The Thyroid gland secretes thyroxin, a hormone that can reduce concentration and lead to irritability when the thyroid is overactive, and cause drowsiness and sluggish metabolism when the thyroid is underactive. Within the thyroid are four tiny organs, the parathyroids, that secrete parathormone to control and balance the levels of calcium and phosphate in the blood and tissue fluids, which affects excitability of the nervous system. The pineal gland secretes melatonin, which helps regulate sleep wake cycles. (Morris and Maisto. 2005) The hypothalamus is a collection of specialized cells that is located in the lower central part of the brain, it is the main link between the endocrine and nervous systems. Nerve cells in the hypothalamus control the pituitary gland by producing chemicals that either stimulate or suppress hormone secretions from the pituitary. (Nemours Foundation. 2009) Although it is no bigger than a pea, the pituitary gland, located at the base of the brain just beneath the hypothalamus, is considered the most important part of the endocrine system. It's often called the "master gland" because it makes hormones that control several other endocrine glands. The pituitary influences blood pressure, thirst, sexual behavior and sexual interest. (Nemours Foundation. 2009) The gonads, which are the testes in males and the ovaries in females, secrete androgens, including testosterone and estrogen. These hormones regulate sexual development and are linked to sexual interest and behaviors. (Morris and Maisto. 2005) Genetics is the study of how living things pass on traits from one generation to the next. Chromosomes are tiny threadlike bodies which are made up of deoxyribonucleic acid, or DNA. Elements that control the transmission of traits are found on chromosomes and they are known as genes. The transmission of traits is referred to as heredity. Members of a gene pair can be either dominant or recessive. In some cases a trait is delayed until later...