Genetic Glossary  

Allele: a gene can exist in different forms as there are variations in the DNA. Innocent variations are called “polymorphisms”, disease-causing variations are called “mutations”.Amino Acids: amino acids are the building blocks of proteins. There are 20 different amino acids. Each amino acid is encoded by a codon in the DNA.

Base: a base is a building block of DNA that forms the genetic code. There are 3 billion bases in human DNA, but only 4 different kinds of bases: adenine (A), thymine (T), guanine (G), and cytosine (C). DNA exists as a double helix where the bases pair: A always with T, and G always with C, hence the terminology base pairs.

Candidate gene: A gene suspected of being involved in a disease.

Carrier: An individual who carries 1 mutant allele ( mutation) that causes disease only when 2 mutant alleles are present. Although carriers are not affected by the disease,  carriers can produce a child who has the disease, and their children can also be carriers.

Cell: The basic subunit of any living organism.

Chromosome: A long wire found in the cell nucleus that contains genes. Chromosomes can be classified according to size and form, and numbered of large to small. These classified chromosomes form a karyotype. In each cell we have 2 copies of each chromosome, of which one originates from our father and one from our mother. So there are 23 chromosome pairs, and the total sum of chromosomes is 46.

Cloning: DNA technology to produce multiple, exact copies of a single gene or other segment of DNA.

Codon: A sequence of 3 consecutive bases which encodes for an amino acids. There arte 64 different codons in DNA.

Congenital: Any trait or condition that exists from birth on.

Deoxyribonucleic Acid (DNA): The genetic material; human DNA is composed of about 3 billion nucleotides, which consist of sugar, phosphate, and a base.

Dominant: A disease-causing mutation can be either “dominant” or “recessive” : a dominant mutation results in a specific physical characteristic or disease even if the second allele of that gene is normal. With a dominant gene, the risk of passing on the mutation, which may cause a condition or disease, to children is 50 % in each pregnancy.

Enzyme: A protein that facilitates a biochemical reaction in the cells, usually speeding it up.

Gene: Genes are separate units of DNA, that usually encode one or more specific proteins. Humans have about 25.000 genes.

Genetic counselling: a professional advise given by a genetic counseller about the risk of occurrence of a birth defect or a genetic disorder.

Genome: All the DNA contained in an organism or a cell, which includes both the DNA located on chromosomes within the nucleus (Nuclear genome), and the DNA in mitochondria (mitochondrial genome).

Genotype: The genetic identity or an individual that does not show ash outward characteristics.

Heterozygous: We are heterozygous for a mutation or polymorphism in a particular gene if we have two different alleles (one with and one without the mutation or polymorphism), one inherited from each parent.

Homozygous: We are homozygous for a mutation or polymorphism in a particular gene if we have two identical alleles of that given gene, one inherited from each parent.

Inherited: Transmitted through DNA from parents to offspring.

Metabolism: The process by which the body builds simple molecules into complex molecules and vice versa.

Mitochondrial DNA: The mitochondria are small cell structures that contain mitochondrioal DNA.

Mutation: A structural alteration in DNA that can cause disease.
Nucleotide: Building blocks of DNA. A nucleotide consists of sugar, phosphate, and a base.

Nucleus: The central cell structure that contains the chromosomes.

Oncogene: A gene that can cause the transformation of normal cells into cancer cells.

Phenotype: The observable traits or characteristics of an organism, for example, hair color, weight, or the presence or absence or a disease.

Polymorphism: A variation in the DNA sequence that does not cause a disease. As there exist many variations  in DNA each individual has a unique genetic make up.

Protein: A molecule made up of a number or amino acids arranged in a specific order determined by the genetic code. Proteins are essential for all life processes.

Recessive: A disease-causing mutation can be either “dominant” or “recessive” : a recessive mutation results in a specific physical characteristic or disease if there is a disease-causing mutation in both alleles of that gene (autosomal recessive diseases). One recessive disease-causing mutation in an X-linked gene can cause disease in males as they only have one allele of an X-linked gene (X-linked recessive diseases).

Sex Chromosome: One or the two chromosomes that specify an organism's genetic sex. Two kinds of sex chromosomes exist, one called X chromosome and the other the Y chromosome. A woman has two X-chromosomes and no Y chromosome, because she gets an X chromosome from both her father and her mother. A man has two different sex chromosomes: an X chromosome that he inherits from his, and a Y chromosome  which comes from its father. A man can make therefore both sperm cells warrants with an X chromosome, and an Y chromosome. A woman has only eggs with an X chromosome. If a sperm cell fertilising the egg (which always contain an X chromosome) contains an Y chromosome the resulting child will be a boy (XY). If a sperm cell contains an X chromosome, a girl (XX) will develop.

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