The Inheritance Model
So, at this point, we have introduced some of the key ideas and terminology used to described the underlying DNA model contained within the larger cell model. In so doing, we have also introduced some the basic tenets of an ‘inheritance model’ based on sexual reproduction and the transfer of XX or XY chromosomes, via meiosis, from parents to offspring. Therefore, we shall proceed from this point, by making some historical reference to the idea of Mendelian inheritance, which is based on work of Gregor Mendel, in the period (1856-1863) using the hybridization of pea plants. While Mendel's initial conclusions were largely ignored at the time, they were ‘rediscovered’ at the beginning of the 20th century, although the model was still controversial as the inherited outcomes of many of the experiments were still not fully understood.
So what is the basis of the Mendelian inheritance model?
Mendel’s work was based on cross-breeding experiments with plants, e.g. white and purple flowered pea plants. Initially, it was assumed that the first (F1) generation hybrid would be a mix of both colours, while the actual result was generally purple-flowered. Even more surprising was that the F1 generation of pea plants resulted in a 3:1 ratio of purple to white flowers. However, the explanation of Mendel’s experiments is often easier to understand using the idea of ‘Punnett Squares’ , as devised by Reginald Punnett in the early part of the 20th century.
What is a Punnett square?
While, in part, Punnett developed this idea to help explain the outcome of Mendel’s experiments, this discussion will use the example of colour-blindness, within the human genome, as it is possibly more relevant to the earlier examples involving XX and XY chromosome inheritance. As already explained, male-female orientation is determined by the combination of XY (male) or XX (female) chromosomes within a fertilised human zygote. It was also highlighted that the X chromosome contains far more genetic information in the way of genes than the smaller and essentially male-orientated Y chromosome, such that genetic ‘defects’ are statistically more likely to be associated with the inheritance of the X chromosome, e.g. colour-blindness. We shall first present a basic Punnett square for the inheritance of the colour-blindness gene in a female offspring, who inherits a X chromosomes from both parents.
The Punnett square above essentially contains all the possible inheritance permutations of both parents passing their X chromosomes to their daughter, where [b] denotes a recessive gene for colour-blindness, while [B] denotes a dominant gene that negates the colour blindness defect. As explained, a daughter inherits two copies of the X chromosome from her parents, where a dominant [B] gene will negate the presence of the recessive [b] gene in the other copy, if present. So, in the example above, we see the shaded squares showing that 3 out of 4 permutations avoid colour-blindness and only when both parents are colour-blind does the defect get passed to the daughter.
We might repeat the logic of the Punnett square, as shown above, for a male offspring with the understanding that only one copy of the X chromosome now exists in the male offspring, as passed by his mother. As such, we now see that a male offspring has a 50% chance of inheriting colour blindness from his mother, while his sister would only have a 25% chance, at least on a statistical basis.
Note: Such results can be used to explain the 3:1 result in Mendel’s experiment using F1 purple-white flowered peas, based on the assumption that the purple gene was dominant over the white gene. In experiments, where both red and white variants are both dominant, hybrid pink variants can be produced. In the case of the other 22 chromosomes, the dominance and recessive ‘battle’ between the allele genes is resolved as per the first 2-by-2 Punnett square above.
It might be worth reiterating some of the terminology already introduced in order to add a few additional terms to our growing database of references. As outlined, each cell has a strand of DNA in the form of 23 chromosomes inherited from the parent gamete cells, as explained by meiosis. Within each set of chromosomes, genes pairs essentially align to the same cell function, but are not necessarily identical due to genetic mutations in both parents. These gene pairs are called alleles and are often assigned the characteristic of being dominant or recessive as in the case of the colour-blindness example. Based on these permutations, the terminology is extended to include the idea of a cell genotype and phenotype, which is explained as follows:
- Genotype identifies the permutations of gene-alleles in the cell,
e.g. BB, Bb, bB or bb.
- Phenotype when based on the dominance of gene [B] over [b] produces a [B] phenotype in 75% of cases.
When the genotype of the alleles is the same, e.g. BB or bb, the cell is said to be homozygous, while opposing genotypes, e.g. Bb or bB, are said to be heterozygous. There is one other permutation of the homozygous condition that can be identified in terms of the cell being homozygous-dominant or homozygous-recessive. Finally, it should be highlighted that the Punnett square examples above are extremely simple in scope, as they are only intended to help introduce the basic concept and the additional terminology as outlined above. In practice, many phenotype characteristics involve complex permutations of genes that interact collectively. As such, the simple two-by-two Punnett square example has to be expanded into larger N-by-N combinations.
Note: The inheritance model outlined above might broadly be described as a neo-Darwinian model based on natural selection of genes, which are mixed by the process of meiosis and random mutation as cells ‘evolve’ through each generation. However, it is unclear whether this description provides an adequate explanation of all the potential causal mechanisms that might be affecting the evolution of the cell. Within the wider context of this overall discussion of human evolution, i.e. past, present and future, changes to the DNA genetic structure may take many forms, both natural and man-made. For example, environmental change can be both natural and man-made, which may come to affect the gene structure by epigenetic mechanisms to be discussed. However, in the future, there is also the probability that man-made changes to the human genome will be made by direct manipulation of the gene structure.
