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What’s a family worth? HEART UK FH GIT TOOLKIT Glossary Allele Alternative forms of a gene at the same locus (position on a chromosome). Autosomal Dominant A genetic condition with autosomal dominant inheritance occurs when only one copy of a gene, located on an autosome, is altered. The other copy is normal. When an individual with a dominantly inherited condition has a child, the child has an equal chance of inheriting the condition or not inheriting it (i.e. a 50% chance or one chance in two) depending on which gene they inherit. Autosome Any of the 22 nuclear chromosomes other than the sex chromosomes. Cascade testing Offering a genetic test to relatives of a person identified as having a gene mutation that causes a particular disease. Chromosomes Chromosomes are tightly packaged bundles of DNA, the chemical that encodes genetic information. Nearly all human cells have a set of 46 chromosomes, identified as 23 pairs; 22 pairs are autosomes with the 23rd pair the sex chromosomes. Human gametes (sperm and egg cells) have 23 chromosomes; one copy of each autosome and one sex chromosome. Each individual therefore inherits one copy of each pair of chromosomes from their mother and the other copy from their father. Consanguineous Where two people share at least one common ancestor (“blood relatives”). Degree of relationship The degree of relationship helps define the proportion of genes shared by two blood relatives. First degree relatives (parents, siblings, children) share ½ (or 50%) of their genes, Second degree relatives (uncles, aunts, nephews, nieces, grandparents, grandchildren and halfsiblings) share ¼ (or 25%) of their genes, and third degree relatives (first cousins, great-uncles, great-aunts, grandnephews, grand-nieces, great-grandparents and greatgrandchildren) share ⅛ (or 12.5%) of their genes. DNA DNA (Deoxyribonucleic acid) is the chemical that encodes genetic information. The code itself is based on four different chemicals, or bases, known as A (adenine), C (cytosine), G (guanine) and T (thymine). DNA testing This describes molecular genetic testing – testing that involves the analysis of DNA. Techniques can involve reading (sequencing) the code of the whole gene or testing for a known mutation. Family mutation This refers to the specific mutation that causes the genetic disease or condition in a family. Identification of a family mutation is often required before cascade testing can be offered if the genetic disease or condition can be caused by different mutations. What’s a family worth? 2 Glossary For example, familial hypercholesterolaemia can be caused by a mutation in one of three genes (LDLR, APOB, PCSK9), and there have been approximately 200 different mutations in LDLR reported in the UK (there is one common mutation in APOB and one common mutation in PCSK9). First-degree relative A person’s biological parent, sister, brother or child. Gene The basic biological unit of inheritance. A gene consists of a segment of DNA which produces a polypeptide chain – usually a specific protein or component of a protein. It includes coding segments (exons), intervening sequences (introns) together with regulatory elements. Human have between 20,000 – 25,000 genes. The genes are arranged in linear order on the chromosomes. Usually an individual has two copies of each gene, one inherited from each parent. Homozygous FH Refers to a person with familial hypercholesterolaemia who has inherited a gene alteration from both parents. This may be the same mutation (i.e. identical alleles), different mutations or mutations in different genes. The term homozygous FH will be used to cover all these possibilities in this toolkit. Index individual The original patient who is the starting point for follow-up of other members of a family when investigating possible genetic factors that are responsible for the presenting condition. Mutation (gene alteration) A mutation refers to any change from the normal to an altered form of a particular gene. This may be disease causing, where the mutation causes the gene or gene product to function differently or cease to function, or a benign, normal variant. Genetic condition A condition or illness caused by changes in a gene or genes, affecting the way the body looks, develops or works. Pedigree A diagram of the genetic relationships and medical history of a family, which uses standard symbols and terminology. Genetic counselling Information and support given to individuals affected by, or at a high probability of a genetic condition. An explanation of probabilities and options may include the findings of specific genetic tests. In the UK nondirective genetic counselling is the accepted practice, i.e. individuals are not told what decisions to make. Phenotype The physical and/or biochemical characteristics caused by a particular gene alteration. Genetic testing A test that gives information about genetic status – for instance whether someone has a condition or is a carrier. Usually taken to mean DNA testing, but other investigations (eg biochemical measurements or ultrasound) can also give genetic information. Proband (index case) The first affected individual studied in an investigation of several related patients with an inherited or familial disorder. Genotype The genetic constitution of an individual. Sex chromosomes The sex chromosomes determine the sex of an individual. These are XX in women and XY in men. The non-sex chromosomes (numbered 1 to 22) are known as the autosomes. Heterozygous Refers to a person who has inherited two different alleles, one from each parent, at a particular chromosome location. Heterozygous FH Refers to a person with familial hypercholesterolaemia who has inherited a gene alteration from only one parent. Homozygous Refers to a person who has inherited two identical alleles, one from each parent, at a particular chromosome location. Prevalence The total number of cases of a condition or disease in a population at a given point in time. Second-degree relative A person’s biological grandparent, grandchild, aunt, uncle, niece, nephew, half sister or half brother. Third-degree relative A person’s biological great-grandparent, great-grandchild, great-aunt, great-uncle, first cousin, grand-niece or grandnephew.