The basics of genetics, how does it work? Part 4

In some cases, there may be cases in which there is high frequency of these diseases.

We’re going to see two cases: the offspring of consanguineous couples and populations. Here you have the offspring of first cousins. This is a case of a consanguineous marriage in which the descendant is an individual with inbreeding meaning that he may be heterozygotes because both copies of the gene may come from the previous, from the common ancestors of his father and mother. This is something interesting in the sense that the offspring of consanguineous marriage will have a high frequency of homozygotes because of this common ancestry.

And another interesting case is in populations that have had a very strong bottleneck in which by chance the individuals that founded a population or survived at the population drop, these individuals do have some specific alleles that are at the base of the genetic diseases and now we know of a few populations in which they have very high frequency of some diseases, random specific populations, specific diseases.

There are the case of the retinitis pigmentosa in Tristan de Cunha, a very distant and isolated place in the middle of the Atlantic in which a very small number of settlers by bad luck they had an allele for the retinitis pigmentosa that has been quite spread among the population or we have the case of the Tay-Sachs disease among the Ashkenazi Jews which, due to the founder effect of the Jewish population in Europe, they had for this and for some other disease a very high frequency. Last, let’s have a look at the X link inheritance in which we are seeing how the heritance is being done in the cases of genes that are in the X chromosome.

We could also work with the Y, but this is a very simple case. In the X chromosome is more interesting because women have two X chromosomes, men have one and a woman may be homozygote, carrier or affected if she is homozygote. And here means that the pattern of transmission will differ depending on having males or females. And here there are many cases in which is interesting to see the transmission of the X chromosome. Colorblindness it’s a very common case all over the world, hemophilia A and B, Duchenne muscular dystrophy, glucose-6-phosphate dehydrogenase deficiency, etc. Here I have shown you my own pedigree.

I am the one in the arrow with colorblindness –and I’m very proud of that– and you can see that in the line of my mother that was a carrier I was the only one with the mutant X chromosome but by luck we have another part of the family in which this is still now being transmitted. So, at the end what we have seen is the transmission, the standard transmission of simple traits –nothing special–, how these simple traits, this Mendel’s pea, and the transmission of genetic diseases is exactly the same and this is the base of the most simple way of transmission. Unfortunately, genetics is much more complex than that.