Do we really inherit acquired traits?
Parenting is full of paradoxical situations. There are times when you hate to see your kids doing things that you also do, or at least did, oftentimes. When it first dawned upon me that my child has developed a habit of sucking her lower lip, I couldn’t decide if I should be concerned or happy. I was aware that my dental structure was ruined because of that very habit during my childhood. I have always been casual about looks, but the habit also affected my jaw function in the long run. Now, like most parents, I do not want my child to repeat my mistakes.
The sort of “happy” side to it was that I saw in this a potential living proof of Lamarck’s theory of evolution. The theory roughly goes like this: an individual acquires some traits during its lifetime and thus adapts to changes in environment; the traits then get passed on to the next generation and become evolutionary changes. Giraffes evolving longer necks and front legs is often cited in books as a likely example of Lamarckism. In my case the lip-sucking habit was acquired purely by my actions in my lifetime. I didn’t inherit it from my parents. No big deal, one might say, many babies suck lower lip. True, some babies do this as a natural reflex of sucking nipple for feeding. However, seldom do babies continue with this habit for months. I was doing that even after I started earning. My daughter at least stopped the habit when she had braces.
Proposed at the beginning of the nineteenth century, Lamarck’s theory has been out of favour for over a century now. Another theory, one by Weismann which caught the fancy of scientific world, killed it. Almost. Let me explain.
Let’s start with the development of an embryo inside mother’s body. Very early during the process, a specialised group of cells of the foetus is kept aside safely to be used later exclusively for sexual fusion after puberty. These forbidden cells are known as germ cells. All other bodily cells (jargon: somatic cells) eventually develop into organs which perform day-to-day activities. As far as exchange of genetic material is concerned, these two types of cells don’t talk to each other. Weismann argues that ongoing life characteristics can at best affect the somatic cells which are involved in the activities. Such changes cannot be transferred to the germ cells. And unless the changes affect the germ cells they cannot be passed on through heredity. This is called Weismann barrier which, most scientists agree, forbids inheritance of acquired characteristics. Lamarck therefore failed to become a legendary figure of evolutionary biology.
The new millennium however saw reemergence of Lamarck’s ideas. Certain acquired characteristics of brain were found to be inherited in rodents (Singer, 2009). The mechanism of transfer of such traits does not involve genome. Rather it’s attributed to things that affect how genes of the DNA are expressed for protein making (jargon: epigenetics). The “things” are chemical molecules acting like tags that tell a gene to be active or inactive. Even though the recipe for a particular protein is written in a gene, whether it will be made or not is determined by such tags. The tags are modified throughout the lifetime as the environment we live in changes. Note that the genome doesn’t change. In that sense it does not break the Weismann barrier. Yet the net effect is one of transfer of acquired traits.
When male and female germ cells fuse after mating to create the new fused cell (jargon: zygote), it was believed that the new formed DNA does not inherit the epigenetic tags. They are meant to be erased to provide the new cell with an “epigenetic blank slate”. It has been discovered recently that the erasure of tags is not complete. Some tags indeed pass through from parent cells to the next generation. Thus two generations are exposed to an environmental change such as smoking, and three generations in case of direct exposure of a pregnant mother. This is because a developing foetus has germ cells ready for contributing to the next generation. So before your baby comes out of your body, the seeds of your grandchild are already there.
Weismann barrier is thus no longer a valid objection to Lamarckian inheritance. There is another problem with Weismann’s theory which stems from lack of clear distinction between germ cells and somatic cells during embryo development. In fact the zygote initially divides into many cells which at that phase are like somatic cells, some of which go on to become germ cells for the next generation. Thus germ cells have a brief period of “somatic” life before they are assigned their role.
With the Weismann barrier gone and evidences of transgenerational epigenetic inheritance emerging, acceptance of Lamarckism is definitely on the rise. That may not necessarily mean however that I have passed on my lip sucking habit to my daughter. Though tempting to the scientist in me, until someone clearly explains the mechanism from end to end as a case study, I am not pleading guilty.
Koonin, EV. (2014) Calorie restriction ? Lamarck. Cell 158, 237-238.
Singer, Emily (2009) A comeback for Lamarckian evolution. MIT Technology Review. https://www.technologyreview.com/2009/02/04/216134/a-comeback-for-lamarckian-evolution/amp/