Genetics, evolution, populations, and communities

• What is a genotype?

  The genetic constitution of an organism
• What is a phenotype?


  The physical characteristic expressed by organism
• How to calculate phenotypic ratio of dihybrid inheritance

  Write the parent genotypes

  Write the gametes they can produce

  Put the gametes in a punnet square

  Identify offspring with different characteristics e.g. short grey fur: long grey fur

  Write number of each in a ratio
• What is allele frequency?

  How often an allele occurs in a population
• Why are males susceptible to recessive monohybrid diseases carried on the x chromosome?

  They only need one copy to inherit
• What is the phenotypic ratio of two heterozygous dyhybrid parents?

  9:3:3:1
• What does a large number of phenotypes similar to the parents suggest?


  Linkage was not complete
• What does a large number of recombinants suggest?

  Linkage was not complete
• Epistasis


  When one gene at one locus masks a different gene at a different locus
• Dominant epistasis


  Where the presence of one dominant allele of one gene at one locus masks the expression of another gene at another locus
• Recessive epistasis


  Where the presence of two recessive alleles of one gene at one locus masks the expression of another gene at a different locus
• Complimentary epistasis

  When one dominant allele is needed at both alleles at two separate loci for a phenotype to show
• What is the Hardy Weinberg principle?

  An equation to calculate frequency of alleles in a population
• Hardy Weinberg equation


  P^2+2pq+q^2=1
• How to calculate percentage frequency

  Population with recessive/ Population= q^2

  Square root = q

  1-q=p

  Pxp=P^2

  OR

  2xpxq= 2pq

  x100 to get percentage
• What is the biggest factor in genetic variation?

  Mutation
• How does trisomy 21 occur?


  The failure of the 21st pair of chromosomes or chromatids to separate during either anaphase I or anaphase 2 results in a gamete with an extra 21st chromosome. If this gamete is fertilised by a normal haploid gamete, which only contains one copy of the 21st chromosome, the resulting zygote has three copies of the 21st chromosome (trisomy).
• What does crossing over involve?

  The swapping of genetic material between non-sister chromatids
• Natural selection process


  1 - Variation occurs as a result of a mutation

  2 - Selection pressure

  3 - Organism with
• Natural selection causing antibiotic resistance

  1. A mutation occurs in a bacterium

  2. This mutation conveys resistance to an antibiotic for the bacterium

  3. When antibiotics are added to a medium, the bacterium with the resistance will survive

  4. Those bacteria without resistance will die

  5. Surviving bacteria reproduce rapidly by binary fission and the gene for antibiotic resistance is passed on to the offspring bacteria

  6. If reproduction is allowed to continue for long enough, this specific species of bacteria may become resistant to this specific antibiotic
• What is stabilising selection?

  Selection that favours the most common phenotype
• What is directional selection?

  Where individuals with extreme allele types are favoured
• What is genetic drift?

  A change in the frequency of an existing allele in a population
• What is the genetic bottleneck theory?

  Sharp reduction in population size due to environmental catastrophe leads to reduced variation
• What problem arises due to the genetic bottleneck?

  Reduction in genetic variation means a population is less likely to cope well with selection pressures
• What is the Founder effect?


  The establishment of a new population isolated from the original population e.g. due to migration
• What occurs due to the Founder effect?

  A change to allele frequency as founders are not entirely representative of the entire species
• What is speciation?

  When one species becomes two different species
• What is the reasoning for speciation?

  Two groups become genetically different enough that they can't produce fertile offspring
• What are two types of speciation?

  Allopatric speciation/ sympatric speciation
• What causes allopatric speciation?


  Geographical isolation > different selection pressures > different allele frequency due to separate populations being selected for in different ways > populations become different + unable to produce fertile offspring
• What are examples of geographical isolation?

  Environmental changes such as mountains/ rivers
• How does allopatric speciation affect genetic variation?

  Reduced gene flow > reduced genetic variation
• What causes sympatric speciation?

  Reproductive isolation due to biological or behavioural changes in an organism
• What are examples of causes of sympatric speciation?

  Changes to genitalia or courtship behaviours
• Community

  All the different populations that live together in an area
• Habitat

  Place where an organism lives
• Ecosystem


  A biological community of interacting organisms and their physical environment
• Abiotic factors

  Nonliving components of environment
• Biotic factors


  All the living organisms that inhabit an environment