Sanja Franic, VU University Amsterdam 2010
Genetics: basic terms and concepts
The human genome consists of DNA (deoxyribonucleic acid), a polymeric molecule
comprised of a chain of monomeric subunits termed nucleotides. The bulk of the human
genome - around 3,200,000,000 DNA nucleotides - is contained in the cell’s nucleus. The
nucleotides are organized into 24 types of linear molecules, each contained in a different
chromosome (22 autosomes, i.e., non-sex chromosomes, and 2 sex chromosomes). Vast
majority of cells are diploid, i.e., they contain two copies of each autosome and two sex
chromosomes (XX for females and XY for males), 46 chromosomes in total. These are
somatic cells, in contrast to haploid sex cells or gametes, which contain a single copy of each
autosome and a single sex chromosome.
A gene is traditionally thought of as a sequence of DNA nucleotides coding for an
RNA and/or polypeptide molecule (ref);1 for the current purpose we will adopt the definition
of a gene as a region of the genomic sequence corresponding to a unit of inheritance. Genes
may correspond to regulatory regions, transcribing regions and/or other functional sequence
regions typically associated with the production of proteins; via this association, genes
influence observable traits, also denoted phenotypes. Locus is the site of the gene on the
chromosome. Alternative forms of a gene occupying a locus are denoted alleles. Genes with
only one allele present in the population are termed monomorphic; those with two or more
alleles are termed polymorphic (or segregating) genes. Despite the large number of alleles
associated with certain genes (e.g., the HLA-B gene has over 400 alleles), the descriptions
that follow will be illustrated using the simplest instance, i.e., diallelic gene (gene with only 2
alleles).
1
It should, however, be mentioned that the definition of a gene is becoming increasingly replete with
controversy; see e.g. Pearson, 2006.
1
Sanja Franic, VU University Amsterdam 2010
In diploid cells, each chromosomal locus consists of a pair of alleles, each allele
contained in one of the chromosomes of a chromosomal pair. The two corresponding alleles
constitute the genotype for that particular locus. For instance, denoting alleles at a single
biallelic locus A1 and A2, the possible genotypes for this locus are A1A1, A1A2, and A2A2.
Considering two biallelic loci, the possible genotypes are A1A1B1B1, A1A1B1B2, A1A1B2B2,
A1A2B1B1, A1A2B1B2, A1A2B2B2, A2A2B1B1, A2A2B1B2, and A2A2B2B2. Homozygosity refers
to the presence of identical alleles at both corresponding loci of a chromosomal pair; e.g.
A1A1 for a single locus, or A1A1B2B2 for two loci. Heterozygosity denotes the opposite (e.g.,
A1A2 or A1A2B1B2). Polygenicity is the phenomenon of a trait being affected by genes at
multiple loci. Pleiotropy is the phenomenon of a gene affecting the expression of more than
one trait.
2