Heredity

I

Introduction

Heredity, similarity between parents and offspring. In biology, offspring resemble their parents because the offspring inherit genes, carried on DNA molecules, from their parents (see Genetics). The word heredity is also used in a non-biological sense, in human affairs, to refer to the inheritance of cultural or material goods, such as religious or political beliefs, or land or money. This article is mainly concerned with biological heredity.

In human beings, and many related forms of life, inheritance occurs by a set of detailed mechanisms, some (but not all) of which are well understood. In molecular terms, heredity is due to DNA. The DNA codes for genes, and the genes specify particular proteins. The DNA acts as a set of coded instructions for building a body, given a particular environment. At a reproductive level, inheritance is a sexual process. The offspring contain two copies of each gene, inherited from their two parents. At a cellular level, inheritance proceeds via meiosis, a special kind of cell division that produces the gametes (eggs in females, sperm in males). In meiosis, the two copies of each gene are reduced to a single copy. When male and female gametes combine, the double set is restored. This pattern of heredity is called Mendelian (see Mendel’s Laws).

However, the particular hereditary mechanism that is used in modern humans is only one of many ways in which heredity occurs in all of life. Viruses, bacteria, and other microbes use different hereditary mechanisms. In this article, we look at the variety of hereditary mechanisms, and how they are related to the forms of life that use them.

Moreover, theoretical biologists have thought up several further ways in which heredity could conceivably occur though in fact it does not. We shall also look at some of these theoretical hereditary mechanisms to see how they shed some light on why life uses the hereditary mechanisms it does rather than some alternative mechanisms.

II

The Mechanisms of Heredity in the Main Forms of Life

Heredity is one of the main defining features of life as a whole. The existence of life probably requires three conditions to be met: reproduction, heredity, and variation. Any entities that possess these three attributes will be able to evolve, and may evolve into something we recognize as life. The three conditions are related. Heredity is impossible without reproduction. However, reproduction without heredity is possible. For example, fires can reproduce: a spark from one fire may ignite a second fire elsewhere. But the attributes of the “offspring” fire—its size, its duration, the pattern of flickering flames—depend on local features such as the supplies of combustible material, oxygen, and the wind, rather than on attributes of the “parental” fire. Fires show reproduction without heredity and do not evolve by natural selection; they are not alive.

In life, reproduction is of a kind that produces heredity; offspring tend to resemble their parents. All life reproduces by “template reproduction”; that is, the parental hereditary molecule acts as a template for the production of the offspring hereditary molecule. Template reproduction is the best-known example of a method of reproduction that produces heredity. However, not all template reproduction takes the same form as DNA replication. Other examples of template reproduction include photocopying, old-fashioned printing with metal-typeset or blocks of woodcut, and industrial processes in which molten metal is poured into a mould. All these types of template reproduction produce some form of heredity. Fires, by contrast, do not spread by template reproduction.

The third feature of life, variation, is also related to the hereditary process. Variation ultimately arises because of errors, or mutations, in heredity. But the amount of variation in a population depends on the shuffling and reshuffling of genetic variants that already exist as well as the creation of new variants by mutation. The shuffling is effected by a process called recombination, and recombination is another feature of the hereditary mechanism. During meiosis, the two gene-sets that an individual inherits from its two parents are shuffled before they are sent into the gametes. The offspring in the next generation show a greater range of variation in consequence. Thus, heredity is an essential property of life—not only is heredity one of the three defining conditions of life but it also influences the other two conditions. Without heredity, life would not exist.

Moreover, the details of the hereditary mechanism influence the form that life takes on. Over evolutionary time, the hereditary mechanism has changed from simple beginnings at the origin of life to the forms seen in modern life. At each stage, the details of how inheritance occurs constrain the form that life can have. Indeed, many biologists think of the history of life as a series of ways in which genetic information is passed on from one generation to the next. Advances during evolution have depended on changes in the way that heredity occurs. (It is worth noting that hereditary mechanisms have not changed in order to cause any future evolutionary events. However, once the hereditary mechanism changes for some reason, certain other evolutionary changes may become more likely in consequence.)

