How is human intelligence inherited

The evolution of human intellectual abilities

But why is the X chromosome so favored in this process? New genetic changes, i.e. mutations, almost always behave recessively.





Figure 3: Scheme of vertebrate evolution





Table 1: “large X chromosome effect“ for intelligence genes


These recessive mutations can only be positively selected if they are homozygous. And this homozygous status is only achieved when two individuals with exactly these mutations mate. This can potentially take an infinite number of generations - but shipping has to be quick. There must be no long “dry spell” with relatively sterile hybrid species with limited vitality. And this is where the importance of the X chromosome comes into play. Because on the X chromosome, which is only simply present in every male individual, new mutations can be selected immediately, either positive if it is an improving mutation (which is rarely the case) or negative in the case of one worsening mutation.





Table 2: Intrafamily correlation data of intelligence

So it is selected via the male sex - a general mechanism that applies not only to humans, but to the entire animal world. In this way, genes that are associated with intrauterine or early death or with general vitality are increasingly concentrated on the X chromosome. The resulting higher male mortality rate, which takes place early in development, is balanced out in nature by a shifted sex ratio. At birth, the male to female ratio, e.g. in humans, is still 106: 100, at conception 140: 100, and new data from in vitro fertilization suggest an even more extreme sex ratio. Much more male fetuses die in early development than female fetuses (Fig. 2). Correspondingly, a three-fold higher concentration of genes for sex and reproduction was found on the X chromosome in humans. Doctors experience this every day in the reproductive clinic, in which 5 to 10% of all men prove to be hypo- to infertile. Both observations, the high rate of male infertility and the higher mortality rate in males, are due to X-linked effects. From these observations it can also be deduced that the speciation process in humans is far from over.

Selection characteristic intelligence

The large X chromosome effect applies universally in nature, however, not only, as shown, to the mechanisms or genes of speciation that are effective after mating, but also to the mechanisms before mating, in particular with regard to sexually selected traits . There is also a special clustering of these characteristics on the X chromosome.
One of these characteristics appears to be intelligence or cognitive abilities in humans. This characteristic must have been selected since prehistoric times, because only one process that has taken place for millions of years could lead to such a comprehensive genetic change that it can be found today as a redistribution of genes on the chromosomes. For more than 100 years, medical genetic findings have shown that the male sex predominates among mentally retarded patients: in the corresponding institutions there are about 1/3 more male patients than female patients. Various socio-cultural reasons have been blamed for this in the past; but research over the past 20 years has shown that genetic reasons in particular are present instead. The genes which, in the mutated state, lead to a mental handicap or a limitation of the cognitive abilities, are involved in the development of these cognitive abilities in the normal state. A data analysis shows that these genes occur 4 times more frequently on the X chromosome than on the other chromosomes (Tab. 1). In other words, the X chromosome contributes about 1/5 to the trait of cognitive function, even though it is only a 20th of the amount of DNA. There is therefore a large X chromosome effect for intelligence or cognitive functions, as was to be expected for a typical speciation trait.
However, since the 1960s there has been another convincing indication of the special importance of the X chromosome for the inheritance of cognitive properties. At that time, the IQ values ​​were examined in families with father, mother, son and daughter and correlations between one another were calculated (Table 2). You can see a high correlation between father and mother, which is not based on genetics but on "associative mating", i.e. the selection of the partner based on characteristics that are similar to one's own (see also Fig. 4) . There is also a high correlation of intelligence between father and daughter and also between mother and daughter and mother and son. However, there is practically no correlation between the intelligence of the father and that of the son, since the father only passes the Y chromosome on to the son, with all the genes for "becoming male" and for spermatogenesis. The father only passes on the combination of his X chromosome, which is particularly favorable for intelligence, to his daughters.




Figure 4: One-sided Z-chromosome evolution in birds in the male sex and the obligatory exchange between the sexes in the X-chromosome evolution in mammals, which is seen as a prerequisite for the evolution of our brain.

Hen or egg?
However, the question arises: Is this enrichment of the characteristics for cognitive functions as well as sex and reproduction on the X chromosome the result of a long-term, continuous selection, or was a certain chromosome with a pre-existing enrichment for? selected these genes as sex chromosomes? By comparing the genome data of many other species, this question can be answered today. As already mentioned, today's X chromosome was introduced about 160 million years ago. This very precise definition is possible because our current X chromosome appears for the first time in mammals (approx. 148 million years ago) and is not yet present in platypus (approx. 165 million years ago) (Fig . 3). A forerunner genome for all mammals is the extraordinarily conserved genome of birds in which there is no X / Y but a Z / W sex chromosome system.
In the case of birds, that of the chicken was sequenced first as an example genome. By comparing genomic observations, one can directly identify the building blocks in the chicken genome from which the X chromosome was later formed (Fig. 1). With the genome-wide expression analyzes using chip technology that are relatively easy to carry out today, it was possible to show that the building blocks were actually selected for the later formation of the X chromosome about 160 million years ago that were the had the highest density of brain-specifically expressed genes. However, this does not apply to the genes for sex and reproduction, which are now also enriched on the X chromosome. These genes were actually selected in a long process and are therefore subject to dynamic changes that the genes for brain functions do not have.

