topic 7 - Genetics, populations, evolution and ecosystems

• Gene


  Section of DNA on a chromosome that controls a feature by coding for formation of one or more specific polypeptides or a functional RNA (including rRNA and tRNA).
• Locus


  The specific linear position of a particular gene on a certain chromosome
• Alleles


  Alternative forms of a particular gene with different base sequences, and therefore different codes
• Homozygous


  Situation or organism in which paired alleles are the same
• Heterozygous


  Situation of organism in which paired alleles are different
• Dominant


  An allele that expresses itself in the phenotype in heterozygous organisms
• Recessive


  An allele that does not express itself in the phenotype in heterozygous organisms
• Co-dominant


  Alleles which both contribute to the phenotype (i.e. produce a blended effect) in the heterozygous condition
• Multiple alleles


  If there are more than two alleles for each gene in the gene pool. NB. Only 2 chromosomes in a homologous pair so only 2 of the three or more alleles can be present in a single organism
• Pure-breeding strain


  Group of organisms in which a certain characteristic is unaltered for generations, indicating that organisms are homozygous for that feature
• Monohybrid


  Inheritance of a single pair of alleles
• Dihybrid


  Inheritance of two pairs of alleles
• Haploid (n)


  Cell or nucleus containing single, unpaired chromosomes
• Diploid (2n)


  Cell or nucleus containing pairs of homologous chromosomes
• Phenotype


  The observable or biochemical characteristics of an organism, resulting from both its genotype and the effects of the environment
• Genotype


  The combination of alleles present within the cells of an organism
• Heritable


  Features determined by genes (rather than the environment) and therefore passed on to offspring
• Homologous chromosomes


  A pair of chromosomes, one maternal and one paternal, that have the same gene loci and therefore determine the same features. They are not necessarily identical as may have different alleles. They are capable of pairing during meiosis.
• Ratio


  A measure of the relative size of two classes that is expressed as a proportion.
• Sex linked


  Any gene that is carried on either the X or Y chromosome.
• Autosomal linkage


  The situation where two or more genes are carried on the same autosome.
• Autosome


  A chromosome which is not a sex chromosome
• Epistasis


  This arises when the allele of one gene affects or masks the expression of another in the phenotype.
• Hardy Weinberg Principle


  The principle predicts that the proportion of dominant and recessive alleles of any gene in a population remains the same from one generation to next provided five conditions: No mutations, population is isolated, no natural selection (alleles are equally likely to be passed on), large population, mating is random
• Gene Pool


  all the different alleles of all the genes of all the individuals in a population at any one time
• Allelic frequency


  the number of times an allele occurs within the gene pool in a population, relative to all others at same locus
• Discontinuous variation


  Distinct forms with no intermediate types.
• Continuous variation


  No distinct categories - likely to obtain a bell-shaped curve known as a normal distribution curve.
• Natural selection


  Darwin's theory to explain the mechanism of evolution. The process by which organisms better adapted to their environment survive and reproduce and pass on their advantageous alleles to the their offspring, whilst those less well adapted fail to do so.
• Directional selection


  Favours one extreme of the range of characteristics and the other extreme is selected against - shift in population curve
• Stabilising selection


  Favours the mean of the distribution because the extremes are at a selective disadvantage - frequency of mean phenotype increases
• Disruptive selection


  Favours both extremes of a distribution, selection occurs against the mean - results in bimodal distribution.
• Genetic Drift


  Causes random changes in allele frequencies in a population due solely to chance factors.
• Genetic bottleneck


  Severe reduction in population size
• Founder effect


  Isolation of a small group of individuals by migration or a physical barrier.
• Speciation


  Evolution of new species from existing ones. Process by which reproductive isolation occurs between two populations so that they evolve along their own separate paths into 2 separate species with different allele frequencies.
• Species


  A group of organisms that have a common ancestry and so share the same genes and are capable of breeding together to produce fertile offspring - are reproductively separated from other species.
• Allopatric speciation


  occurs when populations occupy different environments - they are geographically separated.
• Sympatric speciation


  occurs when populations are reproductively isolated within the same environment.
• Ecosystem


  Self-contained unit made up of the biotic and abiotic factors in an area