Genetic diversity

• Meiosis divides twice. In the first meiotic division the homologous chromosomes pair up and crossing over occurs (causing genetic variation). In the second division chromatids are separated. This overall results in four cells forming. (In humans they usually contain 23 chromosome/ haploid)
• Meiosis causes variation through crossing over between sister chromatids during prophase one and independent assortment or homologous chromosomes during metaphase one.
• Both crossing over and independent assortment cause different combinations of alleles and therefore genetic variation.
• Crossing over is the process by which non-sister chromatids exchange alleles during meiosis one when homologous chromosomes pair up forming a bivalent pair.
• The crossing points of non-sister chromatids when crossing over occurs is called the chiasmata.
• The entanglement when crossing over occurs puts stress on the DNA molecules causing the section of a chromatid from one chromosome to break and re-join with a chromatid from another chromosom.
• The swapping of alleles is significant as it can cause a new combination of alleles on the two chromosome.
• An allele is two or more copies of different genes which share the same genetic locus.
• Non-disjunction is when a chromosome is not separated properly resulting in one cell having no chromosome. It results from an improper seperation of chromosomes during anaphase.
• An example of a disease that results from chromosome non-disjunction is Down syndrome.
• Genetic diversity arises because of differences in what genes are present in an organisms and due to the presence of different alleles of a gene.
• Genetic diversity or variation can be caused by genetic or environmental factors such as mutations crossing over and independent assortment.
• Natural selection is the process by which organisms that are better adapted to their environment survive and reproduce in greater numbers, resulting in the increase of the frequency of the advantageous allele within the population.
• Selection pressure is a factor which determines the spread of an allele within a gene pool.
• A Gene Pool is the total sum of all the alleles of all the genes in the population.
• Stability Selection:

  -Occurs in all populations where the environment is stable

  -selective pressure at both ends of the distribution

  -favours the average

  -tends to eliminate extremes

  -reduces opportunity for evolutionary change
• Directional Selection:

  -mean population represents optimum for existing conditions

  -environmental change may produce new selection pressures that favour an extreme characteristic -over time selection means these will predominate and the mean will shift.
• Bacteria can replicate every 20 minutes so there is a high chance of mutation.
• MRSA (methicillin resistant antibiotic resistant) is one of the most famous antibiotic resistant bacteria.