Sentence:  Note that only recombinantion fractions between 0 and 0.5 are meaningful, and that all <WSD>lod</WSD> scores are zero at (theta)=0.5 (because they are then measuring the ratio of two identical probabilities, and log<sub>10</sub>(1)=0). 
Meaning of the word marked with <WSD> tags:  Logarithm of odds; A measure of likelihood calculated by taking the log of the ratio of the probability of a hypothesis being true given the observed data over the probability that the hypothesis is false.