Genetics Northern California



A condition, either present at birth or acquired, where the head is abnormally large.


A single localized poor formation of tissue that initiates a chain of subsequent defects in the developing fetus.


In reference to neoplasm, having the property of locally invasive and destructive growth and metastasis. Compare to benign.

Maple Syrup Urine Disorder (MSUD)MSUD is a condition that occurs when the body is missing one of the enzymes needed to break down (metabolize) protein. Without this enzyme, parts of the protein build up in the body causing damage to the nervous syste

The ordering of genes on chromosomes according to recombination frequency.

Mean Corpuscular Volume (MCV) A measure of the volume of red corpuscles expressed in cubic micrometers.
Medical geneticistA medical doctor who specializes in the field of genetics. Usually the doctor begins training in pediatrics, OB/GYN, or internal medicine before specializing in genetics.

Two successive nuclear divisions (with corresponding cell divisions) that produce haploid gametes.


A protrusion of the meninges, without brain or spinal cord, through a defect in the spine or skull. The defect is covered with skin. If occurring on the spine, there is often a bulge in the midline of the back where the abnormality is located. Although there is frequently less nerve damage initially, complications can occur later from tethering of the spinal cord.

Mental retardation

The term given to an individual with an IQ score below 70 (average = 100). A disability that affects a person's intellectual functioning and is accompanied by significant limitations in areas such as communication, self-care, work, leisure, health, safety, and interpersonal skills. Onset must occur before age 18 years.

Metabolic disorders

Conditions that affect the body’s ability to use amino acids (from proteins), organic acids (from proteins, fats, and sugars) or fatty acids (from fats) all of which are necessary for energy, growth and repair. To break down or convert these substances, certain enzymes must be present. When there is a deficiency or absence of the needed enzyme (block in the metabolic pathway), metabolites build up in large amounts and may be harmful to the body. Metabolic disorders have varying degrees of severity.


The chemical reaction that occur in living cells, both synthesis and degradation.

Metaphyseal Chondrodysplasia, McKusick TypeAnother name for Cartilage Hair Hypoplasia (CHH).
Metaphyseal dysplasia, Schmid type

A rare form of dwarfism that primarily affects the metaphyses (growing portion of the bone under the cartilaginous growth plate). It causes short stature with abnormally short arms and legs in proportion to the trunk, bowed legs, flared rib cage and waddling gait. There are almost always hip deformities (coxa vara) and leg pain.


The spread of cancer to another organ, usually through the bloodstream.

Methlymalonic Acidemia (MMA)MMA is a condition that occurs when the body lacks an enzyme needed to break down (metabolize) protein. When this enzyme is missing, parts of the protein build up in the body and cause damage to the nervous system, especially the brain. If left untreated, MMA can result in vomiting, anorexia, kidney problems, blood disorders, failure to thrive, developmental delays, mental retardation, and even death.

The way methlymalonic acidemia is inherited is called autosomal recessive.

Microcephalic Osteodysplastic Primordial Dwarfism (MOPD)

There were originally 3 types of this condition, but now there is a discussion to combine Types I and III. All types have very slow growth (adult height < 100cm), small slender bones, and microcephaly (small head). The main difference between Types I (& III) and II are the facial features and development. People with Type I have a prominent nose and sloping forehead and poor development whereas those with Type II have a round face, large-appearing eyes, flat midface and a long, curved nose. Type II also has a better, though not necessarily average, developmental outcome.


Abnormal smallness of the head. Head circumference which is below the 3rd percentile.

MicrocytosisPresence of abnormal numbers of microcytes (small erythrocyte or red blood corpuscles).
MicrodeletionA chromosome deletion too small to be visible under a microscope. May be identified using FISH or array CGH.
Midface hypoplasiaUnderdevelopment or undergrowth of the middle areas of the face (upper jaw, cheekbones, eye sockets).
Missense mutation

A type of mutation that results in a single amino acid change in the translated gene product.


A membrane-bounded structure in the interior of the cell that generates energy for the cell. Generally inherited from the mother.

Mitochondrial DNA

The genetic material of the mitochondria, the organelles that generate energy for the cell.


The nuclear division producing two daughter nuclei identical to the original nucleus. A type of nuclear division that produces two daughter nuclei identical to the parent nucleus normally just prior to cell division.


A diploid cell missing a single chromosome. A cell or individual that has only one copy of one particular chromosome pair.

Monounsaturated fatsFats such as olive oil and peanut oil. They are felt to lower the risk of heart disease when used in moderation and in place of other oils.

Referring to a twin pair in which both members are derived from a single embryo. Compare with dyzygotic.


A tissue containing two or more genetically distinct cell types, or an individual composed of two or more genetically distinct cell lines.

MRNA (messenger ribonucleic acid)

An RNA molecule transcribed from the DNA of a gene, and from which a protein is translated by the action of ribosomes. The basic function of the nucleotide sequence of mRNA is to determine the amino acid sequence in proteins.

MultidisciplinaryUtilizing several disciplines. In a multidisciplinary clinic, there are service providers from several specialty areas (disciplines) of medicine (i.e., genetics, neurology, psychology, etc.)

Genetic disorder resulting from the combined action of alleles of more than one gene (e.g., heart disease, neural tube defects, diabetes, and some cancers) and the environment. Hereditary patterns are usually more complex than those of single gene disorders.

Multiple Epiphyseal Dysplasia (MED)

People with MED are often not diagnosed until ages 2-10 because growth in infancy and childhood is often normal. There is a malformation in the epiphyses (ends) of the long bones which slows growth over time and final adult height ranges from 135cm-155cm. Other characteristics of MED include: hip degeneration, premature osteoarthritis (beginning in the teens), bowed or knock-kneed legs, and short hands with very short fingers.

Multispecialty clinicA clinic involving health care providers from various specialties working together in a single setting to provide coordinated and comprehensive patient care, for complex genetic and metabolic disorders. (e.g. The Craniofacial clinic includes a geneticist, genetic counselor, maxillofacial surgeon, orthodontist, speech pathologist, etc.)

A deleterious change in the sequence of the DNA molecule in the cell of an organism.


Most serious and most common form of spina bifida. The skin, muscle, and bone over the opening in the spinal column fail to close properly. The spinal cord and meninges (fibrous protective sheath/covering) protrude through an opening on the back into a sac. Most infants born with myelomeningocele will have significant spinal cord dysfunction.