Given that our current understanding of the mechanisms of inheritance has undergone major revisions over the past fifty years, I thought a brief review of our current knowledge was in order.
As complex multicellular organisms we know that we have genes for specific traits such as eye color that are predictably passed on to our offspring. This occurs through the processes of sexual reproduction which endow our offspring with a mixture of those genes carried by the two parents.
Cloning (or asexual reproduction) occurs when the offspring are identical to a single parent. In this process there is no mating and mixing of genes. Cloning is a common (but often not the only) means of reproduction in some plants, insects, bacteria, and viruses. Cloning has also been induced artificially in the laboratory by manipulation of the egg cells of higher organisms such as mice and sheep.
Under certain natural conditions, a bacterium can donate some genes to a neighboring bacterium thus changing not only the recipient bacterium but the recipient bacterium’s future progeny as well. In this way new genes (such as for antibiotic resistance) can spread through a bacterial colony very rapidly. In the sense that there is now a mixture of genes in an individual bacterium that came from two different individuals, the results are similar to that following sexual reproduction in higher organisms.
Likewise, when two different viruses infect the same cell they may exchange genes and, if they do, their offspring may emerge having different characteristics from that of either of the parent viruses. Finally, as described previously in this column (and reminiscent of some ancient myths), on rare occasions an organism can either ingest, or become infected by, another organism which, while remaining mostly intact, ends up living permanently in the host organism and replicating in concert with its host, thus producing a novel hybrid organism.
If the offspring of this novel hybrid organism have survival capabilities equal to or superior to either of its parent organisms, the new hybrid will persist over time.
Further complicating evolution in multi-cellular organisms like humans is the need to produce a large variety of cells, each with vital but different functions. Thus, beginning with all the same genes, our embryonic cells differentiate into brain, kidney, liver, and skin cells among others, in order to produce a whole human being.
This process of differentiating is accomplished by accelerating the activity of some genes and suppressing the action of many others. This complex regulation of gene activity is accomplished by a large group of genes which don’t code for proteins (and therefore don’t contribute directly to such obvious characteristics as eye color) but code instead for regulator molecules which have the ability to regulate the activity of other genes.
It is now thought that many of dramatic changes seen during evolution (say from small primate to human) are less the result of random mutations in protein producing genes and more the result of heritable changes in the regulation of the activity of these genes. This could account for the fact that, although our genetic constitution (genotype) is only slightly different from that of a chimpanzee, there are much more marked differences in our appearance and behavioral characteristics (phenotype).
The attributes of the environment in which these processes take place provides much of the selection pressure which determines which organisms perish and which persist. In these multiple ways evolution has proceeded over millions of years to give us the huge variety of organisms we share the Earth with today.
Questions and suggestions from readers are welcomed and will be responded to in future editions of this column. Contact me at firstname.lastname@example.org.