Genetics is the study of heredity and traits that are passed from parents to offspring. A trait is a characteristic like eye color that varies between individuals. Alleles are different versions of genes that determine traits, like brown vs. blue alleles for eye color. Genes are sequences of DNA that code for proteins and determine traits. Gregor Mendel conducted early studies of heredity in pea plants and established that traits are inherited as discrete units and assort independently. Dominant alleles mask recessive alleles when present, and genotypes describe an organism's allele combination while phenotypes describe observable traits. Punnett squares predict offspring genotypes and phenotypes from parental crosses.