Koltsov predicted in 1927 that an organism's inherited traits are determined by gradual changes in a “giant hereditary molecule,” later known as DNA, that is the building block of the genome that determines an organism's genetic makeup. This hypothesis was unproven, for a time, because of limitations in experimental methodology and an inability to do much more than observe qualities of an organism's DNA compared to the traits it expressed.
To determine the nature of the connection between DNA and heritable characteristics, scientists needed to be able to bring about changes in the genome and observe whether they corresponded to physical changes in the organism. An experiment conducted by Zimmer in 1935 indicated that this was possible: radiation applied to living tissue can change the structures of DNA and another nucleic acid, RNA, that are found in the cells of every organism. Most of the exposed molecules go unchanged in this experiment, but some of them respond to experimental pressures. When X-rays are applied to cells, the nucleic acids warp and rearrange themselves, sometimes dramatically altering the traits they cause the organism to express. Because the changes induced by radiation exposure are more rapid and intense than those brought about by natural selection, significantly altered heritable traits can be observed in a single generation.
As a proof of the connection between DNA and heritable characteristics, radiation experiments have two advantages. First, they are universally applicable: the genome of any organism responds to radiation exposure in a way comparable to any other organism. Second, it is a more direct means of genetic intervention than other methods like selective breeding. These advantages mean that radiation experiments can be used to isolate the hereditary influence of DNA from other confounding variables. The results of these experiments demonstrate that DNA exerts a substantial influence on the hereditary characteristics organisms express: DNA is the way that organisms transfer heritable traits between generations.
However, it is important to note that there are other biological mechanisms, such as epigenetics and certain environmental pressures, that also affect the expression of inherited characteristics. The advantage of the Koltsov theory is its broad applicability; DNA is present in every organism on earth, which is not true of the various other factors that might have a role to play in determining the expression of heritable characteristics. That said, the comparative immeasurability of these other potential influences does not make them unworthy of study.