Allele: One of two or more forms of a gene, located in the same position (or locus) of homologous chromosomes.
Amino acid: The building blocks of proteins. There are 20 different kinds of amino acids found in proteins.
Central Dogma of Gene Expression: DNA carries genetic information, which is transcribed into RNA, and RNA is translated to produce a polypeptide.
Chiasma: Attachment point between chromosomes where crossing over occurs.
Chromatin: DNA in eukaryotes occurs as chromatin in the nucleus. Chromatin consists of DNA and histones packaged as nucleosomes, with linker DNA. Contraction and condensation of chromatin with other proteins occurs during mitosis and meisos.
Cloning: Vector: Serve as vehicles for the introduction of DNA into, and replication within in a new host. Usually plasmids which possess an origin of replication, selectable marker and a single recognition site for one or more restriction endonucleases.
Control segment: The upstream segment of a gene that is not transcribed, but regulates activity of the gene. Two important parts of the sequence are the control sequence (ON-OFF switch), and the RNA polymerase binding site (TATA box).
Dihybrid: An individual that is heterozygous at two loci.
Dihybrid Cross: A cross involving individuals that are heterozygous at two different loci; involving the segregation of alleles of two genes.
Diploid: Having two sets of chromosomes (2n).
DNA: A nucleic acid that is the hereditary material of an organism, stored as a coded sequence of nitrogenous bases. Comprised of two complementary double helical strands of nucleotides.
Epistasis: Interactions between two genes affecting the same character.
Gametes: Mature male and female germ cells that fuse to form the zygote.
Gene: Segments of DNA in a chromosome which are capable of transcription at specified times, in specified tissues or in response to a specific stimulus. Includes an upstream control segment, and a downstream transcribed segment.
Genetic Distance: The degree of linkage between two genes can be used to construct a genetic map. The percentage recombination between genes is used as a measure of their distance apart on the chromosome, or "genetic distance".
Genotype: The genetic make-up of an organism, that is , its specific allelic composition.
Genome: In a prokaryotic cell, the genome is the single circular DNA molecule (chromosome); in a eukaryotic cell, the nuclear genome is the chromosomal DNA in the nucleus. Chloroplast and mitochondrial genomes are circular chromosomes.
Haploid: Possessing only one set of chromosomes (n), as in egg or sperm.
Hardy-Weinberg Principle: In an infinitely large, inter-breeding population, in which there is random mating but no migration, mutation or selection, frequencies of genes and genotypes will be constant in each generation.
Heterozygous: Where an individual carries different alleles of a gene.
Heterozygote Advantage: Occurs where the phenotype of the heterozygote is fitter in the Darwinian sense than the phenotypes of either homozygote.
Homologous Chromosomes: In dipoid cells, one chromosome of a pair that associate at the first stage of meiosis; carry equivalent genes.
Homozygous: Where an individual carries two copies of the same allele of a gene.
Histone: Basic proteins which associate with DNA to form nucleosomes in chromatin.
Linked genes: Linked genes usually occur close together on a chromosome, thus they will not show independent assortment. For linked genes, the parental arrangements of alleles will occur with greater frequency then the recombinant arrangement of alleles.
Locus: Is the position of a gene on a chromosome. May be occupied by any of the alleles of a gene.
Meiosis: A type of nuclear division that takes place in germ cells and that results in halving of the number of chromosomes; each daughter cell (gamete) receives on set of chromosomes.
Mutation: Change of the genotype. A point mutation is a change in the base sequence of DNA (base insertion, deletion or substitution). A chromosome mutation is a change causing breakage and rejoining of chromosomes.
Nucleotide: The building blocks of nucleic acids. Comprises a five carbon sugar, a phosphate group and a nitrogenous base. Nucleotides link together by a phosphodiester bond between the sugar and the phosphate group to form nucleic acids.
Nucleosome: A beadlike structure in eukaryotic chromosomes. Comprises 8 histone proteins with associated DNA (about 146 BP) wound around them.
Operon: In bacteria, transcription units that cover >1 polypeptide coding region.
Phenotype: The detectable properties or traits of an organism as a result of both the genotype and the environment.
Polymorphism: The presence in a population or more than one allele of an individual gene at higher levels than could be maintained by mutation alone.
Polypeptide: A molecule composed of a chain of amino acid residues.
Principle of Independent Assortment: Alternative forms of a gene controlling one trait assort into gametes independently of alternative forms of another gene controlling a different trait.
Principle of Segregation: Alternative forms of an inherited trait, controlled by alternative forms of a gene (alleles), segregate into gametes after meiosis with equal probability.
Promoter: Specific sequence of DNA that binds RNA polymerase, promoting transcription of the coding region by the enzyme.
Protein: See polypeptide. Proteins are structurally varied and perform a wide range of functions: enzymes, structure, movement , storage, hormones etc.
Purine: Double ring molecule, forms 2 of the bases of nucleic acids: guanine and adenine.
Pyrimidine: Single ring molecule, forms 3 of the bases of nucleic acids: cytosine, thymine and uracil.
Quantitative characters: Character of an organism, such as height or weight, that shows continuous variation and is controlled by both environment and polygenes.
Recombination: The realignment of gene alleles caused by crossing-over.
Recombinant DNA: Technology involving cutting DNA into fragments using restriction enzymes, isolating the fragments, identifying genes and multiplying them in bacteria, where the gene products can be expressed.
Restriction Endonuclease: A site-specific enzyme that recognises short sequences of DNA and cleaves the double strands either at the recognition site or nearby.
Ribonucleotide: Nucleotide that forms RNA. The five carbon sugar is ribose.
RNA: Ribonucleic acid. Single-stranded nucleic acid characterised by a ribose sugar in each nucleotide and the bases adenine, cytosine, thyamine and uracil; involved in transcribing and translating coded information of DNA during the production of proteins.
Selection: Differential survival or reproductive success of different genotypes.
Selfish DNA: A population is composed of individual organisms which are transitory aggregations of immortal genes. The selfish DNA concept regards individuals and populations as mere vehicles for the transmission of genes.
TATA box: The sequence of bases in the control segment of a gene where RNA polymerase first binds to begin transcription.
Test-cross: A cross between the F1 progeny and the pure breeding, recessive phenotype parent. Note that when two different genes (with two different alleles) are inherited independently, the traits segregate in a 1:1:1:1 ratio when a test-cross is performed.
Trait: An observable phenotypic feature.
Transcription: Process involving copying one strand (the template strand) of DNA to produce a complementary strand of RNA.
Translation: Process in which information stored in the sequence of mRNA specifies the amino acid sequence of poypeptide.