Topic 18 - Populations and evolution

• A population is a group of organisms of the same species that occupies particular space at a particular time and that can potentially interbreed.
• All the alleles of all the genes of all the individuals in a population at a given time is known as the gene pool.
• The number of times an allele occurs within the gene pool is referred to as the allelic frequency.
• Alleles can be dominant or recessive.
• Cystic fibrosis is a disease in humans in which abnormally thick mucus is produced. The gene has a dominant allele (F) that leads to normal mucus production, and a recessive allele (f) that leads to the production of thicker mucus.
• There are three possible combinations of the alleles of the cystic fibrosis : homozygous dominant (FF), homozygous recessive (ff) and heterozygous (Ff).
• The Hardy-Weinburg principle provides a mathematical equation that can be used to calculate the frequencies of the alleles of a particular gene in a population.
• The Hardy-Weinburg principle assumes that the proportion of dominant and recessive alleles of any gene in a population remains the same across multiple generations.
• The Hardy-Weinburg principle can be used, provided that five conditions are met : no mutations arise, the population is isolated (no allele flow), no selection (equal likelihood of alleles being passed on), the population is large, and that mating within the population is random.
• Variation in phenotypes is a result of both genetic and environmental factors. Further genetic variation can arise from meiosis and the random fertilization of gametes.
• Within a population, all members have the same genes, but variation occurs since they have different alleles.
• Genetic variation occurs through meiosis (changes in genes/chromosomes that could be passed onto the next generation), meiosis (new combinations of alleles are produced before they become gametes), and random fertilization of gametes (new allele combinations and different offspring due to sexual reproduction).
• The environment can affect the phenotype of an organism or plant (e.g. climate conditions such as temperature, rainfall and sunlight).
• Polygenes are characteristics controlled by more than one gene.
• Individuals who are genetically predetermined to be the same height actually grow to different heights. This type of variation is caused by the product of polygenes and the environment.
• A bell-shaped curve on a graph is known as a normal distribution curve.