The researchers counted the IQ data of parents and children of more than 2,000 families in the United States, and found that the IQ between mother and child was slightly stronger than that between father and child. For example, the IQ correlation between mother and son is 0.443 (the closer the correlation is to 1, the stronger the correlation is), while the IQ correlation between father and son is 0.4 1 1. But the gap of less than 10% is really tiny. From this perspective, the conclusion that "a son's IQ is determined by his mother" is totally untenable. The IQ of his mother, father and child are all related, but they are not decisive. So my husband's heart can be put in his stomach. But is the statement that "intelligence genes are on the X chromosome" really groundless?

Is the intelligence gene on the X chromosome?

In fact, as early as 1972, some people in the scientific community put forward the statement that "intelligence genes are on the X chromosome". At that time, gene sequencing had not been realized, and researchers only made this guess through the statistical differences in IQ between men and women. Because they found that although the statistical distribution of IQ of men and women is Gaussian distribution, the variance of IQ distribution of men is larger, that is to say, the proportion of men with very high and very low IQ is more than that of women. Moreover, some genetic diseases with mental retardation seem to have a soft spot for men, and there are various indications that the inheritance of intelligence is closely related to sex chromosomes. This speculation was later slowly confirmed by experiments.

At the beginning of this century, with the vigorous development of the human genome project, more and more gene sequence codes have been cracked. Scientists have found that at least 40% of the nearly 1000 protein coding genes on the X chromosome are expressed in the brain, which is higher than that on the autosome and far more than that on the Y chromosome ([2], [3]). In other words, X chromosome has a great effect on brain structure, cognitive ability, intellectual development and so on. For boys, their only X chromosome comes from their mother, that is to say, in theory, the mother should also have a great genetic effect on their son's intelligence, but why is there no significant correlation between IQ statistics? What the hell is going on here?

How do genes on the X chromosome determine the structure of the brain?

This involves two concepts-X inactivation and chromosome recombination in meiosis.

For men, the task of coding the genes of brain structure on the X chromosome is very simple, because there is only one gene, so just take it and use it. For women, they have two sets of X chromosomes. If both sets are used to express protein, it would be a mess. So choosing which set to use to compile the brain has become a difficult problem. In a woman's cell, one of the two sets of X chromosomes will automatically lose its activity, leaving only one set to decode and compile protein. This phenomenon that one of the two X chromosomes loses activity is called X chromosome inactivation. At present, it is generally believed that in most cases, the process of which group of X chromosomes is inactivated by different cells is random. Therefore, about half of embryonic cells come from the mother's X chromosome inactivation, while the other half comes from the father's X chromosome inactivation. So when genes compile the brain structure, they come from the mother's X chromosome for a while and from the father for a while. There is no fixed pattern, and the gene expression of X chromosome is completely mixed. In addition, about 15% genes in human X chromosome can escape mistakes, which makes the already complicated X chromosome compilation more confusing.

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The different fates of X chromosome and Y chromosome.

On the other hand, meiosis is a special way of cell division. When a sexual cell divides, the chromosome is copied only once, and the cell divides twice continuously, so that the number of chromosomes in the cell is halved. Women have two X chromosomes, and each egg cell will get an X during meiosis. When cells divide constantly, chromosomes will recombine many times, and two different X chromosomes from parents will exchange some genes at the same position, which makes the X chromosome assigned to each egg cell mixed with the genes of the original two X chromosomes.

Therefore, it is the result of meiosis and chromosome recombination that the son got the X chromosome from his mother, and it is the result of mixing the X chromosomes of his grandparents. The X chromosome used by his mother to compile the brain was also randomly selected. The brain structure of mother and child may be completely different, or there may be a great degree of similarity. The most likely outcome is that some are similar and some are different. The design of nature is so ingenious. Although the mother is mainly responsible for the inheritance of her son's intelligence, her son's intelligence level can't be completely predicted by her intelligence.

In addition, due to the existence of genomic imprinting, some scientists speculate that compared with the mother's inheritance of intelligence through X chromosome, the father expresses genes through autosomes, thus affecting the brains and behaviors of offspring [4]. However, due to the few gene imprinting, accounting for less than 1%, this effect is not significant.

Evolution of human intelligence and genius genes

A large number of intelligence genes are concentrated on the X chromosome, which sounds unfavorable to men, but many scientists believe that this is actually a great advantage of human intelligence evolution. Professor Gillian Turner of Newcastle University in Australia believes that human intelligence has evolved faster than any other genetic feature. If you want to spread a gene among people as quickly as possible, it is the wisest choice to put it on the X chromosome. As a sex chromosome, X chromosome has a history of more than 300 million years. Most of its genes are well preserved and have always been in a dominant position on sex chromosomes. Therefore, some people speculate that the evolution on the X chromosome does not mainly depend on the generation of new genes, but on the generation of new combinations by old genes to a great extent.

Many scientists also speculate that genius genes are also closely related to the X chromosome. The following figure represents a simple model of the emergence and spread of such a genius gene [5]. This model has only six genes, all on the X chromosome. After the meiosis of the first generation mother's X chromosome, a genius gene combination-the son of ——all-A gene was produced. The son married another woman and gave birth to two children. His genius genes can only be passed on to his daughter, not to his son. However, when his daughter remarried, the genius gene combination was dismantled because of the recombination of meiosis chromosome (the position of X), and the fourth generation was mediocre, but the gene was still spreading, looking forward to the opportunity of genius combination again. Therefore, although the genius gene can continue, the emergence of genius is only an accident.

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If this is true, and the genius gene needs to be passed down by daughters, then talented men should have daughters, otherwise the incense will be broken when the gene is in the son's place, which is a great loss for mankind.