Genetics is the study of how heritable traits are transmitted from parents to offspring. Humans have long observed that traits tend to be similar in families. It wasn’t until the mid-nineteenth century that larger implications of genetic inheritance began to be studied scientifically.
Genes come in different varieties, called alleles. Somatic cells contain two alleles for every gene, with one allele provided by each parent of an organism. Often, it is impossible to determine which two alleles of a gene are present within an organism's chromosomes based solely on the outward appearance of that organism. However, an allele that is hidden, or not expressed by an organism, can still be passed on to that organism's offspring and expressed in a later generation.