OCR A Biology - key definitions

• Allele


  A version of a gene
• Allopatric speciation


  A form of speciation that occurs when two populations become geographically isolated due to a physical barrier
• Apoptosis


  Programmed cell death. This is important for pruning surplus cells and tissues in development
• Artificial selection


  See 'selective breeding'
• Autosomal linkage


  When two or more genes are positioned on the same autosome. They are unlikely to be separated by crossing over during meiosis so are often inherited together
• Autosome


  A chromosome that is not an X or Y chromosome
• Bioinformatics


  The development of the computer tools and software required to organise and analyse unprocessed biological data
• Chi-squared (X2) test


  A statistical test used to determine whether a pattern of inheritance is statistically significant
• Chlorosis


  A condition in which plant leaf cells produce insufficient chlorophyll, resulting in pale or yellow coloured leaves. This may be due to mineral deficiencies, lack of light, or viral infections
• Codominance


  When both alleles for a gene in a heterozygous organism equally contribute to the phenotype
• Computational biology


  The use of computational techniques to analyse large amounts of biodata and build theoretical models of biological systems
• Continuous variation


  A type of variation that cannot be categorised e.g. skin colour, height. It produces a continuous range in which a characteristic can take any value. Multiple genes influence continuous variation
• Degrees of freedom (X2 test)

  The number of categories minus one
• Deletion


  A form of gene mutation in which one or more nucleotide bases are removed from a DNA sequence. This may lead to a frameshift mutation, changing every successive codon
• Dihybrid inheritance


  The determination of a trait by the inheritance of two genes
• Directional selection


  A type of selection that favours one extreme phenotype and selects against all other phenotypes
• Discontinuous variation


  A type of variation that can be categorised e.g. blood group. A characteristic can only appear in discrete values. One or two genes influence discontinuous variation
• Disruptive selection


  A type of selection that favours individuals with extreme phenotypes and selects against those with phenotypes close to the mean
• DNA barcode


  A short sequence of DNA that is used to identify a species. DNA barcodes are common to all species but vary between species
• DNA ligase


  An enzyme that joins the sugar-phosphate backbone of two DNA segments
• DNA profiling


  A technique used to determine the patterns in the non-coding DNA of an individual. It involves five main stages: DNA extraction; digestion; separation of DNA fragments; hybridisation; and observation
• DNA sequencing


  Determining the entire DNA nucleotide base sequence of an organism
• Dominant


  Describes an allele that is always expressed. Represented by a capital letter
• Electrophoresis


  A type of chromatography that separates nucleic acid fragments or proteins by size using electric current
• Electroporation


  A method of transformation in which a small electric current is used to transfer recombinant plasmids into bacterial cells or fragments of DNA into eukaryotic cells
• Epistasis


  Describes a relationship between genes at different loci, where the allele of one gene affects the expression of a different gene
• Etiolation


  A condition in plants characterised by weak stems and small, pale leaves, due to insufficient exposure to light
• Evolution


  The gradual change in the allele frequencies within a population over time. Occurs due to natural selection
• Exon


  A sequence of DNA that codes for an amino acid sequence
• Founder effect


  A type of genetic drift in which a few individuals of a species break off from the population and form a new colony. This results in smaller gene pools and an increased frequency of rare alleles
• Gene mutation


  A change to at least one nucleotide base in DNA or the arrangement of bases. Gene mutations can occur spontaneously during DNA replication and may be beneficial, damaging, or neutral
• Gene therapy


  A therapeutic technique in which a faulty allele is replaced with a functional allele in order to treat or prevent disease
• Genetically modified organism (GMO)


  An organism that has had its genome altered
• Genetic bottleneck


  A drastic reduction in population size leading to reduced genetic diversity within a population
• Genetic drift


  Random variations in allele frequencies in small populations, due to mutations
• Genetic engineering


  The modification of the genome of an organism by the insertion of a desired gene from another organism. This enables the formation of organisms with beneficial characteristics
• Genome


  The complete genetic material of an organism
• Genotype


  An organism's genetic composition. Describes all alleles
• Germ line cell gene therapy


  A type of gene therapy in which a faulty allele is replaced with a functional allele in germ cells or a very early embryo. The effects of this are permanent and can be inherited
• Hardy-Weinberg principle


  A model that predicts that the ratio of dominant and recessive alleles in a population will remain constant between generations if the following five conditions are met: no new mutations; no natural selection; no migration; large population; and random mating. It provides a formula for calculating the frequencies of alleles: p2 + 2pq + q2 = 1.0 where p is the frequency of the dominant allele, and q is the frequency of the recessive allele