Frequency of resistance alleles and homozygous resistant worms
 in a worm population developing anthelmintic resistance
A
In the earliest phase of  AR development
 (around point A) resistance alleles are rare,
and resistance develops very slowly.
This is a theoretical representation of the rate at which anthelmintic resistance might develop in a worm population. Two assumptions are (1) that selection pressure is constant (ie, the same anthelmintic remains in use) and (2), resistance is conferred by two recessive alleles, so only homozygous-resistant parasites survive treatment..
The figure shows that (a) resistance develops slowly at first then more quickly as the frequency of resistance alleles exceeds 5% to 10% of all alleles, and (b) the frequency of homozygous resistant worms lags behind the frequency of resistance alleles.