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i know that the definition of gene is a unit of heredity, also a sequence of nucleotides in DNA that gets copied to RNA and then proteins. Genotype is the genetic composition of an individual consisting of heritable genes. Phenotype is are observable traits that can be seen on the body of the organism. Knowing all this information, what is the difference between genes, genotype and phenotype before and after we found the existence of DNA?

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As a point of order, the existence of DNA was demonstrated first by Miescher long before these terms came up (around 1870). He called it "nuclein" but he pretty fully characterized it chemically. However, it was not until the 1944 Avery-Macleod-McCarty experiment published in 1944 that DNA was shown to be the hereditary material.

Now a partial answer, relating purely to the origin of the terms (before DNA was known as the hereditary material):

  • here is a translation that I did of the lecture that defined "genotype" and "phenotype", by Johannsen in 1909. I don't think it was previously translated.
  • This definition was critiqued by another author (Woltereck in 1909), I translated that paper too. I don't think it was previously translated.
  • Johannsen came back with this lecture in 1911 (the first time the debate happened in English rather than German). This is now pretty much thought of as the canonical definition of that time.
  • I wrote a short essay discussing this back and forth with more sources.

The basic idea of the early consensus definition is that the genotype is the thing that you inherit, which consist of genes, which control your development. The physical nature of genes was speculative, though a lot of people suspected they were similar to enzymes. However, this genotypic control of development is partial and can be affected by environmental influences etc. The outcome of this developmental process involving both genotype and environment is the phenotype (morphology etc., the things that you can see with the eye, or with a microscope, or maybe a chemical measurement).

For further detail I strongly suggest that you read the papers I link to.

I will leave the question of the definitions post 1944 (or 1953, or post solution of the genetic code, or whenever) to an expert of that history.

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