Science practice 50

Ever since Darwin proposed his theory of natural selection in 1859, biologists have regarded the gene as the sole unit of inheritance, and the discovery of DNA in the twentieth century only served to reinforce this view, known as “hard inheritance.” The idea that environmental factors could produce heritable changes in an organism without altering the organism’s DNA (as occurs in gene mutation due to radiation exposure, for example), known as “soft inheritance,” had long been written off as an impossibility. “Inheritance of acquired traits” became a biological fallacy associated with pre-Darwinians like Jean-Baptiste Lamarck and fanatical Soviet scientists like Trofim Lysenko. But the very recent discovery of what has been termed epigenetics—biological mechanisms that leave DNA unchanged but can alter the ways in which individual genes express themselves—has opened the door on the possibility of “soft inheritance” once again, although many biologists remain skeptical.

Scientist 1

Although study of the so-called “epigenome” is worthwhile and possesses the potential to answer questions about a host of problems from diabetes to cancer, it would be premature to call true “soft inheritance” a reality, at least in animal species. Isolated situations in which environmental factors could affect an organism’s immediate descendants—a mother with a zinc deficiency producing children and grandchildren with weakened immune systems, for example—had already been documented without anyone ever suggesting that there was more to heritability than the gene. Immediate successive generations may exhibit observable effects, but the bloodline always reverts to the true expression of its DNA. A map with dust on it may be hard to read, but the information on the map remains unchanged, and it is simply a matter of how long it takes for a strong wind to blow away the dust.

Scientist 2

Someone who is determined never to see “true soft inheritance” will never see it, but that doesn’t change the fact that it occurs, and indeed has been occurring all along without our realizing it. Epigenetic mechanisms may not alter DNA, but they possess the ability to turn genes “on” or “off” for the duration of an organism’s bloodline, at least until such time as the genes are affected by another epigenetic mechanism. There is already compelling evidence, for example, that tobacco smoking poses risks not only to the individual smoker, but to all of his or her descendants. Imagine a three-way light bulb with three possible brightness settings being put into a three-way lamp on which one of the settings is broken. The fact that the bulb still possesses three settings is immaterial if it is permanently set into a broken lamp. It will continue to “express itself” in a limited way. And the same thing can happen to a gene. If environmental factors can permanently alter the way in which an unaltered gene expresses itself down along a bloodline, resulting in observable effects in the members of that bloodline, then what can we possibly call this other than true soft inheritance?

Figure 1 shows the average sleep pattern of a child, Figure 2 shows the average sleep pattern of a young adult, and Figure 3 shows the average sleep pattern of an elderly person. At Stage 0, the person is awake. As sleep moves from Stage 1 to Stage 4, it grows progressively deeper. REM sleep, commonly associated with dreaming, is predominant in the final third of a sleep cycle.

Figure 1

Figure 2

Figure 3