in G + E_p.
• Environmental variances
  • The respective environmental variances are referred as V_Ep or V_Eg and V_Et or V_Es.
    • Where
      • V_Ep or V_Eg refers to permanent or general environmental variance contributing to the between-individual component and
      • V_Et or V_Es refers to temporary or special environment variance arising from temporary or special circumstances within individual.
  • Heritability indicates the relative contribution of V_A to V_P.
  • In a similar manner, repeatability indicates the relative contribution of V_G + V_Ep to V_P.
  • In terms of causal components of variance, repeatability is expressed as
    
  • Because of this, we can conclude that for repeatable character, heritability is never greater than repeatability.