Glossary of Terms

We’ve created a list of scientific and medical terms that might be useful to help you understand your diagnosis better.


Autosomes are the “numbered” chromosomes (numbered from 1 to 22). Chromosome 20, for example, is an autosome.


Bands are the light and dark stripes that can be seen on chromosomes when they are karyotyped, or stained with special dyes and looked at under a microscope. Each chromosome pair has a distinctive pattern of bands. Each band is given a number and a letter (p or q, depending on the arm). The numbers go up as you move away from the centromere. Labelling the bands helps scientists describe the location of a specific gene or any changes to the structure of a chromosome. 


Cells are the basic building blocks of all living things. The human body is made of trillions of cells.


Centromeres are where the chromosomes appear narrow or pinched in, near the center of each chromosome. The centromere separates the two arms (p arm and q arm) on each chromosome.


Chromosomes are long string-like structures that store all of our DNA. Each cell in the human body usually has 23 pairs of chromosomes (or 46 chromosomes in total).

Chromosome microarray analysis

Chromosome microarray analysis is also known as CMA, microarray, or array. Chromosome microarray analysis can test for extra or missing genetic regions, or it can test for specific mutations (changes in the DNA code).


A deletion is a region of a chromosome that is missing.


DNA stands for deoxyribonucleic acid. This is the hereditary material in humans and most other living things. The information in DNA is stored as a code, based on the order of the 4 “building blocks” of DNA, called nucleotide bases (adenine, thymine, guanine, cytosine – the As, Ts, G, and Cs of DNA).


A duplication is a region of a chromosome that has an extra copy.


Exons are the segments of DNA that code for a protein. Together axons make up the exome


The exome is the DNA in genes that code for proteins (the exons). The exome represents only about 1.5% of our total DNA


FISH (Fluorescence In Situ Hybridization) is a special test that can detect and locate specific chromosome regions. The targeted chromosome region “lights up” if it is present, due to a fluorescent tag. FISH can be used to help scientists learn if a chromosome region is present or absent in a sample, and if it is in its proper position or not. FISH therefore can help us learn about the number and structure of certain chromosome regions, including the regions at the ends of chromosome 20, which are arranged in a different structure and may be missing if someone has r(20).


Genes provide coded instructions that tell our bodies how to develop, grow, and function. They also help determine many of our unique traits, like height and eye color. Genes are made of DNA and they are inherited from our mother and father. Each person has around 20,000 – 25,000 genes stored in our chromosomes.


The genome is the complete set of all of the genetic material


Introns are the segments of DNA that do not code for a protein. Introns used to be thought of as “junk DNA” but we now know that some introns play important roles, like turning genes “on” and “off” and controlling how much protein is made from a gene.



Karyotyping is also known as chromosome analysis or cytogenetic testing. Karyotyping is a test that examines the chromosomes under a microscope, after special preparation, in a laboratory. It is most often performed on a sample of blood, but it can also be performed on other types of samples, either before birth (from the placenta or the amniotic fluid surrounding a baby) or after birth (from muscle, skin, and other tissues). Karyotyping looks at the number, size, and structure of your chromosomes. It does not look at individual genes or the DNA code of the bases (the As, Ts, Gs, and Cs).



Monosomy is a single copy of a chromosome in a cell. Some individuals with r(20) may have a cek=ll line with just one chromosome present.


Mosaicism is the presence of more than one “cell line,” meaning that some cells in a person have a different genetic make-up than other cells. Many individuals with r(20) are mosaic (or have mosaicism). This means that the ring 20 chromosome is present in some of their cells, but not all of their cells. In general, individuals with mosaic r(20) tend to be less severely affected compared to people who have r(20) in all of their cells. The “level” of mosaicism, meaning the amount of cells with r(20) compared to the amount of cells without r(20), can be different in different parts of the body.



The nucleus is a special structure within each cell that contains and protects the chromosomes.



Nucleotide bases are the four “building blocks” of DNA: adenine, thymine, guanine, cytosine (the As, Ts, Gs, and Cs of DNA).



Autosomes are the “numbered” chromosomes (numbered from 1 to 22). Chromosome 20, for example, is an autosome.Proteins…explain what proteins do in our bodies…?

P Arm

The p arm is the “short arm” of each chromosome above the centromere. In this image, this segment is called 20p (p arm of chromosome 20).


Q Arm

The q arm is he “long arm” of each chromosome below the centromere. In this image, this segment is called 20q (q arm of chromosome 20).


Ring Chromosomes

Ring chromosomes are formed when the two ends of a chromosome become joined or “fused.” This causes the chromosome to be a circular structure (a ring) rather than a long string-like structure. 

A ring can form in this way and not have any deleted (missing) chromosomal material. All of the DNA and genes are there, they are just arranged or packaged in a different structure. A ring chromosome can also be formed when there is a “break” on one or both ends of the chromosomes, and then the ends of the chromosome become joined or “fused.” In this kind of ring, some chromosomal material is deleted or missing.



Sequencing refers to determining the order of the 4 bases in a segment of DNA (all of the As, Ts, Gs, and Cs).

Sex Chromosomes

Sex chromosomes are the “X” and “Y” chromosomes. Males usually have one X chromosome and one Y chromosome. Females usually have two X chromosomes.


A small supernumerary marker chromosome (sSMC) is an extra copy of a chromosome that is usually a small chromosome piece with an abnormal structure. For example, someone with a supernumerary ring 20 chromosome has two “normal” copies of chromosome 20, plus an additional (third) copy of chromosome 20 in a ring shape.




Telomeres are the very ends of the chromosome. Here, the arrow is pointing to the telomere on the p arm of chromosome 20.


Whole Genome Sequencing

Whole genome sequencing (WGS) is sequencing the entire genome. WGS determines the order of all of the bases in a person’s DNA (all of the As, Ts, Gs, and Cs).

Whole Exome Sequencing

Whole exome sequencing (WES) is sequencing just the exons (the DNA that gets “expressed” and codes for proteins).