Y (in genetics): The Y chromosome, the sex chromosome found in normal males, together with an X chromosome.

Y chromatin: Brilliantly fluorescent body seen in cells stained with the dye quinacrine which lights up the Y chromosomes most brightly.

YAC: Yeast artificial chromosome.

Yard: In length, 3 feet or 36 inches or, metrically, 86.44 centimeters. The yard, along with the foot and inch, are English creations to which the USA has stubbornly clung. The yard was originally a unit of measurement of land and was about 5 meters (now termed a rod). In the 14th century, the yard emerged as 3 feet, about the length of a riding stick or sword.

Yawn: Involuntary opening of mouth often caused by suggestion and accompanied by breathing inward then outward. Repeated yawning can be a sign of drowsiness or depression.

Yaws: A tropical infection with berry-like bumps on the skin of the face, hands, feet and genital area. The infectious organism (a spirochete, Treponema pertenue) enters the skin, often where it is scraped, and causes a painless bump that heals but is followed by crops of bumps which may be pus-filled and ulcerate. In time, yaws can deform and destroy skin, bones and joints. Because of the berry-like bumps, jaws is also called frambesia tropica from the French framboise for raspberry.

Y chromosome: The sex chromosome found in normal males, together with an X chromosome. Once thought to be a genetic wasteland, the Y now is known to contain at least 20 genes, some of them unique to the Y including the male-determining gene and male fitness genes that are active only in the testis and are thought responsible for the formation of sperm. Other genes on the Y have counterparts on the X chromosome, are active in many body tissues and play crucial "housekeeping" roles with the cell.

yd.: Abbreviation for the measure of length, a yard. For example, an American football field is 100 yds. long.

Yeast: A group of single-celled fungi that reproduce by budding. Although most yeast are harmless (some are used in baking and brewing), other yeast can cause disease in humans. For example, the yeast Candida (once called Monilia) causes thrush (oral infection) and diaper rash in infants, fingernail infections, vaginal area infections after puberty, and a host of problems in patients with immune deficiency.

Yeast artificial chromosome (YAC): A vector created and used in the laboratory to clone pieces of DNA. A YAC is constructed from the telomeric (end), centromeric, and replication origin sequences needed for replication in yeast cells.

Yeast syndrome: The yeast Candida has been thought to cause a syndrome with a number of nonspecific problems including fatigue, loss of appetite, headache, short-attention span, depression and all manner of intestinal irregularities. There is no scientific evidence to support the existence of the yeast syndrome (also called the yeast connection).

Yellow fever: An acute systemic (bodywide) viral illness with in severe cases a high fever, bleeding into the skin and death of cells (necrosis) in kidney and liver, the liver damage causing intense jaundice (yellowing). Yellow fever once ravaged port cities in the U.S. including Philadelphia and New York. It occurs in tropical areas of Africa and the Americas. The mosquito vector (carrier) was found through the research of the Cuban physician Carlos Juan Finlay and the American army doctor Walter Reed.

Yellow fever vaccination: A live attenuated (weakened) viral vaccine recommended for people traveling to or living in tropical areas in the Americas and Africa where yellow fever occurs.

Yellow jacket stings: Stings from yellow jackets and other large stinging insects such as bees, hornets and wasps can trigger allergic reactions of varying severity. Avoidance and prompt treatment are essential. In selected cases, allergy injection therapy is highly effective. (The three "A’s" of insect allergy are Adrenaline, Avoidance and Allergist.)

Yersinia: A group of bacteria some of which are responsible for the bubonic plague (see Yersinia pestis), intestinal infections and a condition called mesenteric lymphadenitis that can mimick appendicitis.

Yersinia pestis: The bacterial cause of the bubonic plague which in the year 541 (as the Black Death) and later in the Middle Ages decimated Europe. The effects of the plague are described in the nursery rhyme "We all fall down." It is transmitted to humans by the bite of fleas that have fed on infected animals, mostly rodents. Plague occurs in the U.S. It is treatable with antibiotics but, if not treated promptly, can promptly lead to death.

Y-linked: A gene on the Y chromosome. A Y-linked gene is by necessity passed from father to son.

Y-linked inheritance: Inheritance by genes on the Y chromosome. Also called holandric inheritance.

Y map: The array of genes on the Y chromosome. Once thought to be a genetic wasteland, the Y now is known to contain at least 20 genes, some of them unique to the Y including the male-determining gene and male fitness genes that are active only in the testis and are thought responsible for the formation of sperm. Other genes on the Y have counterparts on the X chromosome, are active in many body tissues and play crucial "housekeeping" roles with the cell.

Yogurt: A common dish made of milk curdled and fermented with a culture of Lactobacillus (the milk bacillus). The word was acquired in the 1620s from Turkey. It can be spelled myriad ways including yogurt, yoghurt, yaghourt, yooghurt, yughard, and yaourt. The most popular spellings in the Anglo-Saxon world are yogurt and yoghurt while in France one eats yaourt.

Yolk sac: Not all yolk has to do with birds’ eggs. Human embyros have a yolk sac, a membane outside the embryo but connected by a tube (the yolk stalk) though the umbilical opening to the embryo’s midgut. The yolk sac serves as an early site for the formation of blood and in time is incorporated into the primitive gut of the embryo.

Yolk stalk: A narrow tube present in the early embryo that connects the midgut of the embryo (through the umbilical opening) to the yolk sac outside the embryo. Later in development, the yolk stalk is usually obliterated but a remnants of it may persist, most commonly as a finger-like protrusion from the small intestine known as Meckel’s diverticulum. Found in 2-4% of people, Meckel’s diverticulum may become inflamed much like the appendix and require surgical removal. The yolk stalk is also called the umbilical duct, vitelline duct, or oomphalomesenteric duct.

Youth: The time between childhood and maturity. (Unfortunately, as the songwriter Sammy Cahn noted, "youth is wasted on the young.")