When a structure is homologous, it refers to body parts that might not necessarily have the same function, however, the structure is very similar leading to the conclusion that these organism's shared common ancestry. Due to different environments and natural selection, this organ experienced structural changes over time. For example, a cat paw, human arm, bat wing, and whale flipper have the same underlying bone structure, although modified. Some bones have become elongated or shortened, or fused. But, because the same body structure is present, it suggests evolution from a common ancestor. However, in genetics, two chromosomes located on the same position on a karyotype are said to be homologous chromosomes. For example, the two chromosomes located at pair number one in a human karyotype are homologous chromosomes. The term homozygous in genetics refers to the situation where two genes or alleles for a particular trait are the same on a pair of homologous chromosomes. For example, in pea plants, if both height genes for the trait of height in a particular plant are tall, then that plant is said to be homozygous tall.