7. Genetics, Populations, Evolution and Ecosystems

• Genotype: The genetic constitution of an organism
• Phenotype: The expression of an organism’s genetic constitution, combined with its interaction with the environment
• Allele: Different forms of a particular gene, found at the same locus (position) on a chromosome. A single gene could have many alleles
• Diploid organisms carry two alleles per gene
• Dominant allele: An allele whose characteristic will always appear in the phenotype, whether one or two are present
• Recessive allele: An allele whose characteristic only appears in the phenotype if no dominant allele is present, meaning two must be present
• Codominant alleles: Two dominant alleles that both contribute to the phenotype, either by showing a blend of both characteristics, or the characteristics appearing together
• Homozygous: Both alleles are dominant, or both alleles are recessive. Heterozygous: One allele is dominant, the other is recessive
• Monohybrid inheritance: Where one phenotypic characteristic is controlled by a single gene
• Punnett square for monohybrid cross with parental genotypes of GG and gg:

  G G

  g Gg

  g Gg
• 100% of offspring will express the characteristic determined by allele G
• Punnett square for monohybrid cross with parental genotypes of Gg and Gg:

  G g

  G GG

  g Gg

  g gg
• 75% of offspring should express the characteristic determined by allele G
• Genetic diagram example:

  Parental phenotypes: Brown eyes, Blue eyes

  Parental genotypes: Bb, bb

  Gametes Offspring genotypes: Bb, Bb, bb, bb

  Offspring phenotypes: 2:2 brown eyes:blue eyes

  B b

  b b
• Dihybrid inheritance: Where two phenotypic characteristics are determined by two different genes present on two different chromosomes at the same time
• Punnett square for dihybrid cross with parental genotypes of RrGg and RrGg:

  RG Rg rG rg

  RG RRGG RRGg RrGG RrGg

  Rg RRGg RRgg RrGg Rrgg

  rG RrGG RrGg rrGG rrGg

  rg RrGg Rrgg rrGg rrgg
• 9 out of 16 offspring should have the same phenotype as their parents. 6 will match their parents on one characteristic, but differ on the other. 1 will differ on both characteristics
• Sex-linkage: Where an allele is located on one of the sex chromosomes, meaning its expression depends on the sex of the individual
• Males are more likely to express a recessive sex-linked allele because they only have one X chromosome, so if the allele is on that X chromosome, it will be expressed
• Males are more likely to express a recessive sex-linked allele because most sex-linked alleles are located on the X chromosome. Therefore, males only get one copy of the allele, so will express this characteristic even if it’s recessive. Since females get two alleles, this is less likely
• Males inherit sex-linked characteristics from their mother, since the Y chromosome can only come from their father. Therefore, if the mother is heterozygous for sex-linked alleles, she is a carrier and may pass on the trait
• In a sex-linked cross with parental genotypes of X H X h and X H Y, the four possible phenotypes of the offspring are: Normal female, carrier female, normal male, affected male
• Autosomal linkage refers to when two or more genes are located on the same (non-sex) chromosome. In this case, only one homologous pair is needed for all four alleles to be present
• In a multiple alleles cross with parental genotypes of I A I O and I B I O, the offspring could express the alleles AB, A, B, O
• Epistasis is where two non-linked genes interact, with one gene either masking or suppressing the other gene
• The two types of epistasis are:

  • Recessive epistasis= where two homozygous recessive alleles mask expression of another allele
  • Dominant epistasis= where one dominant allele masks expression of multiple other alleles
• In an epistasis cross with parental genotypes of AABB and aabb, genotypes BB or Bb allow expression of gene A, while genotype bb masks gene A. 25% of the offspring will have gene A masked
• The chi-squared test is a statistical test to find out whether the difference between observed and expected data is due to chance or a real effect
• Criteria for the chi-squared test:

  • Data placed in discrete categories
  • Large sample size
  • Only raw count data allowed i.e. not percentages
  • No data values equal zero
• A chi-squared test is performed by using a formula that results in a number, which is then compared to a critical value (for the corresponding degrees of freedom). If the number is greater than or equal to the critical value, we conclude there is a significant difference between the observed and expected data and that the results did not occur due to chance
• We can use a chi-squared test to compare expected phenotypic ratios with observed ratios to test our understanding of how different genes and alleles are inherited
• Species is a group of organisms that can interbreed to produce fertile offspring
• Population consists of all the organisms of a particular species that live in the same place
• Gene pool is the range of different alleles existing for a particular locus within a population
• Allele frequency is the proportion of a certain allele within a gene pool, expressed as a decimal or percentage
• The Hardy-Weinberg principle allows us to estimate the frequency of alleles in a population and determine if allele frequency is changing over time
• Assumptions of the Hardy-Weinberg principle:

  • No mutations occur to create new alleles
  • No migration in or out of the population
  • No selection, so alleles are all equally passed on to the next generation
  • Random mating
  • Large population
• Hardy-Weinberg equation for calculating allele frequency:

  • The frequencies of each allele for a characteristic must add up to 1.0
  • Equation: p + q = 1
  • Where p = frequency of the dominant allele, and q = frequency of the recessive allele
• Hardy-Weinberg equation for calculating genotype frequency:

  • The frequencies of each genotype for a characteristic must add up to 1.0
  • Equation: p^2 + 2pq + q^2 = 1
  • Where p^2 = frequency of homozygous dominant, 2pq = frequency of heterozygous, and q^2 = frequency of homozygous recessive
• Genetic factors that cause phenotypic variation within a species:

  1. Mutation of alleles

  2. Random fertilisation by gametes

  3. Random assortment of genetic material during meiosis