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How we know there is a heritability? Part 2

When we try to understand genetic diseases, we see that in many cases they do not follow single ways of inheritance.
To begin with that we need to recognize to which extent a given trait is inherited or not. And for that we have to work out what we call the heritability which is the proportion of phenotype variants, or of the variation of a given trait, that is accounted by the genome, by the differences in the genome. It is the degree of variation in a phenotype trait in a population that is due to genetic variation between individuals in that population. In general, it is written as heritability (small h square, h2), equal to the variance due to the genes (VA) –A is for additive effect of several genes– divided by the total.
In this case if we have an h square equal one means a high heritability. Indicates that the environment doesn’t have strong importance or very, very, very little importance in the phenotype variation. And h equal zero is driven basically by that environment. In this case the genome doesn’t account for the variation of the trait. To work out, to calculate the heritability of a trait is to give an idea of to which extent this trait comes from genes, comes from the environment.
We don’t have to have a strict idea if we have a heritability of point five saying fifty percent, no, no, no… Means that this value depends on how it is calculated in the population in the genetic background, so it’s a general idea of the relative importance of genetic factors in relation to environmental factors. In many cases this is being studied through comparing monozygotic and dizygotic twins. When we compare them what we take into consideration is that the monozygotic share genes and environment and the dizygotic twins share one half of the genome but share the environment in the same way as the monozygotic.
And this allows us to calculate through the difference of the recent resemblance of ones in relation to the others to calculate the heritability. In this way heritability will be from zero to one depending on the trait in general. And also remember on the way it is calculated and on the population that is calculated. These are a general view of general cases in which we have to the left the cases of very strong genetic base, so these are the Mendelian like Duchenne Muscular Dystrophy or Cystic Fibrosis.
And if we go to the right it is more and more importance of the environment, so this line is a rough idea of the balance of h (heritability) and here what we have are different conditions in which we have varying the risk of importance of genes versus environment. It is interesting to see that there is strong heritability for some human traits and this is interesting because this means that we have to work, to deal, to dissect, to recognize which is exactly the genetic base of that. If we have for a given disease, diabetes let’s say, a given heritability then we have to go and do studies to dissect this genetic of the diabetes.
It is interesting to note that there are many studies in which people have been trying to understand the existence of heritability for physical traits, for diseases, for all kinds of conditions but also for psychiatric diseases for which still now we do not have a deep knowledge of the genetic bases of it. For example, there are psychiatric conditions for which we know that we have a heritability let’s say of point seven which is quite high but for which we do not know yet which are the specific genes at the base of that, but something there must exist.
And here you see also for psychological conditions meaning that we have lots of interesting issues in which the genome has to play an important part and being that complex phenotypes in most of the cases we are not able yet to work out in a proper manner the relationship between the genotype and the phenotype and then to recognize which are the genetic variants at the base of this specific heritability.

When we try to understand genetic diseases, we see that they often do not follow a single mode of inheritance.

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