If we initially ignore the potentially worrying implications in the note above, there is nothing obviously controversial in any of the description of the inheritance model as outlined, although some advocates of political correctness might still question the description of colour-blindness as a genetic ‘defect’. However, there are some aspects of genetic inheritance, which might start in the accepted idea of the selective breeding of animals, but end up straying into the far more controversial issue of eugenics.
Note: It is realised that alarm bells may start ringing on seeing the word ‘eugenics’ because it is often immediately associated with a philosophical ideology based on some form of social hierarchy, especially in terms of its many historical manifestations. This is NOT the context intended in this discussion, although the issue of the possibly diminishing role of natural selection in modern human society cannot be avoided.
We shall start with a limited definition of the word ‘eugenics’ as simply a process that seeks to create improvements in the offspring of a given species. When generalised in terms of all species, the definition might be equated to the idea of selective breeding, which has underpinned farming since the dawn of civilisation in term of both crops and livestock improvements, e.g. selecting the best seeds, a faster horse versus a stronger horse etc. In this respect, selective breeding has helped turned wild species into domesticated crops and livestock. However, it is realised that this interpretation can quickly change into the more sinister inference of the word ‘eugenics’ when directed towards humanity. As such, the alarm bells of political correctness might well start to sound again after reading the following passage taken from Charles Darwin’s seminal work entitled ‘ The Descent of Man’ :
“With savages, the weak in body or mind are soon eliminated; and those that survive commonly exhibit a vigorous state of health. We civilised men, on the other hand, do our utmost to check the process of elimination; we build asylums for the imbecile, the maimed, and the sick; we institute poor-laws; and our medical men exert their utmost skill to save the life of every one to the last moment. There is reason to believe that vaccination has preserved thousands, who from a weak constitution would formerly have succumbed to small-pox. Thus the weak members of civilised societies propagate their kind. No one who has attended to the breeding of domestic animals will doubt that this must be highly injurious to the race of man. It is surprising how soon a want of care, or care wrongly directed, leads to the degeneration of a domestic race; but excepting in the case of man himself, hardly any one is so ignorant as to allow his worst animals to breed.”
Clearly, if perceived in terms of social eugenics, the paragraph above might be seen as an argument that advocates improving the genetic quality of the human population by some form of selective breeding, which inevitably leads to the question:
Who does the selecting?
However, before attempting to address any of the issues surrounding the question above, let us consider Darwin’s words above carefully, for his inference is not one of selection based on any ideology of social hierarchy, but rather the maintenance of a ‘vigorous state of health’. In this respect, Darwin had only recognised the diminishing role of natural selection given the protection that human society now extends to its population and, as the next follow-on paragraph suggests, he accepted the responsibility of society to protect ‘the helpless’ albeit with one key caveat to be discussed.
“The aid which we feel impelled to give to the helpless is mainly an incidental result of the instinct of sympathy, which was originally acquired as part of the social instincts, but subsequently rendered, in the manner previously indicated, more tender and more widely diffused. Nor could we check our sympathy, if so urged by hard reason, without deterioration in the noblest part of our nature. The surgeon may harden himself whilst performing an operation, for he knows that he is acting for the good of his patient; but if we were intentionally to neglect the weak and helpless, it could only be for a contingent benefit, with a certain and great present evil. Hence we must bear without complaining the undoubtedly bad effects of the weak surviving and propagating their kind; but there appears to be at least one check in steady action, namely the weaker and inferior members of society not marrying so freely as the sound; and this check might be indefinitely increased, though this is more to be hoped for than expected, by the weak in body or mind refraining from marriage.”
Today, Darwin’s suggestion that individual liberty might be overruled, such that certain individuals would be deny the right to have children irrespective of circumstances might be seen as immoral and, at the very least, politically incorrect. However, the protection that society has extended to the individual has negated much of natural selection, which has in-turn created other problems that may one day overwhelm political correctness and the ability of society to protect everybody. The arguments behind this statement will not be expanded here, but relate to two earlier discussions: ‘Population & Resources’ plus ‘Political Evolution’ . However, the primary purpose of this diversion away from simply discussing the biological mechanisms of genetics was to highlight that continued technical advances will allow humanity to identify and therefore possibly eradicate many of the diseases that continue to affect humanity. Of course, the response to even this idea might result in a legitimate question:
Exactly what genetic ‘defects’ might be classified for ‘eradication’?
While we will leave this debate for now to continue with the outline of more technical issues, it is clear that developments in the field of genetic biology will have the potential to impact the direction of human evolution into the future and that this future may be much closer than most people realise and not necessarily confined to the direct manipulation of our DNA, if changes to our environment do this for us.