The Genetic Model
In part, this discussion is essentially a footnote to the previous inheritance model, but seeks to further clarify an important aspect of inheritance known as ‘genetic recombination’, which also requires, yet again, a further extension of the terminology database. In practice, the relationship of the genotype to the phenotype cannot always be explained in terms of genetic dominance as perceived by Mendel. In today’s terminology, Mendel’s work can be explain as a heterozygote offspring showing the same phenotype as the parent homozygote, although this can still be described in terms of some traits dominating over other inherited traits. However, allele interactions are not exclusively recessive or dominant and further research began to highlight a variety of relationships between alleles that code for the same trait.
So where do chromosomes fit into all this?
As previously outlined, humans have 46 chromosomes, which are broadly said to have been inherited from our mother and father, i.e. 23 chromosomes from each. From the perspective of a simplified labelling mechanism, each set of 23 chromosomes might be identify by a number, e.g. 1-22, followed by the 23rd chromosome being labelled as [X] or [Y] to signify the female or male sex chromosome respectively. So humans have a total of 46 chromosomes, but only 22 pairs of homologous chromosomes, which are chromosomes that contain the same genes in the same order. The additional 23rd pair of X/Y chromosomes are the sex chromosomes and not homologous in their gene content or size. As such, we might consider each chromosome type, e.g. 1-22, as a separate container of genetic information subdivided into genes that can be associated with different inherited traits. Therefore, each individual inherits a gene pair, which are referred to as alleles, which might have the characteristics of being dominant or recessive. If we initially assume a random distribution of dominant genes being inherited from our parent and previously from their parents and so on, we might perceive that an inherited trait being broadly passed down through the ancestral tree.
So is this enough to explain the basic nature of inheritance?
While this is not a bad approximation, such that you might rightly conclude that a zygote-egg cell has some chromosomes from your mother and some from your father, e.g. chromosome-1 from your mother, chromosome-2 from your father and so on. However, there is also a process called ‘recombination’ that shuffles genetic information in each chromosome and results in a unique pairing of genes in chromosomes 1-22, which is a mix of your parent chromosomes with the broad exception of the gender specific [X-Y] chromosomes. This then leads us to the next question.
Why can you possess traits neither of your parents have?
In order to start addressing this question requires, at least, an outline of the idea of complete and incomplete dominance. Gene dominance can affect the phenotype without necessarily affecting the way these genes are inherited. Complete dominance occurs when the heterozygote phenotype is indistinguishable from that of the homozygous parent. However, sometimes the heterozygote displays a phenotype that is an intermediate between the phenotypes of both homozygote parents, i.e. one of which is homozygous dominant and the other homozygous recessive. This intermediate phenotype is an illustration of incomplete dominance that can result in a range of phenotypes within the offspring.
For simplicity, we will revert back to a Mendel-like experiment with plants, where the diagram above shows a genetic cross between two flowering plants. The plant, top-left, has red flowers with an A1A1 genotype, while the plant, top-right has white flowers and an A2A2 genotype. The offspring plants shown both have a genotype A1A2, but different colours, i.e. red or white, based on the dominance of the red-white dominant allele. If neither red or white flower colour is fully dominant, when homozygous red flowers (A1A1) are crossed with homozygous white (A2A2), a variety of pink-shaded phenotypes can result. However, in addition to partial dominance, there is also the idea of co-dominance, which is a situation in which both alleles are equally strong and both alleles are visible in the hybrid genotype. An example of co-dominance can be found in chickens, when white chickens are crossed with black chickens, the result is not a grey chicken, but a chicken with both black and white feathers.
So, in eukaryote cells, recombination typically takes place during meiosis and may be described in terms of a process called chromosomal crossover. This crossover process leads to offsprings having different combinations of genes from those of their parents, and can occasionally produce new alleles. The shuffling of genes brought about by genetic recombination produces increased genetic variation. Chromosomal crossover involves recombination between the paired chromosomes inherited from each parent, i.e. during meiosis. The probability of recombination can depend on the location along chromosome as the frequency of recombination between two locations depends on the distance separating them. Therefore, for genes sufficiently distant on the same chromosome, the amount of crossover is high enough to destroy the correlation between alleles. So while genes determine most of our physical phenotype characteristics, the exact combination of genes we inherit is subject to the process of genetic recombination.
So what effect have these genetic mechanisms made to the variation of human traits?
In many respects, the day-to-day variation within the human genetic code is relatively insignificant in biological terms. In fact, probability alone may suggest that the differences that do occur in the DNA sequence may have little impact if major portions of the total sequence have no known function. Of course, some specific changes are known to have had both positive and negative effects, especially in changing environments, e.g. the mutation for sickle haemoglobin provides a selective advantage in areas where malaria is endemic. There is also the suggestion that certain gene mutations have provided protection against AIDS. Therefore, such changes highlight the significance of genetic variations can depend on the environment and that evolution would not proceed in the absence of any genetic variation within a species, although the timescale for significant change may be long. Of course, genetic variation can also have a downside when linked to single-gene disorders, such as sickle-cell anaemia, cystic fibrosis and muscular dystrophy. Research is also uncovering genetic variations associated with the more common diseases, such as heart disease, cancer, diabetes and psychiatric disorders, e.g. schizophrenia and bipolar conditions. While disorders, such as cystic fibrosis can result from a mutation in a single gene, more common diseases usually result from the interaction of multiple genes and environmental variables. Genetic variations that underpin single-gene disorders are relatively recent and often appear to have a major detrimental impact, although from a hereditary perspective may be less damaging as disorders often exact their toll early in life, i.e. before puberty. In contrast, multiple gene diseases generally are older in origin and have a more gradual effect on overall health, usual later in life.
So what is the nature of human evolution?
As a science, evolution is concerned with how DNA works and adapts to change, where some of this change is down to the permutation of inherited chromosomes combined with effects of genetic recombination as outlined. In this respect, humans are just another species that is defined by its genetic makeup and subject to various mechanisms of genetic change. As such, DNA provides an evolutionary mechanism for encoding the characteristics of life plus a mechanism for this code to be changed with no specific limit on the amount of change that can take place. However, in the context of future evolutionary progress, the amount of time required by natural selection may now be seen as an intrinsic limitation, which is the issue to be expanded in the next section of discussions covering man-made models.