|MadSci Network: Genetics|
Thanks for the interesting question Carol,
The Rh in Rh factor stands for rhesus, so named because the Rh factor is an antigen that was first discovered in the blood of rhesus monkeys. The Rh antigen is called D, and is encoded by a gene called the RHD gene. Because humans are primates, many of our genes are the same or similar to those of other primates, and the RHD gene produces the same antigen in both humans and rhesus monkeys. Most people produce the D antigen, but a few people are missing functional copies of the RHD gene, and they produce no D antigen. These are the people who have Rh negative blood types, and the non- functional version (allele) of the RHD gene is called the RHD silent allele.
Because we each have two copies of the RHD gene (one copy inherited from each of our parents), it is possible for a person to have one non-functional RHD gene and one working copy. In these cases, such people still have Rh positive blood types because the functional allele directs the synthesis of the D antigen. Because your mother was Rh positive, we know that she had at least one functional copy of the RHD gene. Because you have siblings who are Rh negative, we know that they have two copies of the RHD silent allele, therefore they had to have inherited one non-functional copy from each of your parents. So, we can infer from this that your mother had one functional RHD allele, and one RHD silent allele. We can also infer that your father had at least one RHD silent allele, because he had to pass that on to your siblings.
In general, the RHD silent allele occurs at a frequency of about 40-45% in Europeans, and people of largely European ancestry. So, thats the basic answer to your question. In non-European populations the frequency of the RHD silent allele is much lower. In people of largely African ancestry, this allele occurs at a frequency of about 3%, and in people of Asian, Pacific Islander, and Native American ancestry, the RHD silent allele occurs at or less than a frequency of 1%. In addition, it seems like the mutation in the RHD silent allele in Europe is different from the mutation in the RHD silent allele found
Given these numbers, we can use population genetics make some predictions about the incidence of Rh negative blood types in various populations. In Europeans, we expect that about 16% of the population will have Rh negative blood types. In the other populations of the world, the frequency of Rh negative types will be much lower; in Africans, only 9 people in 10,000 will be Rh negative, and in the non-African, non-European portion of the world, only 1 person in 10,000 will be Rh negative.
The high frequency of the RHD silent allele in Europe tells us that the mutation that generated the non-functional RHD silent allele arose in Europe. Analyses of European populations tells us that this allele is found at the highest frequencies in western European populations, especially in Spanish and French Basques, suggesting that this allele arose long ago in some western European population. In the United States, about 15% of the general population is Rh negative; almost 20% of European Americans are Rh negative (because many Americans have western European ancestry), and approximately 5-10% of African Americans are Rh negative (because of the flow of European genes into the African American population). Fewer than 1% of Asian and Native Americans are Rh negative.
Based on the frequencies of the RHD silent allele in Europe, we can also estimate that about 55-60% of Rh positive Americans have one RHD silent allele, as your mother must have had. In addition, there is a greater than 50% chance that your father was Rh positive (with one RHD silent allele), and in general there is about a 1 in 3 chance of two Rh positive Americans (of predominantly European ancestry) both having one RHD silent allele.
I hope this helps you in your researches into your family's history.