Conservatism in Evolution

Contrary to logical expectations, the neurological functions are therefore subject to the fewest evolutionary changes. The later X chromosome, which encodes this, is already present as a normal pair of chromosomes in the genome of the platypus (Fig. 1), which itself has a very complicated chromosomal sex determination mechanism. In a second step, a more or less random gene for the sex-determining SRY was developed on a partner of this chromosome pair in a dynamic process. This step in the development of human sex chromosomes, which has been studied many times, is presented in a new light. The decisive step was the new combination of existing chromosome components with a special concentration of brain-specific genes to form a new chromosome. The development of the SRY gene into the male-determining gene, on the other hand, is a subsequent, secondary step that has taken place many times in the animal world in a similar way - and may still take place if the current Y chromosome actually degenerates completely at some point in the future should.
This evolutionary history of the X chromosome emphasizes the extremely conservative character of evolution. The genome analysis initially showed that hardly any new genes were made available for specifically human development. Evolution into humans uses practically the same genetic tool kit that is used for all other mammals and even for all vertebrates, including birds. The same genes are used for the functions in the most highly developed organ in humans, the brain, as were used for the same function in much earlier organisms. It has long been a fascination for geneticists that this finding of a highly conserved gene function for the important neurological processes corresponds to a highly conserved gene arrangement on the X chromosome, which is found in elephants and primates as well as in humans - and lies in between more than 100 million years of evolution. Today it has been found that it was very likely to be particularly useful to preserve the valuable neurological qualities on the chromosome which, through its function as a sex chromosome, was most stable against chromosomal remodeling.

Advantages of the XX / XY chromosome system

The introduction of the XX / XY sex chromosome system has a second advantage over the previously existing ZW / ZZ sex chromosome system in birds (Fig. 4). Once a particularly favorable gene combination has been selected, it is obligatorily exchanged between the sexes in the X / Y system, i.e. this combination favors both men and women. In the previous sex chromosome system of the birds, however, a direct transmission only takes place in the male sex. In humans, however, women potentially have two copies of the valuable gene arrangement, while men with only one copy are practically disadvantaged, since mutations in these genes in particular become immediately noticeable as X-linked defects.



Figure 5: IQ distribution in male and female sex

In fact, exact IQ measurements for girls and boys will give a different IQ curve. The average IQ value as it is determined today is slightly higher for women than for men, and the IQ curve for women is much more symmetrical (Fig. 5). In the male sex, these average IQ curves prove to be markedly more variable. As expected, this is particularly the case in the lower IQ range with male patients more frequently affected by intellectual disabilities. But even in the higher IQ range of> 135, the frequency in the male sex exceeds that in the female sex (Fig. 5). One possible interpretation of this data is that a certain and particularly favorable gene arrangement works particularly well, but is very rare, so that, from a purely statistical point of view, inheriting two copies of this gene arrangement at the same time, as would be necessary in the female sex, is even less likely.
It should not be concealed here that this IQ distribution is politically extremely explosive in today's discussion. When the then president of the famous Harvard University, Larry Summers, was asked about 10 years ago why so many men are still appointed to high positions at Harvard, he gave this IQ distribution as evidence and subsequently had to assume his position as Harvard - Give up the president - which, however, did not prevent him from starting a political career afterwards. It should also be added at this point that there are repeated attempts to find out through genetic examinations which genetic variants are responsible for high intelligence. A recent study showed that such a gene variant actually exists. The possession of this gene variant gives the carrier an average of 1.3 points more on the IQ scale, which, however, cannot be measured in an individual. A combination of many genes and not a single gene is probably responsible for really outstanding intelligence.On the other hand, a mutation and thus malfunction of a single gene can very well cause an intellectual disability, as demonstrated by the many genes, especially on the X chromosome whose mutations are detrimental.

Conclusions and Outlook

Finally, it should be pointed out once again how much the conservative principle prevails in evolution. It was found that humans do not have any new, additional genes for the special development of mental abilities that their closest relatives do not have. No fundamental changes have been made to this gene pool for around 450 million years. The studies of gene expression patterns that have only been possible in recent years also show a high degree of conservation of these patterns in the brain. The mechanisms for stimulus reception and response, once developed, have been retained over the many hundreds of millions of years of evolution. The complex neurological abilities of our brain are made possible by the finely regulated interaction of gene expression, and thus the circle closes with the epigenetics discussed in earlier articles. Epigenetic regulatory mechanisms are the most widespread and most highly developed for brain function, which in turn can be derived from the now known genes, the mutations of which lead to an impairment of intellectual abilities. Many of these genes have something to do with the epigenetic mechanisms of chromatin modification and configuration. This also applies to the gene variant that has now been described for the first time, which increases the IQ value a little.