Genetics is a branch of biology concerned with the study of genes, genetic variation, and heredity in organisms. Though heredity had been observed for millennia, Gregor Mendel, a scientist and Augustinian friar working in the 19th century, was the first to study genetics scientifically. Mendel studied "trait inheritance", patterns in the way traits are handed down from parents to offspring. He observed that organisms (pea plants) inherit traits by way of discrete "units of inheritance". This term, still used today, is a somewhat ambiguous definition of what is referred to as a gene.