Introduction

Many phenotypes (traits) of biomedical, agricultural, or evolutionary importance are quantitative in nature. Examples include blood pressure (to study hypertension), milk output (in dairy breeding), and number of seeds produced per plant (to study evolutionary fitness). Many phenotypes such as coat color of mice, or cancer tumor aggressiveness, may not be strictly quantitative, but may be studied by a derived quantitative measure. We may classify mice by whether or not they have an agouti coat color, a 0/1 measure, or grade tumors by aggressiveness on a scale of 1 to 4 by examining tumor biopsies.