Table 1.
A glossary of some genetic terms.
| Term | Definition |
|---|---|
| Alleles | One of two or more versions of a specific location on the DNA sequence. An individual has two alleles, one from each parent |
| Base | Also called nucleotide. Bases are the ‘building blocks’ of DNA. DNA consists of four bases: adenine (A), cytosine (C), guanine (G) and thymine (T). It is the sequence of these four bases that encodes information |
| Chromosome | A continuous piece of DNA that carries a collection of genes. Every cell in the human body contains 46 chromosomes |
| DNA | Deoxyribonucleic acid (DNA) contains the genetic instructions used in the development and functioning of all living organisms. The DNA segments that carry the genetic information are called genes. The double-helix structure joins two strands of DNA, where the base A binds with T, and G binds with C |
| Gene | A section on the chromosome that comprises a stretch of DNA |
| Genotype | The specific set of two alleles inherited at a particular location on the DNA sequence. If the alleles are the same, the genotype is homozygous. If different, it is heterozygous |
| Homozygous | When the two alleles at a particular locus are the same |
| Heterozygous | When the two alleles at a particular locus are different |
| Heritability | The proportion of the total variance that is explained by genetic factors. It is most commonly calculated from twin studies by comparing intra-pair correlations for the characteristic in monozygotic (MZ) with intra-pair correlation in dizygotic (DZ) twins. The heritability is of a characteristic is calculated as twice the difference between MZ and DZ intra-pair correlations (h2=2⁎(rMZ−rDZ)) |
| Linkage disequilibrium (LD) | The correlation between alleles at different loci within the population that occurs due to the co-inheritance of alleles. Alleles that are in LD are not independent of another. The extent of LD is a function of the distance between the alleles on the chromosome |
| Phenotype | An organism's observable characteristic or trait, such as its biochemical or physiological properties. Phenotypes result from the expression of genes as well as the influence of environmental factors and the interaction between the two |
| Pleiotropy | The potential for variants to have more than one phenotypic effect. If a SNP is pleiotropic, it influences multiple phenotypes |
| Polymorphism | Locations where DNA varies between individuals |
| Population stratification | The presence of a systematic difference in allele frequencies between subpopulations within a population. The most common example is population stratification due to ethnicity |
| Single-nucleotide polymorphism (SNP) | A genetic variation in which a single base/nucleotide on the DNA is altered, e.g. the nucleotide T is changed to A |