Molecular Genetics

Genetic characters can be linked, exchanging partners by recombination. The degree of linkage produces a consistent linear relationship and thus linear genetic maps.

Facts

Mendel's second law claimed that traits segregate independently. In 1911, T. H. Morgan noted that for some pairs of sex-linked characters, parental combinations were much more frequent in the F2 generation than other combinations, not segregating independently. Morphologists had noted a chromosome that was correlated with the sex of the organism. Rudimentary wings (m) and vermillion eyes (p) are recessive sex-linked traits. A. H. Sturtevant, following the lead of T. H. Morgan, crossed an Mp female with an mP male. The offspring (F1) were allowed to mate with each other. In the F2 progeny there were two types of females and four types of males (shown at left). Unexpected types were found. Identify them! Interpretation

• Sturtevant performed similar crosses involving many sex-linked characters.
• Results with characters b, p, and r were representative. Males with non-parental phenotypes were found with frequencies represented by the length of the bars:

• Invariably for sex-linked markers, the largest of the three frequencies was about equal to the sum of the other two frequencies.

Interpretations

• Not all genes are unlinked. In particular, sex-linked genes are linked to one another.
• The morphologists' results led geneticists to believe that in the male fruit fly all sex-linked characters are on a single transferrable unit, the observable sex chromosome.
• In Sturtevant's first experiment, MP and mp (the non-parental types) in males can not be explained assuming the chromosomes remain intact and that there is a single sex chromosome in males.
• This discrepancy led to the concept that chromosomes can recombine with their homologues. The recombined chromosomes consist of part of the mP attached to part of the Mp chromosome.
• Sturtevant's further experiments provided quantitative support to the deduction (made by T. H. Morgan) that recombination frequencies measure a genetic "distance" between genes.
• Such genetic distances are additive (bp +pr = br).
• Markers that have measurable recombination frequencies are said to be linked.
• Markers related through a chain of linkage constitute a linkage group.

Further information

• Recombination frequencies can also be measured for autosomal genes. The strategies for their calculation are more complex than for sex chromosomes.
• As more and more markers are studied, linkage groups become larger (and the number of groups smaller) until the number of linkage groups equals the number of chromosomes.
• The experiment cited above was confirmation of an early experiment of T. H. Morgan using white eyed, long winged and red eyed, rudimentary winged flies. The vermilion eye-color locus and the miniature wing locus provided further evidence of recombination.
• The p, r, and b loci were used by Sturtevant in one of the first genetic mapping experiments. The p, r, and b loci are located on the genetic map of the Drosophila chromosome. The present symbol for the p locus is v, and that for b is y.
• A genetic map is constructed from recombination frequency data. Extensive genetic maps are available for the chromosomes of many eucaryotic organisms including:

  Drosophila melanogaster (fruit fly)
  Caenorhabditis elegans(nematode)
  Arabidopsis thaliana (thale cress)
  Mus musculus (mouse)
  Homo sapiens (human)
  Oryza sativa (rice)
  Zea mays (corn)
  Saccharomyces cerevisiae (baker's yeast)
  Danio (Brachydanio) rerio (zebra fish)