The main changes that have occurred during evolution in the form of heredity are now discussed in detail.

A

The Origin of Heredity

The origin of heredity, together with reproduction, represents the transition from chemistry to biology. We have only inferential and incomplete knowledge of how the transition occurred. No one has observed the origin of life; indeed it is not even known to the nearest hundred million years when life on Earth originated. Heredity and reproduction in all life forms depend on base pairing. In modern DNA the bases are four molecules symbolized by A, C, G, and T. A binds to T, G binds to C. A base sequence such as GCTT will reproduce into CGAA, which in turn reproduces into GCTT. (Modern DNA can only reproduce with the assistance of many enzymes; but the earliest replicating molecules probably reproduced without enzymatic assistance.) The earliest replicating molecules probably relied on pairing between molecules other than A, G, C, and T; but we have several reasons to think that some kind of base pairing was used. One reason is that all life uses base pairing now. Indeed, the evolutionary biologist Leslie Orgel once remarked that if you imagine stripping away all the details of Earthly life one by one to try to define what is essential to all life, you can remove almost everything—skin and bones, eating and breathing, cells, even enzymes—but finally you are left with base pairing, like the smile of the Cheshire cat.

Secondly, base pairing has the right theoretical attributes for a hereditary system that can allow the evolution of Earthly life. Base pairing allows what is called unlimited heredity. Theoreticians distinguish between mechanisms of limited heredity, which permit inheritance for only a small number of states, and mechanisms of unlimited heredity, which permit heredity among a large, or practically infinite, number of states. As an example of a system with limited heredity, consider autocatalytic cycles (cycles that catalyse themselves). Freeman Dyson has discussed how autocatalytic cycles may have arisen near the origin of life. Thus, a set of chemicals that can be symbolized by X, Y, and Z may catalyse one another in a cycle of the form X → Y, Y→ Z, Z → X. A system of X, Y, and Z can perpetuate itself, and generate more of X, Y, and Z.

However, only very simple evolution is possible here. There might be a second autocatalytic cycle, X’ → Y’ → Z’. Then XYZ systems might compete with X’Y’Z’ systems, and one or other might increase in frequency depending on the local conditions. But there are only two inherited states (XYZ or X’Y’Z’), and evolution is confined to fluctuations in their relative frequencies. In general, the number of states of autocatalytic systems is likely to be limited, because only certain sets of chemicals will form autocatalytic cycles. The evolution of life, in its modern complexity and variety, requires unlimited heredity. Base pairing permits this property. For example, with four different bases, a sequence of only five bases can have over 1,000 forms: AAAAA, AAAAC, AAACG, and so on. With a longer sequence of bases, the number of heritable states soon becomes astronomical. The evolution of life with the kind of complexity that we see in life on Earth would have required unlimited heredity, and that probably required base pairing or something like it.

B

The RNA World

The earliest living systems probably had heredity but no metabolism. (In genetic terms, they had genotypes but no phenotypes.) The molecule just replicated, without doing anything more. The next stage is for life to catalyse reactions, in the way that modern enzymes do, altering the conditions around the replicating molecule such that it can copy itself more effectively. Biologists suspect that, early in the history of life, there was an “RNA world”. In the RNA world, RNA acted as the hereditary molecule and also catalysed reactions. In most of modern life, DNA is the hereditary molecule and it codes for proteins that catalyse metabolic reactions. In life forms with DNA, heredity is separate from catalysis, and the hereditary molecule contains coded information. In the RNA world, the RNA molecules acted as ribozymes: that is the RNA molecule itself acted as an enzyme rather than coding for a protein that in turn acted as an enzyme. A system in which heredity and catalysis are two functions of the same molecule is a simpler system than one in which the two functions are performed by separate molecules. The simpler system is likely to have come first in evolution. However, the possibilities in the RNA world would have been limited. The need for an RNA molecule to act as a catalyst constrains what shape it can have, and therefore its base sequence.