Chromosome abnormalities occur when there are defects in chromosome number or structure. They can be numerical, such as having an extra or missing chromosome, or structural, with changes to the chromosome composition. Down syndrome is a numerical abnormality caused by trisomy of chromosome 21, resulting in three copies instead of the typical two copies. This leads to developmental delays and other health issues. Other examples of chromosome abnormalities include Turner syndrome, Wolf-Hirschhorn syndrome, and Jacobsen syndrome, each caused by defects or deletions of different chromosomes. Modern technologies allow for more precise identification of chromosomal imbalances compared to earlier microscope methods. Chromosomal abnormalities can impact development and functioning depending on the genes involved in the affected chromosomes.

1. Erin Bosman
Chromosome Abnormalities and Down syndrome
Genetic make-up influences both physical and behavioral traits, which help to
form the phenotype. Imbalances in genetic material tend to result in complex phenotypes,
observable characteristics based on the genotype. Nervous system disorders can be
caused by irregularities in a chromosome. Chromosomes are found in the nucleus of a
cell and hold our genes, which help our nervous system, know how to develop and
function.
In general, a human cell contains 46 chromosomes or 23 pairs of chromosomes.
One pair comes from the mother and the other comes from the father. Also, they are
categorized according to approximate size from largest to smallest. Chromosome pairs 1
through 22 are called autosomes and make up most of our physical appearance as well as
behavioral functions while chromosome pair 23 contains the sex chromosomes.
A chromosome abnormality reveals a defect of chromosome number or structure.
Often time’s chromosome abnormalities occur as an accident during development before
birth causing the abnormality to potentially be present in every cell. They also sometimes
can be inherited from a parent. However, they often occur when there is an error in cell
division. There are several types of chromosome abnormalities that can be organized into
two basic groups: numerical abnormalities and structural abnormalities. A numerical
abnormality occurs when there is a chromosome missing from a pair or when a pair has
an extra chromosome while a structural abnormality occurs when the structure of the
chromosome is altered in some way.
Down Syndrome is a numerical abnormality associated with developing mental
retardation among other abnormalities that affects roughly 1 out of 700 children. Some of

2. Erin Bosman
the abnormalities associated with Down syndrome are learning disability, heart defects,
and early onset Alzheimer’s disease. In general, the nucleus of a cell contains 23 pairs of
chromosomes while in individuals with Down syndrome there typically is an abnormality
of a trisomy of the chromosome 21. Chromosome 21 is the smallest human chromosome.
Often it is the mother that passes down the extra chromosome. Supposedly, this extra
chromosome results in an increased expression of some of the genes on the chromosome.
Three different variations of Down syndrome exist: trisomy 21, translocation, and
mosaicism. Trisomy 21, or nondisjunction, is an error in cell division where an embryo
has three copies of chromosome 21. Nondisjunction is the case for ninety five percent of
Down syndrome cases. Translocation occurs when part of chromosome 21 separates
during cell division and attaches to another chromosome, which is often chromosome
14. The number of chromosomes in the cells remains 46 but the occurrence of an extra
part of chromosome 21 causes the abnormality of Down syndrome. In addition,
mosaicism occurs when nondisjunction of chromosome 21 takes place in one of the
initial cell divisions. This causes there to be a mixture of two types of cells, some with
the usual 46 chromosomes and others with 47 chromosomes. Those cells with 47
chromosomes have an extra copy of chromosome 21 leading to Down syndrome. Those
with mosaic Down syndrome may have fewer characteristics than those with other types
of Down syndrome.
Down syndrome is a widely known chromosome abnormality however there are
quite a few chromosome abnormalities in existence such as Turner syndrome, Wolf-
Hischhorn syndrome, and Jacobsen syndrome. Turner syndrome occurs when one of the
x chromosomes in females is incomplete. Symptoms of Turner syndrome can be

3. Erin Bosman
incomplete sexual development and infertility. In addition, Wolf Hirschhorn syndrome is
caused by the deletion of the short arm of chromosome 4, which results in delayed
development and seizures while Jacobsen syndrome results in delayed development as
well. Jacobsen syndrome is caused by a loss of genetic material from the chromosome 11.
Usually, chromosome abnormalities are found through inspection of the
chromosomes under a microscope. The use of molecular cytogenetic technologies has
allowed for the identification of imbalances that aren’t visible with the microscope. This
has allowed for the discovery of new conditions. Many chromosomal changes are large
and include numerous genes. Individuals with overlapping may allow for further
research.
A chromosomal abnormality can affect the number of chromosomes and the
composition of chromosomes. If the chromosome is balanced so that the expected
amount of material is found in each cell the abnormalities do not occur. If the balance is
altered then chromosomal abnormalities occur and can cause a wide range of
malfunctions.

4. Erin Bosman
Chromosome Abnormalities Fact Sheet. (n.d.). Retrieved January 24, 2015, from
http://www.genome.gov/11508982
Theisen, A., & Shaffer, L. (n.d.). Disorders caused by chromosome abnormalities.
Retrieved January 24, 2015, from
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3681172/
Roper, R., & Reeves, R. (n.d.). Understanding The Basis For Down Syndrome
Phenotypes. PLoS Genetics, E50-E50.
What Is Down Syndrome? (n.d.). Retrieved January 24, 2015, from
http://www.ndss.org/Down-Syndrome/What-Is-Down-Syndrome/
Wiseman, F., Alford, K., Tybulewicz, V., & Fisher, E. (n.d.). Down Syndrome--recent
Progress And Future Prospects. Human Molecular Genetics, R75-R83.