Education

Glossary

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A

Allele

One of two or more forms of a particular gene at a specific position on a chromosome. For example, blue and brown eyes in human beings are determined by different alleles of the eye colour gene.

Allelic Heterogeneity

Different mutations in the same gene lead to the same disease phenotype. In other words, deletions and point mutations produce the same end result. This is true for Duchenne Muscular Dystrophy, where 70% of mutations are due to large deletions in the dystrophin gene but the rest of the mutations are a mixed bag. Interestingly, the current genetic testing only finds the deletions; so even if the testing is negative, other mutations can still exist. In this case, a linkage analysis has to be done to determine if you’re at risk for a mutated dystrophin gene.

Alternative Splicing

The process where the initial strand of RNA copied directly from DNA is cut up (spliced) into different mRNA strands without introns. Basically, different combinations of exons are joined to create different proteins from the same initial RNA (and same DNA sequence). Thus, one gene can be the source of several proteins that differ in function, timing or place of action. For example, alternative splicing can make one isoform ‘brain-specific’ and another ‘muscle-specific’. It is estimated that at least half of the genes in the human genome undergo alternative splicing, which may help explain why our genome has fewer genes that originally expected.

Amino Acid

Amino acids are building blocks for proteins. Composed of carbon, hydrogen and nitrogen, these organic molecules all contain a basic amino group (NH2) and an acidic carboxyl group (COOH). Hence the truly logical moniker, amino acid. To keep on living and loving, we humans need 20 of them. Our bodies make 12 and we get the other 8 (the 8 essential amino acids) from our food.

Amniocentesis

Amniocentesis is a prenatal procedure that examines the chromosomes of fetal cells. It involves removing a small amount of fluid from around the developing baby (amniotic fluid) with a needle through the woman’s abdomen into the uterus. The amniotic fluid has some of the baby’s cells that can undergo various lab tests. It is commonly used to diagnose chromosome conditions (like Down syndrome) and to diagnose genetic conditions (like cystic fibrosis) in the pregnancy. It can be done at 15 weeks and later in a pregnancy. There is about a 1 in 200 risk of miscarriage associated with this procedure.

Amniotic Fluid

The fluid surrounding the fetus in the uterus. It is mostly water but also has cells sloughed off from the fetus, secretions from the placenta and fetal urine. These fetal cells can be examined for chromosomal and genetic disorders.

Apoptosis

The normal method for disposing of unwanted cells. Otherwise known as programmed cell death, apoptosis is a deliberate ‘suicide’ where a cell dies in an organized way and no harmful bits are released. The cell shrinks, its contents dissolve, its DNA is fragmented and the neighboring cells take up (phagocytose) the leftovers. Apoptosis can be triggered by damage and aging, and it’s also normal in fetal development. The cells between our fingers and toes, for example, undergo apoptosis so we are not born with webbed hands and feet. Handy. (Note: The terms 'programmed cell death' and apoptosis are used interchangeably; but technically speaking, ‘pcd’ refers to the whole process, while apoptosis refers to the structural changes the cell undergoes while dying.)

Autoimmune Disease

In autoimmune diseases, the immune system has been misdirected to attack the body’s own cells and tissues. In rheumatoid arthritis, for example, autoimmune response causes damage in the joints; and an autoimmune response in the gut accompanies celiac disease. Although the reason why the response happens in the first place is unknown, it appears that some people may have a genetic predisposition to autoimmune disorders.

Autosome

A fancy word for the chromosomes other than the sex chromosomes. We human beings have 22 pairs of autosomes (chromosomes 1-22) and two sex chromosomes, two Xs if you’re lucky enough to be female and XY if you’re equally lucky enough to be male.

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B

Backcross

If you want to increase the frequency of a characteristic, you can cross (mate) a hybrid back to a parent or the genetic equivalent of a parent. This is what a backcross is. The term is also used to describe the two step process of an outcross (to get the hybrid) and then a backcross.

