It is important to stress that having a hereditary intelligence or any other hereditary character does not mean following a linear ratio or a predetermination of a particular value. The combination of genes offers a wide array of possibilities.
For this purpose, just remember that pink flowers may have red, pink, or white descendants in the typical examples of Mendel's laws.
The chart relating to the concordance in identical twins and just twin brothers can help us see the difference more clearly.
The comparison between these percentages of concordance not only allows us to perfectly differentiate between hereditary character and predetermination of a particular character but rather also to deduce information about the number of chromosomes or chromosomes involved and the number of possible different expressions or characteristics.
For example, it seems that the blood group depends on just one chromosome while schizophrenia depends on various, possibly six or seven, although it is difficult to know with just this information. Also, for schizophrenia, the heritability of parents to children is very low even though it has a genetic concordance of 69%
The concordance of intelligence is not presented because it changes with the different studies. Although from the information available, I would say that it is mainly found on one chromosome.
Another problem as we have seen is the definition of intelligence. Multiple functions and operatives modes configuring the cognitive processes affect the ability of measurement of intelligence test
However, it is not as serious as it seems; the fact that intelligence can be formed by various elemental faculties that are somewhat independent does not mean that they cannot be hereditary brain functions. Furthermore, if it turns out that one group of them has this characteristic; they will probably all have it.
On the other hand, the lack of a generally accepted definition of intelligence is very inconvenient given that I really think that there are many ways to deal with this concept. Even so, a way of avoiding this byzantine discussion is to reverse the concept so that it meets our needs:
-We will study the heritability of intelligence by understanding it as a group of faculties of intellectual reasoning measured by what is commonly called the intelligence test-
Afterwards, we may analyze if the results of the study can be generalized.
Intelligence is a continuous variable of Nature, which complicates the quantitative research of its heritability.
Normally, studies performed on continuous variables are based on the treatment of these variables as a sum of more elemental discreet variables. You have to take into account that, in general, the change of continuous variables is not easily compatible with the idea of change by random mutations.
Intelligence can be understood as a sum of relational functions such as above / below, larger / smaller, general / specific, etc. Although the appearance of a new relation could be understood due to random causes, the improvement or greater precision of an existing relation that could be produced in a similar fashion could turn out to be incomprehensible.
We find ourselves with two large problems when measuring these intellectual abilities. The first is derived from the fact that the display of these brain functions is very much influenced by the moment in which they are performed.
Tiredness, moods, and other various factors can significantly affect their expression from one day to the next or between the beginning and end of the same day.
The second is that each particular measurement uses a specific test and the results can vary according to: whether the individual has done a similar ability test in the past, the individual's personality or cultural affinity with the ability test. However, if it is well designed, this aspect should be fairly reduced.
In the quantitative data used from the longitudinal Young Adulthood study for the statistical analysis included in the EDI Study, the correlations between the different measurements of the same children of intellectual abilities are very low. The maximum is 33% in spite of the fact that the same people are being measured.
A simple conclusion would be to think that the brain functions of intelligence are not stable throughout life and that they are highly influenced by the environment. However, some experts believe that intellectual abilities are greatly stable throughout life, especially from 6 years of age on.
Popular language, as an expression of the collective unconscious, clearly expresses that intelligence is a fixed quality given that it uses the verb to be referring to permanent not transitory characteristics in expressions such as, that person is very intelligent. Well, at least in Spanish this aspect is obvious.