13.1 Chromosomal Theory and Genetic Linkage

13.1 Chromosomal Theory and Genetic Linkage

  • The father of modern genetics began studying heredity in the 19th century.
    • Cell biologists were able to stain and visualize subcellular structures with dyes and observe their actions during cell division and meiosis with improved techniques in the late 1800s.
    • The X-shaped bodies of the identical sister chromatids migrated to separate cellular poles when the chromosomes were replicated.
  • The speculation that chromosomes might be the key to understanding heredity led several scientists to reexamine his model in terms of chromosome behavior.
    • Theodor Boveri observed in the 19th century that proper sea urchin development does not occur unless chromosomes are present.
    • During meiosis, Walter Sutton observed the separation of chromosomes into daughter cells.
  • The Theory of Inheritance was consistent with the laws of the law.
  • The sorting from each pair into pre-gametes appears to be random.
  • Each parent makes gametes that only contain half of their chromosomal complement.
  • The male and female gametes have the same number of chromosomes, suggesting equal genetic contributions from each parent.
  • During fertilization, the gametic chromosomes combine to produce offspring with the same chromosomes as their parents.
  • Critics pointed out that individuals had their own way of segregating their genes.
    • Thomas Hunt Morgan provided experimental evidence to support the theory of inheritance after carrying out crosses with the fruit fly.
  • According to the work of Mendel, traits are not inherited from each other.
    • Morgan found a correspondence between a segregating trait and the X chromosomes, suggesting that random chromosome segregation was the basis of the model.
    • This showed that linked genes disrupt predicted outcomes.
    • Individuals with many linked genes can have more than one trait.
    • Morgan's laboratory suggested that alleles on the same chromosomes were not always the same.
    • During meiosis, some genes became unlinked.
  • chiasmata is the point at which chromatids are in contact with each other and may exchange segments.
    • He suggested that all genes become unlinked.
    • The chromosomes appeared to interact at different points.
    • The points correspond to regions where segments exchanged.
    • We now know that synapsis does more than just organize the homologs for migration to separate daughter cells.
  • If you want to understand the type of experimental results that researchers were obtaining at this time, consider an individual that has two genes on the same chromosomes and one that is dominant in maternal and paternal alleles.
    • One would expect this individual to produce gametes with a 2:1 ratio if the genes are linked.
    • If the genes are not linked, the individual should produce Ab, Ab, aB, and Ab gametes with the same frequencies.
  • Morgan and his colleagues found that when they crossed such individuals to a parent who was aaBb x aabb, there were both parental and nonparental cases.
    • 50 offspring would result in either Aabb or aaBb, but 950 offspring would be recovered that were either AaBb or aabb.
    • A majority of offspring were products of recombination, according to the results.
  • The figure shows unlinked and linked genes.
    • Independent assortment occurs when two genes are located on different chromosomes.
    • The offspring have an equal chance of being either the parent type or nonparental type.
    • Two genes are very close together on the same chromosomes so that they don't cross over.
    • All of the offspring are the parents' type.
  • The genes will be the same as if they were separate.
    • Sometimes the genes are on the same chromosomes and sometimes they are not.
  • The idea of crossing over was an abstract idea that scientists did not believe in.
    • Scientists didn't understand how chromosomes could break and rejoin when chiasmata were a variation on synapsis.
    • The data was clear that linkage did not always occur.
    • It took a young undergraduate student and an "allnighter" to figure out the problem.
  • One night in 1913, Alfred Sturtevant, a student in Morgan's laboratory, took the results of the research from the lab and took them home to study.
    • The first "chromosome map" was created by the next morning.
  • Figure 13.4 shows how we can use recombination Frequency to predict genetic distance.
    • The values correspond to the map distances in centimorgans.
    • The genes for body color and wing size were separated by 17cM, indicating that the maternal and paternal all genes recombine in 17 percent of offspring.
  • To make a map of the chromosomes, Sturtevant assumed that genes were ordered on threadlike chromosomes.
    • He assumed that the incidence of recombination could happen anywhere along the chromosomes's length.
    • Under these assumptions, Sturtevant believed that alleles that were far apart on a chromosomes were more likely to split during meiosis.