4. Genetics, Variation & Interdependence: Key Terms

• Adaptation**:** A characteristic that enhances an organism's survival in its environment. Adaptations can be anatomical, physiological, or behavioural.

• Allele**:** A variant form of a gene.
• Anticodon**:** A set of three nucleotide bases on a tRNA molecule that pairs with a complementary mRNA codon.

• Arithmetic mean**:** The average value of a dataset, found by dividing the sum of all values by their count.
• Artificial classification**:** Categorising organisms based on observable traits like leaf shape or leg number, often ignoring evolutionary relationships.
• Binomial system**:** A universal naming system for organisms, using two parts: the genus (e.g., Homo) and the species (e.g., sapiens).
• Biodiversity**:** The variety of genes, species, and habitats within a specific region.
• Cellular proteome**:** The full range of proteins expressed in a specific type of cell.
• Chromatid**:** One of two identical strands of a replicated chromosome.
• Chromosome**:** A structure made of DNA and proteins that carries genetic information.
• Chromosome mutation**:** A change in the structure or number of chromosomes, occurring spontaneously.
• Classification**:** The grouping of organisms into categories. It can be artificial or based on evolutionary relationships (phylogenetic).
• Codon**:** A triplet of bases on mRNA that codes for a specific amino acid.
• Conservation**:** The protection and management of ecosystems and biodiversity to sustain natural resources.
• Courtship**:** Behavioural patterns that enable species to select mates, ensure breeding readiness, and strengthen pair bonds.
• Crossing over**:** The exchange of genetic material between homologous chromatids during meiosis, generating genetic variation.
• Degenerate**:** A property of the genetic code where multiple codons can specify the same amino acid.
• Deletion**:** A gene mutation where nucleotide bases are removed, potentially altering the entire amino acid sequence.
• Directional selection**:** A form of natural selection favouring traits at one extreme, shifting population characteristics.
• Ecosystem diversity**:** The variety of habitats within a given area.
• Eukaryotic DNA**:** DNA in eukaryotic cells that is linear, associates with histones, and forms chromosomes; mitochondrial and chloroplast DNA is circular.
• Exon**:** A DNA sequence that codes for amino acids in proteins.
• Fertilisation**:** The random fusion of haploid gametes to form a diploid zygote, introducing genetic variation.
• Gene**:** A segment of DNA coding for a protein or functional RNA.
• Gene mutation**:** A change in a DNA base or its sequence, occurring during replication.
• Generic name**:** The first part of a species' scientific name, representing its genus (e.g., Homo).
• Genetic code**:** The set of rules where DNA triplets determine the amino acid sequence in proteins. It is universal, non-overlapping, and degenerate.
• Genetic diversity**:** The variety of alleles in a population, measurable by comparing characteristics, DNA, or protein sequences.
• Genome**:** The complete set of genes in a cell.
• Histones**:** Proteins that package and organise DNA into chromosomes in eukaryotic cells.
• Homologous chromosomes**:** A pair of chromosomes, one from each parent, containing the same genes but potentially different alleles.
• Independent segregation**:** The random separation of homologous chromosomes during meiosis, producing genetic variation.
• Index of diversity**:** A measure of species diversity in a community, calculated as:

where $N$ is the total number of organisms and n is the number of individuals of each species.

$$d = \frac{N(N-1)}{\sum n(n-1)}$$

• Intron**:** A non-coding DNA sequence.
• Locus**:** The specific location of a gene on a chromosome.
• Mean**:** A measure of central tendency, represented by the peak of a normal distribution curve.
• Meiosis**:** A type of cell division that produces four genetically unique haploid gametes through two divisions.
• Messenger RNA (mRNA)****: A single-stranded RNA molecule that carries genetic information from DNA to ribosomes for protein synthesis.
• Mitosis**:** A process of cell division producing two genetically identical diploid cells.
• Mutagenic agent**:** Any factor that increases the likelihood of mutations, such as radiation or chemicals.
• Natural selection**:** The process where advantageous alleles become more common in a population over time.
• Non-coding sequence**:** DNA regions that do not code for proteins, such as introns and repetitive sequences.
• Non-disjunction**:** The failure of homologous chromosomes to separate during meiosis, resulting in an abnormal chromosome number.
• Non-overlapping**:** A feature of the genetic code where each base is part of only one triplet.
• Phylogenetic classification**:** Organising organisms based on evolutionary relationships, using a hierarchical system.
• Phylogeny**:** The evolutionary history and relationships among species or groups.
• Prokaryotic DNA**:** Circular DNA not associated with proteins, found in prokaryotic cells.
• Random sampling**:** A method of sampling that avoids bias, such as using random coordinates in a grid.
• Recombination**:** The exchange of genetic material between chromatids during crossing over.
• Ribosomes**:** Cellular structures where proteins are synthesised, composed of a large and small subunit.
• RNA polymerase**:** The enzyme that synthesises RNA by joining nucleotides along a DNA template strand.
• Species**:** A group of organisms capable of interbreeding to produce fertile offspring.
• Species diversity**:** The variety and abundance of species within a community.
• Species richness**:** The number of different species in a community.
• Specific name**:** The second part of an organism's scientific name, identifying its species (e.g., sapiens).
• Splicing**:** The removal of introns and joining of exons in eukaryotic pre-mRNA to form mature mRNA.
• Stabilising selection**:** A type of natural selection favouring traits close to the mean, reducing variation.
• Standard deviation**:** A statistical measure of data spread around the mean, shown as the curve's width in a normal distribution.
• Substitution**:** A gene mutation where one base is replaced by another, possibly altering the amino acid sequence.
• Taxon**:** A group within a phylogenetic classification hierarchy.
• Transcription**:** The synthesis of RNA from a DNA template during the first stage of protein synthesis.
• Transfer RNA (tRNA)****: RNA molecules that transport specific amino acids to ribosomes during protein synthesis.
• Translation**:** The second phase of protein synthesis where ribosomes use mRNA as a template to assemble amino acids into a polypeptide chain.
• Triplet**:** A set of three DNA or RNA bases that specifies an amino acid.
• Universal**:** A feature of the genetic code; it is the same across almost all organisms, supporting the theory of evolution.
• Variation**:** Differences among individuals due to genetic or environmental factors.