Principles Module 44


  • Describe the conditions under which characters fail to assort independently
  • Explain a test cross and how one is carried out
  • Analyze the results of a test cross and calculate genetic distance

Key points


  • sometimes characters don’t assort independently during meiosis
  • these characters show a high probability of being inherited together
  • characters that travel together during meiosis are linked
Genetic mapping
  • linkage can be identified and quantified by careful observation of inheritance patterns
  • the basis of genetic mapping is recombination due to crossing over
  • characters that are located very close to each other on the chromosome have a lower probability of being separated by crossovers than those that are farther apart
  • the frequency of crossovers can be used as a measure of distance between characters
Performing a test cross
  • cross a known heterozygous individual with a known homozygous recessive individual
  • score the number of offspring having each of the 4 possible phenotypes
Example test cross
  • cross a homozygous recessive mutant for body color (black, b) and wing shape (vestigial, vg) with a fly heterozygous for both characters: bb vgvg  x  b+ vg+  (the + represents the wild-type allele)
  • phenotypes of offspring
    • parental: black body, vestigial wings (405)
    • parental: wild-type body and wings (415)
    • recombinant: wild-type body, vestigial wings (92)
    • recombinant: black body, wild-type wings (88)
  • calculate recombination frequency
    • (92 + 88) / 1000 = 0.18 or 18 map units
  • notes
    • the only way to produce offspring with wild-type bodies and vestigial wings is via recombination
    • the only way to produce offspring with black bodies and wild-type wings is via recombination

In-class activities

  • work through example test crosses

Questions for Practice

  • Describe how you would test two characters for linkage. What predictions would you make about number of offspring for each phenotype if the genes are linked or not?