-
Asked by to Tomas, Judith, Gemma, Aoife on 29 Dec 2018. This question was also asked by .
-
Judith Sleeman answered on 29 Dec 2018:
Recessive genes increase variation. If you inherit the same recessive gene from both parents, then it makes one of your characteristics. The more common the recessive gene, the more likely it is to be seen in people. The gene for the O blood group, for example, is recessive but it still ends up as the commonest blood group.
-
-
Tomas Fitzgerald answered on 3 Jan 2019:
As Judith says having recessive alleles increases the possible variation of traits. Most people who have studied genetics will know of Gregor Mendel’s pea plants and the punnet square where the offspring of two heterozygous parents for the tall allele (Dominant) and the short allele (Recessive) will turn out with allele proportions DD 25%, DR 50% and RR 25%. This is a good example of the inheritance of Dominant and Recessive alleles however for most traits the situation is rather more complicated. This is because most traits are effected by multiple alleles (e.g. height in humans) and it is the combination of multiple alleles (that can individually be dominant or recessive) that govern the observed trait. Furthermore most genes are transcribed into protein and for certain traits in may not be the case that having the dominant allele fully compensates for the recessive form. For example, in sickle cell anemia, the observation of this debilitating disease is only seen in individuals with the double recessive allele combination (homozygous for the recessive (single) allele form). In this example carriers who are heterozygous with the dominant and recessive alleles will actually produce some sickle shaped red blood cells, however the dominant allele will produce sufficient amounts of the non sickle shape that no visible disease trait is observed and they will most often be unaware of their carrier status.
-
Gemma Chandratillake answered on 3 Jan 2019:
Changes in DNA just happen all the time (either by chance, or due to exposure to something that damages DNA). Whether these alterations stick around in the population depends on whether they are harmful, beneficial, or neutral to the organism, and also just due to random chance.
Recessive genetic alterations usually cause a gene not to function properly; the thing that it codes for (protein or RNA) either doesn’t get made or gets made but doesn’t function. Since we have two copies of our genes, having one copy that doesn’t work often doesn’t matter, and we don’t see any effect because we still have a working copy of the gene to get the job done. It is only when we have two non-working copies of a recessive gene that we see the effect of that gene not working. The effect of a gene not working isn’t always bad, it can just mean that there is a different outcome (e.g. different hair colours, the O blood group) than when the gene is working which provides variation in a particular characteristic on which selection can act.
If it is harmful to the organism for a particular gene not to be working, then recessive genetic alterations will generally be quite rare in the population because selection will work against them. Lots of alterations that cause recessive genetic diseases are very rare for example.
There are some recessive alterations that are useful under certain circumstances though, and so are seen in the population quite frequently. An example would be a genetic alteration in the beta-globin gene that causes beta-globin not to get made. If you have two non-working copies of the beta-globin gene, you have beta-thalassemia which is a serious disease. Therefore, we would expect such genetic alterations to be rare in the population because they are harmful. However, we actually see this type of alteration quite commonly because having one copy of the gene not working seems to protect people from getting malaria. So, in this case, there is selection working *against* this change because beta-thalassemia is a serious disease but it is balanced in malarial regions by selection working *for* this change because having one non-working copy of the gene protects against malaria.
Comments