Rh factor and dominance of gene

Question

"Formation of Rh antigen is controlled by dominant gene(R) and its absence by recipient gene(r).People having this antigen with genotype (RR or Rr) are called Rh positive and those whose blood is devoid of it with genotype (rr) are called Rh negative. About 85% in world are Europe and 97% human being in India are Rh positive."

My questions are:-

Which factors result into dominance of gene(R) and absence of recipient gene(r)?

With difference seen in %ratio in India and Europe, can it be concluded that environmental factors affects it.?

Answer 1

There are various factors which decide the dominance of one gene over the other. Owing to the complex biochemical pathways it is difficult to predict which gene is dominant unless we know exactly how the gene is translated to protein and how the protein interacts with the body. These are the few answers that could help:-

• What makes a gene dominant or recessive
• What determines if an allele is dominant or recessive?

As for your question;

There are two genes:

1. D- The allele which is dominant
2. d-The allele which is recessive.

A recessive gene is a gene that is nonfunctional or expresses its characteristic phenotype in presence of an identical allele.

For a person to have Rh negative blood group he must inherit 'd' allele from both the parents as it is a recessive trait.And as @swbarnes2 mentioned in the comments 'd' allele is more common in European countries leading to a wide range of population possessing Rh negative blood groups.

Answer 2

In this specific case the dominant allele is the presence of a gene (RHD), and the recessive allele is (usually) the absence of that gene. This is why you will see the statement "there is no d gene". There does indeed appear to be an environmental factor which governs the frequency of the various genotypes - Toxoplasma gondii.

I find this topic quite a challenge. Here is a distillation of my research.

It is thought that the RHD gene may have arisen through a duplication of the adjacent RHCE gene. It seems that the possession of this gene (a D allele) confers a selective advantage in individuals suffering from toxoplasmosis (infection by the protozoan Toxoplasma gondii. Toxoplasmosis has an effect upon what is termed psychomotor performance, which is usually measured as reaction time. So, for example, it has been reported that there is an increased incidence of road traffic accidents amongst those suffering from (asymptomatic) toxoplasmosis.

Heterozygosity at RHD seems to reduce this effect of toxoplasmosis. So, the current theory is that the D allele appeared in humans because of the prevalence of toxoplasmosis, with the selective disadvantage of haemolytic disease of the newborn being balanced by the heterozygote advantage (similar to the malaria/sickle cell anaemia interaction). There is currently no biochemical explanation of why the RHD gene product has this effect.

In a region with a very low risk of toxoplasmosis there would be a strong selection for the (ancestral?) dd genotype since these individuals have the best reaction time of all (when not infected). The definitive hosts (i.e. where the parasite reproduces) of Toxoplasma gondii are cats, and these were rare in Europe before the domestication of cats. This could explain why the dd genotype is more frequent in this area.