Non-Mendelian inheritance occurs on occasion when genetic material is introduced into bacteria by nonsexual means. In bacterial transformation, DNA extracted from one strain can be put in a medium that contains another strain of bacteria.
These organisms then absorb some of the foreign DNA and thereby acquire some of the genetic traits of the original DNA donors. Similar in principle is bacterial transduction, in which viruses, not human experimenters, accomplish a transfer of DNA from one bacterial type to another.
When a virus reproduces in an infected bacterium, pieces of bacterial DNA occasionally become incorporated in the offspring viruses. If one of these offspring then infects a new bacterial host, additional bacterial gens are introduced into that host.
As instances of non-Mendelian inheritance, bacterial transformation and transduction have strictly limited significance. Far more important, universally significant in all organisms, are mutations.
Any stable, inheritable change in the genetic material of a cell is a mutation. The most common type is a point mutation, a stable change of one gene. In such cases it is not necessary that all the DNA of a whole gene become altered; a change in not more than a single pair of nucleotides in a double DNA chain can amount to a mutation.
Such a change can alter the genetic code for a single amino acid in a protein, and one different amino acid often suffices to affect the function of the protein: if the protein is an enzyme, for example, a particular metabolic reaction could become altered or even blocked, and the consequences in a cell could be very significant.
The smallest portion of a gene that can produce a mutational effect is called a muton; the smallest mutton would be a single pair of nucleotides in DNA. A “gene” consisting of numerous muttons in linear series, can therefore be defined as a unit of mutation; a section of a chromosome that, after becoming altered in at least one of its mutons changes just one trait of a cell.
Traits are affected not only by point mutation but also by various kinds of so called chromosome mutations. These include, for example, invasions-a piece breaks off a chromosome and reattaches itself in inverted position; translocations-a piece breaks off a chromosome and attaches itself to another chromosome; duplications-a section of a chromosome doubles; and deletions-parts of chromosomes break off and become lost.
All such chromosome mutations alter the nucleotide sequences of DNA and thus the genetic messages transmitted to RNA. Mutational changes can be induced by high energy radiation such as X rays, and the frequency of mutation has been found to be directly receives.
Some of the naturally occurring mutations are probably produced by cosmic rays and other space radiating and by radioactive elements in the earth. But this unavoidable natural radiation is not sufficiently intense to account for the mutation frequency characteristic of genes generally, about one per million replications of a given gene.
Most of these mutations probably represent errors in gene reproduction. Others are undoubtedly caused by man-made radiation, which adds to and increases the natural “background” radiation. Mutations can also be produced experimentally by physical agents other than radiations, and by various chemical agents. Mutation occurs at random. Any gene can mutate, at any time and in unpredictable ways.
It can mutate several times in rapid succession, then not at all for considerable periods. It cans mutate in one direction, then back to its original state or in new directions. Every gene existing today undoubtedly is a mutant that has undergone many mutations during its past history. The effect of a mutation on a trait is equally unpredictable. Some are “large” mutations that affect a major trait in a radical, drastic manner.
Others are ‘small, with but little effect on a trait. Some mutations have dominant effects and produce immediate alterations of traits. Others have recessive effects, and in diploid cells they remain masked by normal dominant alleles. In view of the structural and functional complexity of a cell it might be expected that nearly any permanent change in cell properties would be disruptive and harmful.
Indeed, most mutations are disadvantageous, and if they have dominant effects they tend to impair cellular functions. Most mutations with dominant effects actually tend to be eliminated as soon as they arise, through death of the affected cell.
In some cases, however, a dominant effect of a mutation (particularly a “small” dominant effect) can become integrated successfully with cellular functions. Such a cell then survives with an altered trait. Yet most of the mutations in surviving diploid cells have recessive effects, which are masked by the effects of the normal dominant alleles.
A small percentage of mutants produce advantageous traits or new traits that are either advantageous or disadvantageous. In man, for example, many trillions of cells composed the body and, in view of the average rate of mutations, several million mutations are likely to occur in each individual. Many of these are lethal to the cells in which they occur, and many others remain masked by normal dominants.
But some produce nonlethal dominant traits. Such new traits arising in individual cells are then transmitted to all cells formed from the original ones by division. For example, “beauty spots” probably develop in this manner. Gene changes that occur in body cells generally are somatic mutations.
They affect the heredity of the cell property-a patch of tissue at most. In multicellular organisms such mutations usually have little direct bearing on the heredity of the whole individual.
An entire multicellular organism is likely to be affected only by germ mutations, stable genetic changes in immature and mature reproductive cells. Such mutations will be transmitted to all cells that ultimately compose the offspring’s.
To the extent that germ mutations are recessive and masked by normal dominants, the traits of the offspring will not be altered. But if the offspring is haploid, or is diploid but homozygous recessive for a mutation, or if a mutation is dominant, then a particular trait can be expressed in altered form.
Provided such a new trait is not lethal or does not cause sterility, it will persist as a non-Mendelian variation. Mutations can therefore affect the adaptation of an individual as much as sexual recombination of genes.