6.2.1 - Genetic Variation

• The appearance of a living organism, its phenotype, is influenced by both its genotype and its environment.
• Continuous Variation - Individuals in a population vary within a range and there are no distinct categories such as height or fur length.
• Discontinuous Variation - Two or more distinct categories - each individual falls into one, and there are no intermediates, such as blood type or flower colour.
• Mutations have contributed to the process of evolution. A mutation is a change to the genetic material and may involive changes to DNA structure or number/gross structure of chromosomes.
• Certain physical and chemical agents described as mutagens can increase the rate of mutation

  Physical - X-Rays, Gamma Rays, UV light

  Chemical - Mustard gas, nitrous acid, benzopyrene, free radicals

  Biological - Viruses, transposons (jumping genes), food contaminants.
• Mutations may be harmful, advantageous or neutral and mutations that occur during gamete formation are:

  Persistent - they can be transmitted throughout many generations without change

  Random - They are not directed by a need on the part of the organism in which they occur
• Chromosome mutations may occur during meiosis.
• Deletion - Part of a chromosome, containing genes and regulatory sequences is lost.
• Inversion - A section of a chromosome may break off, turn 180 degrees and join again. Although all the genes are still present, some may be too far away from regulatory sequences to be expressed.
• Translocation - A piece of one chromosome breaks off and becomes attached to another chromosome. This may also interfere with regulation of genes on the translocated chromosome.
• Duplication - A piece of chromosome may be duplicated. Overexpression of genes could be harmful as too many of certain proteins or gene - regulating nucleic acids may disrupt metabolism.
• Non - Disjunction - One pair of chromosomes or chromatids fails to separate, leaving one gamete with an extra chromosome. When fertilised by a normal haploid gamete, the resulting zygote has one extra chromosome and cause Down Syndrome or Trisomy 21.
• Aneuploidy - The chromosome number is not an exact multiple of the haploid number for that organism. Sometimes, the chromosomes/chromatids fail to separate during meiosis.
• Polyploidy - If a diploid gamete is fertilised by a haploid gamete, the resulting zygote will be a triploid. The fusion of two diploid gametes can make a tetraploid zygote, and many cultivated plants are polyploid.
• Sexual reproduction is not an example of mutations, but genetic variation resulting from sexual reproduction has contributed to evolution.
• Meiosis produces genetically different gametes. During meiosis, genetic variation may result from:

  Allele Shuffling - Swapping of alleles between non-sister chromatids in prophase 1.

  Independent Assortment - of chromosomes in metaphase/anaphase 1.

  Independent Assortment - of chromatids in metaphase/anaphase 2.
• Gametes produced by meiosis are genetically different/dissimilar and are therefore individual. They are also haploid and contain just one of each pair of homologous chromosomes and one allele for every gene.
• The random fusion of gametes creates more genetic diversity as any male gamete can potentially combine with any female gamete of an organism of the same species.
• The random fertilisation of gametes creates more genetic diversity, as they are already genetically unique, producing extensive genetic diversity in the resulting offspring.
• Some phenotypic variation is caused by the environment and not passed on through genes:

  • Speaking with a particular regional dialect - a person's offspring would not inherit the dialect through their genes, although they might learn to speak in this way by listening to others.
  • Losing a limb/digit, or having a scar following an injury.
• If plants are kept in dim light after germination, or if the soil in which they are grown contains insufficient magnesium, then the leaves do not develop enough chlorophyll and are yellow/yellow-white:

  • The plant is described as chlorotic or suffering from chlorosis and the plant cannot photosynthesise.
  • Chlorotic plants have the genotype for making chlorophyll, but environmental factors are preventing the expression of these genes.