Genetics is a branch of biology that deals with the study of genes, genetic changes and inheritance in living organisms. Mendel studied the inheritance of traits, which is a model of how traits are passed from parents to children. He observed that organisms inherit discrete units of traits. This term, which is still used today, is a somewhat vague definition of what is referred to as a gene. Inheritance of traits and molecular genetic mechanisms of genes are still the main principles of genetics in the 21st century, but modern genetics has gone beyond inheritance to study the function and behavior of genes. Gene structure and function, diversity and distribution are studied in the text of the cell, organism and in the context of the population. Genetics has created various fields such as molecular genetics, epigenetics and population genetics.