Heritability or h²:

The proportion of the phenotypic (observed) variation due to genetics:

h² = genetic variance/(genetic + environmental variance)

Note that genetic + environmental variance = phenotypic variance.

Selection differential (SD):

The difference between the selected individual and his contemporaries:

45 lb - 30 lb = 15 lb

Estimated breeding value (EBV):

(This also is the Genetic improvement per generation when all selected animals are mated.)

Only the genetic part of the selection differential (selection variation). Thus, multiply by the heritability for weaning weight of 0.2:

0.2 x 15 lb = 3 lb

Expected progeny difference (EPD):

Only half the genes are transmitted to the ram's offspring. Therefore, divide the EBV by 2:

3 lb/2 = 1.5 lb

Generation interval:

Average age of the parents when their progeny are born (3 to 4 years in sheep).

Genetic improvement per year:

Divide the genetic improvement per generation by the generation interval:

3 lb/4 = 0.75 lb per year

Note: This only applies if all selected individuals averaged 15 lb heavier weaning weights. If only the ram has a 1.5 lb increase, then the genetic improvement per year would be:

1.5 lb/4 = 0.375 lb per year

Use of EPD's:

Suppose ram A has an EPD for weaning weight of +1.5 lb and ram B has an EPD for weaning weight of -0.1 pounds. The 1.6 lb difference in EPD's indicates the expected difference in weaning weight between the progeny of the two rams. At current market price, this difference is worth about $2.40 per lamb.