Chromosome Disorders: clinical conditions caused by an abnormal “chromosome” constitution in which there is extra or missing chromosome material (either a whole chromosome or a chromosome segment). (MeSH) 

Occurs if the number of chromosomes in a cell is not 46, or if individual chromosomes have extra, missing, or rearranged genetic material. (Lewis, 243) A structural chromosomal abnormality (may be) caused by the spontaneous or induced breakage of a chromosome. It may result in “DNA sequence” deletions, chromosomal “translocations,” or chromosomal “inversions.” (NCIt)

Aneuploidy: the occurrence of one or more extra or missing chromosomes leading to an unbalanced chromosome complement, or any chromosome number that is not an exact multiple of the “haploid” number (which is 23). (GeneReviews) Missing a chromosome or having an extra chromosome. Includes “monosomies” and “trisomies.” (Lewis, 244)

Monosomies: possessing only one copy of a particular chromosome instead of the normal two copies. (HGPIA) The presence of only one chromosome from a pair; partial monosomy refers to the presence of only one copy of a segment of a chromosome. (GeneReviews) Monosomy is the state of having a single copy of a chromosome pair instead of the usual two copies found in “diploid” cells. Monosomy can be partial if a portion of the second chromosome copy is present. (NHGRI)

Trisomies: the presence of a single extra chromosome, yielding a total of three chromosomes of that particular type instead of a pair. (GeneReviews) Possessing three copies of a particular chromosome instead of the normal two copies. (HGPIA) The most common chromosomal abnormality. (Lewis, 246) Editor’s note - trisomy disorders include “Patau syndrome,”  “Down syndrome,” and “Edward syndrome.”

Down Syndrome: a disorder caused by the presence of an extra chromosome 21 and characterized by mental retardation and distinguishing physical features. (NCI1) The extra chromosome causes problems with the way the body and brain develop. Down syndrome is one of the most common causes of human birth defects. (PubMedHealth2) Symptoms associated with the syndrome include mental retardation, distinctive facial characteristics, and increased risk for heart defects and digestive problems, which can range from mild to severe. The risk of having a child with Down syndrome rises with the mother's age at the time of conception. (NCGRI) Can be caused by “translocation.” Can be revealed by “fluorescence in situ hybridization.” 85% of conceptions survive 1 year after birth. (Lewis, 246-247) Also referred to as ‘Trisomy 21.’

Edwards Syndrome: a genetic disorder in which a person has a third copy of material from chromosome 18, instead of the usual two copies. A somewhat common syndrome. It is three times more common in girls than boys. (PubMedHealth2) Major abnormalities include heart defects, a displaced liver, growth retardation, and oddly clenched fists. Most cases are traced to “nondisjunction” in “meiosis II” of the “oocyte.” (Lewis, 247) Less than 5% of conceptions survive 1 year after birth. (Lewis, 246) Also referred to as ‘Trisomy 18.’

Jacobs Syndrome: male with an extra Y chromosome. One male in 1,000 has an extra Y chromosome. Can arise from nondisjunction in the male, producing a sperm with two Y chromosomes. Symptoms may include great height and aggressive or violent behavior. 96 percent of XYY males are apparently normal. (Lewis, 249) Although males with this condition may be taller than average, this chromosomal change typically causes no unusual physical features. Most males with the syndrome have normal sexual development and are able to father children. Syndrome is associated with an increased risk of learning disabilities and delayed development of speech and language skills. This condition occurs in about 1 in 1,000 newborn boys. Five to 10 boys with ’47, XYY’ syndrome are born in the United States each day. (GHR) Also referred to as ‘XXY syndrome’ and ’47, XYY.’

Klinefelter Syndrome: a chromosomal condition that affects male physical and “cognitive” development. Its signs and symptoms vary among affected individuals. (GHR) Klinefelter syndrome is when a boy is born with at least one extra X chromosome. This would be written as ‘XXY.’ Klinefelter syndrome occurs in about 1 out of 500 to 1,000 baby boys. Women who get pregnant after age 35 are slightly more likely to have a boy with this syndrome than younger women. (PubMedHealth2) Males with this disorder may have larger than normal breasts, a lack of facial and body hair, a rounded body type, and small testicles. They may learn to speak much later than other children and may have difficulty learning to read and write. Klinefelter syndrome increases the risk of developing germ cell tumors and “breast cancer.” (NCI1) Also referred to as ‘XXY Syndrome’ and ’47, XXY.’

Nondisjunction: uneven distribution of chromosomes in meiosis. Causes aneuploidy. (Lewis, 244) An error in cell division (that) results in a reproductive cell with an abnormal number of chromosomes. (GHR)

Patau Syndrome: a genetic disorder in which a person has three copies of genetic material from chromosome 13, instead of the usual two copies. Rarely, the extra material may be attached to another chromosome (“translocation”). Occurs when extra DNA from chromosome 13 appears in some or all of the body's cells.'Trisomy 13 mosaicism’ is the presence of an extra chromosome 13 in some of the cells. ‘Partial trisomy’ is the presence of a part of an extra chromosome 13 in the cells. The extra material interferes with normal development. (Patau syndrome) occurs in about 1 out of every 10,000 newborns. Most cases are not inherited. Instead, the events that lead to trisomy 13 occur in either the sperm or the egg that forms the fetus. (PubMedHealth2) Major abnormalities affect the heart, kidneys, brain, face and limbs. A few individuals have survived until adulthood, but they do not progress developmentally beyond the 6-month level. Less than 5% of conceptions survive 1 year after birth. (Lewis, 246-248) Also referred to as ‘Trisomy 13.’