Base Pair

A base is a basic (duh) component of a nucleic acid. Bases come in two basic versions: pyrimidines and purines. DNA contains the pyrimidines thymine and cytosine plus the purines adenine and guanine. Attached like snap fasteners, these bases form base pairs along the length of the DNA helix. Adenine (A) pairs with thymine (T), while guanine (G) pairs with cytosine (C). In RNA uracil replaces thymine in the pairing with adenine.

Bioinformatics

Bioinformatics is the science of managing and analyzing biological information, usually with advanced computing techniques.

BLAST

BLAST stands for Basic Local Alignment Search Tool. (Blast! Yet another acronym!) Anyway, BLAST is a computer program that compares sequences and calculates the statistical significance of matches. When you enter your query sequence, the program searches public biological sequence data bases for sequence similarities. The program has several versions, two of them being N (a nucleotide search) and P (a protein search). Although we can’t list all the ways researchers use this search tool, we can say that, in general, researchers use BLAST to find out more information about a sequence. If a BLAST search matches your sequence, you will have a portal into all the available information about it, including various scientific databases, scientific publications and expression and protein information. If your sequence is not available in the organism you’re interested in, there’s a good chance it will match a sequence in a different organism. Because you can gain information about the homologous gene, BLAST is still valuable to you. Plus, if your sequence is a member of a gene family, you could gain information about its possible function, as a result of the motifs it shares with its family members.

BRCA1 and BRCA2

BReast CAncer 1 and 2 genes are examples of tumour suppressors that normally help to restrain cell growth. When these genes are mutated and inactivated, cells can grow too much and cancer develops. Someone who has inherited a mutation in BRCA1/BRCA2 has an increased risk of breast (for both women and men), ovarian or prostate cancer.

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C

Candidate gene

A gene located in the chromosome region suspected to cause or contribute to a disease. The gene’s function and place of action should suggest a role in the disease in question. For example, the gene coding for a protein that works in the brain and regulates brain transmitters is a good candidate for depression.

Cell

The smallest, membrane-surrounded dollop of diversified matter capable of independent existence. We humans are walking, talking communities made up of billions of them. With the exception of viruses and prions, all living organisms are made up of one or more of these little powerhouses.

Centromere

When peered at through a microscope, the centromere looks like a cinched-in waist in the chromosome. During cell division, the centromere determines how chromosomes pair up and divide. It is, in fact, the “nerve centre” of the chromosome, directing cell movement during meiosis and mitosis. If the centromere is lost (i.e. chromosome breaks), that portion of DNA without a centromere is lost because it can’t find its way during cell division. Occasionally, a DNA fragment without a centromere can form a new centromere (a neo-centromere). This fragment can survive cell divisions.

Chorionic Villus Sampling

Chorionic villus sampling (CVS) is a prenatal procedure that examines the chromosomes of fetal cells. Fetal cells from the fetal side of the placenta (chorionic villi) are extracted and analyzed for chromosomal and biochemical defects by inserting a needle through the women’s abdomen or vaginally. The sample of tissue (chorionic villi) is sent to the laboratory where various tests can be done. CVS is commonly used to diagnose chromosome conditions (like Down syndrome) and to diagnose genetic conditions (like cystic fibrosis) in the pregnancy. It is typically done at around 12 weeks in the pregnancy. There is about a 1 in 100 risk of miscarriage associated with the procedure.

Chromosome

The structure in the cell's nucleus carrying the genes. Each human being has 46 (23 pairs), twenty-three from dad and the other 23 from mom. Males have pairs of chromosomes 1-22 plus an X and a Y chromosome. Females likewise have pairs of chromosomes 1-22 plus two X chromosomes. Each chromosome is one extremely long twisted and folded up strand of DNA.

Clone

An identical copy of an organism. Identical twins have the same nuclear DNA, so they are clones of each other. The same egg and sperm combination has produced two babies during development. Poetically speaking, clones are Identical in every way, they are One in DNA. When gardeners take cuttings from a plant, they are producing clones because the cuttings grow into plants that are genetically identical to the plant they were cut from. See Somatic Cell Nuclear Transfer for a more modern technique.