inheritance

• gene


  A length of DNA that codes for a single polypeptide or protein is called
• loci


  position of a gene on a chromosome
• • The chromosomes of eukaryotic cells occur in homologous pairs (there are two copies of each chromosome)
  • As a result cells have two copies of every gene
  • As there are two copies of a gene present in an individual that means there can be different allele combinations within an individual
• genotype


  alleles of a gene possessed by an individual, different alleles can be represented by letters
• homozygous


  two allele copies that are identical in an individual
• heterozygous


  two allele copies that are different in an individual
• genotype affects phenotype
• phenotype


  physical characteristics of an organism
• dominant alleles

  always expressed in phenotype

  so is expressed in heterozygous and homozygous
• recessive alleles


  only expressed when two recessive alleles present and no dominant alleles are present

  so recessive only expressed when homozygous recessive
• codominance


  both alleles are expressed

  so if heterozygous both alleles are expressed
• linkage

  two different types:

  • sex linkage
  • autosomal linkage
• sex linkage

  • Some genes are found on a region of a sex chromosome that is not present on the other sex chromosome
  • As the inheritance of these genes is dependent on the sex of the individual they are called sex-linked genes
• autosomal linkage

  • This occurs on the autosomes (any chromosome that isn’t a sex chromosome)
  • Two or more genes on the same chromosome do not assort independently during meiosis
  • These genes are linked and they stay together in the original parental combination
• Monohybrid inheritance looks at how the alleles for a single gene are passed on from one generation to the next
• • Dihybrid crosses look at how the alleles of two genes transfer across generations
• Epistasis

  • In some cases one gene can affect the expression of another gene
  • Epistasis: when two genes on different chromosomes affect the same feature
  • If epistasis is present it needs to be taken into account when determining the phenotypes of individuals
  • The whole combination of alleles from the different genes dictates the phenotype
• A statistical test called the chi-squared test determines whether or not there is a significant difference between the observed and expected results in an experiment

  • If the difference between results is statistically significant this suggests the presence of a factor that isn’t being accounted for
  • The chi-squared test is carried out when the data is categorical, i.e. falls into distinct groups
• the formula for calculating chi-squared

  A) sum of

  B) observed value

  C) expected value

  
• degrees of freedom = number of classes - 1