Polyploidy: the chromosomal constitution of a cell containing multiples of the normal number of chromosomes. (MeSH) An entire extra set of chromosomes. (Lewis, 243)

Translocation: a “mutation” in which a large segment of one chromosome breaks off and attaches to another chromosome. (HGPIA) Chromosomal translocations can be detected by analyzing “karyotypes” of the affected cells. (NHGRI) This results in changed and often flawed chromosomes. Genetic translocations can cause serious disorders. (PubMedHealth2) Sometimes pieces from two different chromosomes will trade places with each other. Translocations may lead to medical problems such as “leukemia,” breast cancer, “schizophrenia,”  “muscular dystrophy,” and Down syndrome. (NCI1) About 50 translocations have been identified, with most associated with disruptions in genes involved in the control of “cell division.” (Micklos, 244)

Balanced Translocation: translocations in which there is no net loss or gain of chromosome material. Usually not associated with “phenotypic” abnormalities, although gene disruptions at the breakpoints of the translocation can, in some cases, cause adverse effects, including some known genetic disorders. (GeneReviews)

Inversion: a chromosomal defect in which a segment of the chromosome breaks off and reattaches in the reverse direction. (NCI1) A chromosomal rearrangement in which a segment of genetic material is broken away from the chromosome, inverted from end to end, and re-inserted into the chromosome at the same breakage site. (GeneReviews)

Philadelphia Chromosome: a small, unusual chromosome found only in leukemia cells. Result of a translocation. (Lewis, 358) An exchange between the long arms of chromosome 9 and 22. Identified in 1972 by Janet Rowley in 95% of “chronic myelogenous leukemia” (CML) patients. Named after the city in which it was first identified in CML patients. (Micklos, 243) First chromosome abnormality to be linked to cancer. (Lewis, 358)

Unbalanced Translocation: translocations in which there is loss or gain of chromosome material. Nearly always yields an abnormal phenotype.(GeneReviews)

Triplo-X: female with an extra X-chromosome. Example of “X-inactivation” and female nondisjunction. (Lewis, 248) Associated with an increased risk of learning disabilities and delayed development of speech and language skills. Seizures or kidney abnormalities occur in about 10 percent of affected females. Occurs in about 1 in 1,000 newborn girls. Five to 10 girls with triple X syndrome are born in the United States each day. (GHR) Also referred to as ‘triple X syndrome,’  ’XXX Syndrome’ and ’47, XXX.’

Turner Syndrome: female lacking a second X-chromosome. Example of female or male nondisjunction. (Lewis, 45) Cells are missing all or part of an X chromosome. The condition only occurs in females. Most commonly, the female patient has only one X chromosome. Others may have two X chromosomes, but one of them is incomplete. Sometimes, a female has some cells with two X chromosomes, but other cells have only one. Turner syndrome occurs in about 1 out of 2,000 live births. Possible symptoms in young infants include swollen hands and feet, and a wide and webbed neck. (PubMedHealth2) Also referred to as ‘Ulrich syndrome,’ ‘Bonnevie-Ullrich syndrome,’  ‘Monosomy X,’  ’XO syndrome’ and ’48, X.’

XX Male Syndrome: a person who has a chromosome composition of 46, XX and who is phenotypically male. (NCIt) Condition in which individuals with two X chromosomes in each cell, the pattern normally found in females, have a male appearance. Results from an abnormal exchange of genetic material between chromosomes (translocation). This exchange occurs as a random event during the formation of sperm cells in the affected person's father. The translocation affects the gene responsible for development of a fetus into a male - the “SRY gene.” The SRY gene, which is normally found on the Y chromosome, is misplaced in this disorder, almost always onto an X chromosome. A fetus with an X chromosome that carries the SRY gene will develop as a male despite not having a Y chromosome. People with this disorder have male external genitalia. Affected children are typically raised as males and have a male “gender identity.” At “puberty,” most affected individuals require treatment with the male sex hormone “testosterone” to induce development of male secondary sex characteristics such as facial hair and deepening of the voice. (GHR) Also referred to as ’46, XX’ and ’46, XX testicular disorder of sex development.’

XXYY Syndrome: a rare sex chromosome abnormality in which a male child has an extra X and Y chromosome. (NCIt) Disrupts male sexual development. Adolescent and adult males with this condition typically have small testes that do not produce enough testosterone, which is the hormone that directs male sexual development. A shortage of testosterone during puberty can lead to reduced facial and body hair, poor muscle development, (and) low energy levels. Males with ’48, XXYY’ syndrome have an inability to father children. This condition is not inherited. it usually occurs as a random event during the formation of reproductive cells. Estimated to affect 1 in 18,000 to 50,000 males. (GHR) Also referred to as ’48, XXYY.’