My notes while reading The Gene by Siddharth Mukherjee.
Part 2: In the sum of the parts there are only parts
The study of genes turned from statistical analysis to cell biology.
What problem was Thomas Morgan trying to solve? How do complex organisms arise out of a single cell. How was his research question related to genetics? Understanding how complex embryological information is “stored inside the cell” could explain how complex organisms arise out of a cell.
Thomas Morgan started his research from the assumption that genes are present in a chromosome. In order to discover how genes were arranged on a chromosome, he started studying mutations in fruit flies. He discovered that certain genes in fruit flies would always appear together as if they were “linked” suggesting that the individual genes were moving in groups. This is unlike Gregor Mendel‘s peas, where the genes behaved like completely independent entities.
Thomas Morgan postulated that genes can appear together only and only if they are physically and permanently connected to each other on the chromosome itself.
Thomas Morgan‘s newly discovered rule had one exception. Even though genes normally move in packs and stay close to their neighbours, sometimes, a gene may unlink from its neighbouring genes and swap places from one chromosome to another. Morgan imagined the genes literally crossing over from one chromosome to another.
The likelihood of two genes to unlink/cross over, is proportional to how far they are from each other on a chromosome. This means we can use the strength of genetic linkage to find out where genes are present relative to one another on a gene. Thomas Morgan‘s student, Strutevant, created the first linear genetic map in 1911 using this technique.
Truths and reconciliation
For genes to be the carrier of biological information, they had to explain: variation of individuals within a specie, evolution of that specie and the development of an individual.
Variation in species
Ronald Fisher, in 1918, wrote a paper titled, “The Correlation between Relatives on the Supposition of Mendelian Inheritance” where he explained how Mendelian inheritance can give rise to normally distributed traits in a population.
Evolution of species
Ukrainian geneticist, Theosodius Dobzhansky, discovered that changes in the environment act as selection pressure, testing the fitness of different individuals in a group. The fitness depends on the individual’s genetic makeup because of naturally occurring variance.
Dobzhansky discovered that the phenotype was a combination of genotype + environment + triggers + chance.
Natural selection indirectly seeks the fittest genotype by applying selection pressure on the phenotype.
Dobzhansky also discovered how genetic variation in a natural population can lead to speciation.
Geographical separation which causes sexual separation which causes genetic separation which causes reproductive separation.
Transformations
Discoveries from Ronald Fisher and Theosodius Dobzhansky lead to the creation of the Modern Synthesis.
Frederick Griffith was a British bacteriologist studying pneumonia causing bacteria. He noticed that a non-virulent strain of Streptococcus pneumoniae could be made virulent when exposed to heat-killed virulent strains.
Transformation is a method of horizontal gene transfer. A bacteria can “ingest” the genetic material in its environment and incorporate it into its own genetic material.
Henry Muller, a former student of Thomas Morgan, was trying to produce mutant fruit flies. He discovered that X-rays in low dosages produced dozens of mutants.
Lives Unworthy of Living
Trofim Lysenko claims to have produced stronger wheat strains by putting them through “shock therapy”. He gave rise to “Lysenkoism” in the Soviet Union.
That stupid molecule
Oswald Avery was able to reproduce Frederick Griffith‘s result of horizontal gene transfer. He discovered that the hereditary information was stored in the chromosomes as DNA.