The basics of genetics, how does it work? Part 2

Mendel did the first, the initial law crossing between homozygotes. In this case what he did is to have, for example, big R big R cross with small r small r. In this case a big R could be whatever, could be any human trait. In this case again is the shape of the seeds, R means round, small r means wrinkled. And in this case if we cross big R big R with small r small r what we get is the first generation. All the individuals’ heterozygotes (Rr), is like having disappeared the phenotype of small r small r, the wrinkled seeds in this initial generation; and this is what is considered initially the first Mendel law.

If we cross these individuals, these heterozygotes, the wrinkled phenotype reappears in the second generation and here we can see that the offspring of the heterozygotes is one fourth big R big R, one half big R small r and one fourth small r small r. So, these are the typical ways of understanding a transmission from one generation to the other of the alleles. And in this case, these alleles, they have very clear the phenotype typed to them.

For example, we can do the crossing of the heterozygotes with homozygotes and we get this result which is very easy to understand, and Mendel worked out also the transmission not just of a single trait, but two different traits and he announced a law of independence of the transmission of one and the other. In his case he did for example the seeds rounded or wrinkled and color yellow or green and he saw that the transmission of one of the transmission of the other was totally independent. And here you have the transmission, the results of the crossing of the individuals that are both heterozygotes for the two characteristics.

So, if you have this individuals that are heterozygotes, the first thing have to do is to work out how these individuals are going to do the gametes.

The gametes may be of four kinds: big R, big Y; big R, small y; small r, big Y; and small r, small y. So, these four (gametes) encounter the other ones that are exactly the same and you have the crossing of all the possibilities here,

and here you have the proportions in which you find this generation that is the well-known proportional 9:3:3:1 of the sixteen cases. Here you can see that there is the apparition of the double homozygotes. So, the strange, the less frequent case in which they appear

just in one proportion of the sixteen: the double homozygote recessive. So, these are the general Mendel’s law, so the first law we have seen the dominants –all the first generation are the same–, the segregation –individuals of this first generation when mating together give rise to the ancestral phenotypes–, and the independent transmission of the characteristics. Why these matters? Why at this stage we are looking at Mendel’s peas? The idea is that there are many cases in which this may matter for humans.

Example: a couple that do not have any of the two, any genetic disease, have had a child with a very rare genetic disease. What are the chances of having the disease in a future child? So, this is very easy to calculate because these two individuals must be normal heterozygotes –we call that “carriers”– meaning that both are carriers, meaning that independently of the offspring they have already had, in the future they have a possibility of recurring of one fourth which is very high and no doubt they have to look for genetic counseling in the future.