The invention provides a method for sub grouping animals according to genotype wherein the animals of each sub-group have a similar polymorphism or combination of polymorphisms in the leptin gene, the single nucleotide polymorphisms being selected from the group consisting of UASMS1, UASMS2, UASMS3, EXON2-FB, and E2JW. The combination of single nucleotide polymorphisms of the leptin gene, especially combinations comprising alleles of the E2JW locus, may also indicate an increase in the tenderness of meat as well as indicating the quality of other traits of the animals. The invention also provides methods for identifying an animal having a desirable phenotype relating to certain feed intake, growth rate, body weight, carcass merit and composition, and milk yield, as compared to the general population of animals of that species, comprising determining the presence of a single nucleotide polymorphism or combination of single nucleotide polymorphisms in the leptin gene of the animal.