Genetic Variation and Change

Fundamentals
1. Chromosomes are long strands of DNA. Chromosomes have coding and non-coding regions
2. A gene is a section of DNA that codes for a specific protein. A trait is expressed when this happens.
  1. An allele is an alternate form of a gene e.g. the eye colour gene can come in the form of green eyes, brown eyes, hazel eyes, etc.
3. A gene pool is all of the alleles present in an interbreeding population
4. Dominant: Allele that's expressed when one copy is present.
  1. Recessive: Allele that is expressed only when two copies are present.
5. Genotype: The combination of alleles that an individual/plant has.
  1. Phenotype: Physical features of an individual/plant that is the result of the genotype.
6. Generations
  1. P: Parental - first animals/plants crossed
  2. F1: Filial 1 - offspring of P
  3. F2: Filial 2 - offspring of F1
7. Allele Frequency: Percentage of population of a species that has a particular allele
8. Genetic Biodiversity: The greater the variation within a species, the greater the genetic biodiversity is.
9. Mendel's Laws
  1. 1st: Law of Segregation: Pairs of alleles seperate or segregate and randomly unite at fertilisation.
  2. 2nd: Law of Independant Assortment: In Meiosis, chromosome line up at equator of cell and are arranged independantly of others. This creates a huge variety of combinations in the gametes.
  3. 3rd: Crossing Over: Homologous Chromosomes line up at the equator and exchange sections of genetic material between the chromatids. This shuffles the alleles, which is called recombination
10. Proteins are coded for with 3 base pairs called triplets.
11. Evolution: Genetic change in a population over a period of time
Mutations
1. Permanent change to DNA base sequence. Happens during DNA replication or protein synthesis. Mutations in somatic cells can cause cancer, but are not passed on to offspring. Only gametic mutations can be passed down through generations.
2. Some mutations natural, but can be caused by mutagens. Those are: Ionizing radiation, ultraviolet light, x-rays, chemicals like formaldehyde and nitrous acid, viruses, alcohol/diet, and temperature in extreme cases.
  1. Spontaneous Mutation: Natural mutation caused by DNA replication failure. Some mutations can be reversed by repair enzymes. These types of mutations increase with age.
  2. Induced Mutation: Caused by exposure to mutagens.
3. Point mutations
  1. One base altered in DNA code
  2. Substitution
    1. One base is swapped out for another
    2. Could result in a different amino acid being coded for
    3. Could result in a stop codon, so DNA polymerase will stop reading the code after that point. That means that protein being coded for is incomplete
    4. Could result in a similar protein being coded for, so no change is observable. And some amino acids can be coded for in multiple ways, so the change could have no consequence at all
      1. Silent mutation: same protein is made from mutation.
      2. Conservative mutation: protein made by mutation is very similar to the original protein that should've been made.
  3. Frame Shift
    1. Occurs in deletion and insertion mutations
    2. Changes all of the triplets after the point mutation
    3. Different proteins coded for
  4. Deletion
    1. Deleting a nucleotide will cause a frame shift
  5. Insertion
    1. Adding a nucleotide causes a frame shift
4. Creates new alleles, thus increasing the gene pool. Increased genetic variation. Leads to genetic change
5. Chromosomal Mutation
  1. Deletion: Part of a chromosome breaks off, reducing its length. Genes are lost.
  2. Inversion: Rearranges genetic information on chromosome. No effect on length of chromosome.
  3. Translation: Movement of genetic material from one chromosome to another. This will affect the length of the chromosomes as one chromosome will be shorter and another longer as that part of the chromosome is physically removed from the chromosome and attached to the other one
  4. Duplication: Some of the genetic code on a chromosome is duplicated, so some genes are featured twice on a chromosome.
6. Aneuploidy
  1. Results in abnormal amount of chromosomes in a gamete.
    1. Examples: Down Syndrome - Extra 21st chromosome, Turner's Syndrome - Missing a 23rd sex chromosome, Klinefelter's Syndrome - Extra sex chromosome
    2. One gamete will have an extra chromosome and one will be missing a chromosome
  2. Caused by the non-disjunction of chromosomes in Meiosis
    1. It happens when the spindle fibre network fails to pull apart a homologous chromosome pair during Anaphase. This is because the spindle fibre network is malformed.
Natural Selection
1. Populations can produce large numbers of offspring with inherited variation.
2. Those with variations best suited to the environment they reside in are more likely to survive and breed. These individuals have more offspring than those without the suitable variation and pass down their genes to their offspring.
3. Acts on the phenotype of individuals in populations. Fitter individuals have more offspring. Gene pool change = Genetic change
4. 3 Types of Natural Selection
  1. Stabilising Selection
    1. Individuals that greatly vary from the mean are selected against and die out. The mean in this case are the individuals with the favourable genetics
  2. Directional Selection
    1. Individuals at one side of the mean are selected for and will increase in number. They now have the favourable genetics. The rest will decrease in number. This usually happens in environmental changes.
  3. Disruptive Selection
    1. Individuals on either side of the mean and have the desirable genetics. The individuals on the mean are selected against. This can be the start of speciation (new species being made)
5. Artificial Selection
  1. Humans breeding animals for desired phenotypes. Changes allele frequency in gene pool. E.g. dogs.
Crosses
1. Monohybrid Crosses
  1. Two or more alleles for each trait which is inherited from each parent.
  2. Alleles can be dominant or recessive. Inherited genotype = phenotype
2. Pure breeding
  1. When a breeder wants to know if their animal is homozygous dominant and not heterozygous, they'll breed their animal with a homozygous recessive animal and observe the offspring's phenotype
  2. If the animal is heterozygous, with a large enough sample, half of the offspring will express the recessive trait. If the animal is homozygous dominant, then none of the offspring will express the recessive trait.
3. Incomplete/Co-dominant.
  1. Incomplete dominance is where both phenotypes are mixed e.g. red and white flowers crossed make pink flowers. No allele is dominant or recessive
  2. Co-dominance is where a gene has multiple dominant alleles and neither allele is more dominant than the other. This means that both phenotypes are expressed equally in the individual e.g. a cow with the black allele and the white allele has the phenotype of black and white patches.
4. Multiple Alleles
  1. Two or more alleles for a trait e.g. blood group is determined by 3 alleles. A and B are co-dominant, and O is recessive to both A and B.
  2. Multiple alleles are shown with superscript e.g. Lⁿ
  3. Can use punnett square for these situations
  4. Some of these alleles can be recessive, dominant, or codominant.
5. Linked Genes
  1. Genes located on the same chromosome. The closer they physically are on the chromosome, the less likely they'll be separated during crossing over.
  2. Sex linked genes
    1. Linked genes carried on the X chromosome. Males only have one of these alleles, so they are more susceptible to recessive diseases.
6. Lethal Alleles
  1. Usually caused by a mutation in a gene/genes that are needed for growth or development. They can be dominant or recessive.
  2. They cause the death of the individual
  3. Examples: Tay Sach's disease - a recessive allele that causes the accumulation of lipids in the nervous system, Huntington's disease - a dominant allele that is a progressive brain disorder and its symptoms appear in an individual's 30's or 40's.
7. Dihybrid Crosses
  1. Not linked so one trait's probability of inheritance does not affect the probability of another trait's inheritance.
  2. Involves inheritance of two independent genes. Uses complete dominance so uppercase is dominant and lower case is recessive
  3. When given genotype of the parents, use FOIL (First Outside, Inside Last) to figure out what to put into the boxes on the first row and first column. Think of the way that you expand a quadratic equation; it's the same process.

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Genetic Variation and Change

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