Genetics Northern California


HaploidA single set of chromosomes (half the full set of genetic material) present in the egg and sperm cells. Human beings have 23 chromosomes in their reproductive cells. All other cells are diploid.
HDLHigh density lipoprotein or the "good cholesterol," as it is known to take plaque off the walls of the arteries and have the fat broken down in the liver.
Head circumference

A measurement around the largest part of the head.

HemoglobinopathiesInherited disorders in which the production of normal hemoglobins is partly or completely suppressed or partly or completely replaced by the production of one or more abnormal hemoglobin variants.
HemolysisDestruction of red blood cells.

A hernia occurs when part of an internal organ bulges through a weak area of muscle. Most hernias occur in the abdomen. There are several types of hernias, including:

Inguinal -  the most common type, is in the groin
Umbilical -  around the belly button
Incisional - through a scar
Hiatal - a small opening in the diaphragm that allows the upper part of the stomach to move up into the chest.


An individual that has two different alleles or genes encoding a protein; one copy encodes a normal protein whereas the second copy encodes a nonfunctional or abnormal protein. Also, two different copies of normal alleles.


Presence of excessive bodily and facial hair.


Hereditary nonpolyposis colorectal cancer. Autosomal dominantly inherited disease predisposing to colorectal cancer. May be in association with carcinoma of the endometrium, ovary, stomach, small bowel, and pancreas.

Homeobox gene

A short stretch of nucleotides whose base sequence is virtually identical in all the genes that contain it. Found in many organisms from fruit flies to human beings.

HomocysteineThis molecule is an intermediate in the synthesis of cysteine. If homocysteine levels are raised in an individual, this metabolic defect can lead to elevated cholesterol and other symptoms (such as mental retardation) known as homocystinuria. Found in many organisms from fruit flies to human beings.

An individual that has two identical alleles or genes encoding a protein. Genes may encode functional or nonfunctional proteins.

Housekeeping genes

Genes whose protein products are required for cellular maintenance or metabolism.

Human chorionic gonadotropin

Human chorionic gonadotropin (hCG) is a hormone produced by the placenta. The levels rise rapidly in early pregnancy and then decline between the 10th and 20th weeks. Second trimester maternal blood levels of hCG may be higher in a pregnancy in which the baby has Down syndrome. The level of hCG may be lower than usual in a pregnancy if the baby is affected with trisomy 18 or SLOS.

Human genome project

A large international project to map and sequence all the DNA in human chromosomes.


The process of joining two complementary strands of DNA or one each of DNA and RNA to form a double-stranded molecule. 

Hydrocephalus (Hydrocephaly)

A condition that is caused by the build up of fluid inside the developing brain due to the partial blockage of the normal flow of spinal fluid out of the brain.

Hydrogenated fatsThese are fats that have been processed so that liquid polyunsaturated fats are changed into solid or saturated fats. The higher the saturation, the more likely it can increase blood cholesterol levels.
HyperextensibilityThe ability to extend a limb or part to extreme or excessive extension - i.e., thumbs or fingers.
HyperlipidemiaA term used to describe an individual with elevated cholesterol and/or triglycerides in their blood stream.
HypoalphalipoproteinemiaLow HDL in an individual confers an increased risk for coronary artery disease; trigylcerides may also be slightly elevated.
HypochondroplasiaThis condition is a mild form of achondroplasia.

A defect of the underside (ventral) portion of the penis where the urethra canal is open in places.

HypotoniaA descriptor for diminished tone of the muscle tissue.