Inheritance

• Genotype


  The genetic constitution of an organism
• Phenotype


  The expression of an organism's genetic constitution & 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 2 alleles per gene
• Allele combinations in diploid organisms

  • Homozygous (both alleles are dominant, or both alleles are recessive)
  • Heterozygous (one allele is dominant, the other is recessive)
• Dominant allele


  An allele whose characteristic will always appear in the phenotype, whether 1 or 2 are present
• Recessive allele


  Allele whose characteristic only appears in the phenotype if no dominant allele is present, so 2 must be present
• Codominant alleles


  2 dominant alleles that both contribute to the phenotype, either by showing a blend of both characteristics, or the characteristics appearing together
• Monohybrid inheritance


  Where one phenotypic characteristic is controlled by a single gene: expected ratio = 3:1 - not always the case due to random fertilisation or sample not large enough
• Dihybrid inheritance


  Where 2 phenotypic characteristics are determined by two different genes present on 2 different chromosomes at the same time: expected ratio = 9:3:3:1 ~ if 2 heterozygous parents for both genes are crossed. Unless crossing over or autosomal linkage
• Sex linkage


  Where an allele is located on one of the sex chromosomes, so its expression depends on the sex of the individual
• Males


  • More likely to express a recessive sex-linked allele
  • Inherit sex-linked characteristics from their mother, since the Y chromosome can only come from their father
• Autosomal linkage


  Where 2 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. For genes that aren't linked, two homologous pairs are needed
• If genes are linked, it means they have the same logo on the same chromosome
• Epistasis


  Where 2 non-linked genes interact, with one gene either masking or suppressing the other gene. There are 2 types: Recessive epistasis (expected ratio = 9:3:4) and Dominant epistasis (expected ratio = 12:3:1)
• Chi-squared test


  A statistical test to find out whether the difference between observed & expected data is due to chance or a real effect. We can compare expected phenotypic ratios with observed ratios to test our understanding of how different genes & alleles are inherited
• 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
• How is a chi-squared test performed

  The formula 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 & that the results did not occur due to chance