The dominant mutant naddne3256shows no response to amphetamine, but a normal initial place preference. (a) Conditioned place preference (%) of 24 individuals of a naddne3256 family (generation F3), showing 12 mutants and 12 siblings. Mutants were defined as showing no or a negative change in place preference. Siblings were defined as having a change in place preference of 5% or over. The last two bars represent the means for both groups. The difference between the means is statically significant (t-test with unequal variances; P = 2.3E-07). (b) Initial place preference (%) for the same 24 individual fish. The last two bars represent the means for both groups. There is no significant difference between the two means (two sample unequal variance t-test P = 0.45). Error bars represent the one fold of the standard error.