Concept 11 Genes get shuffled when chromosomes exchange pieces.
Alfred Sturtevant describes gene mapping.
I'm Alfred Sturtevant. I was a graduate student in T. H. Morgan's lab, and in 1913, I published the world's first genetic map as part of my Ph.D. thesis. The idea of mapping genes came from a discussion I had with my "boss" about the work of Belgian cytologist F. A. Janssens. He found that, early in meiosis, homologous chromosomes intertwine and exchange pieces. This process later became known as "crossing over." To help visualize crossing over, let's colorize this micrograph and label parts of these homologous chromosomes. This is considered a double-crossover: two points of the chromatids overlap, and the section between the crossover points is exchanged. My boss realized that a crossing over event can recombine alleles between homologous chromosomes. He used this ball diagram to illustrate what he meant. When a crossover happens, alleles that are far apart have a greater chance of being recombined. Alleles that are closer have a lower chance of being recombined. I came up with the idea of using recombinant data to construct a map of genes on the X chromosome. I mapped genes for three recessive traits – yellow body (y), white eyes (w), and miniature wings (m). First I made heterozygous female strains, in which the recessive alleles are all on one X chromosome (y, w, m) and the dominant alleles are all on the other (B, R, L). I mapped genes for three recessive traits – yellow body (y), white eyes (w), and miniature wings (m). First I made heterozygous female strains, in which the recessive alleles are all on one X chromosome (y, w, m) and the dominant alleles are all on the other (B, R, L). Then I crossed these females with males, whose single X chromosome carried the recessive alleles (y, w, m). [MALE Yellow Body White Eyes MIniature Wings] The phenotypes of male hybrids are due to the X chromosome contributed by the mother. The mother's X chromosome also determines the phenotype of all female hybrids – against the "neutral" background of the recessive X inherited from the father. I found, as expected, that half the flies showed the dominant and half the recessive trait. If alleles on the same chromosome were always inherited as an unit, I would have found that all hybrid offspringshow either a purely dominant or purely recessive phenotype. I examined 10,495 flies; in fact, I found that only about two-thirds of the offspring showed the pure dominant or recessive phenotypes. Each of the mixed phenotypes can be explained by crossing over between the two X chromosomes during egg formation. The largest number of flies were either phenotypically dominant or phenotypically recessive for all three traits. This is whatMendel's laws predict if no crossovers occurred. The relatively high frequency of crossover between eye color and wing size indicates that they are distant on the chromosome. The low frequency recombination between body color and eye color indicates relatively close linkage. Double crossovers – between body color and eye color and between eye color and wing size – are most rare.
After discussing the genetic implications of crossing over with Morgan, Sturtevant went home and drew the first gene map of the X chromosome — rather than do his college homework.
Based on his experimental results, Mendel derived the Law of Independent Assortment. Genes are inherited separately from one another. Why didn't Mendel see evidence for "linked" genes?