Hormones can be divided into two categories based on their binding with the receptors: Some hormones are water soluble like peptide hormones and catecholamines, they cannot cross the cell membrane of the target cell and so they bind to the receptors on the plasma membrane. Binding to membrane receptors further leads to activation of intra-cellular enzymes leading to physiological change. These hormones can also be referred as non-steroid hormones. One good example is insulin hormone. Insulin binds to the membrane receptor and brings about the physiological change by activation of protein kinase inside the cell.
The other group of hormones are called steroid hormones and thyroid hormones which can cross the cell membrane and bind to the intra-cellular receptors. The hormone-receptor complex then binds with or activates the specific portion of DNA strand, which in turn initiates transcription and translation of specific genes for bringing the physiological change.
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.
