Final answer:
The most likely explanation for a D positive individual producing anti-D antibodies is that they are a partial D, which carries a variant form of the D antigen that can be recognized as foreign by other individuals.
Step-by-step explanation:
If a D positive person makes anti-D antibodies, the most likely explanation is that this person is a partial D. Partial D individuals have a variant form of the D antigen that does not prompt anti-D antibody production within themselves but can be recognized as foreign if transfused into D negative individuals, who may then produce anti-D antibodies. This differs from D negative individuals who do not have the D antigen, and they would only produce anti-D antibodies upon exposure to Rh positive blood. A weak D phenotype generally does not elicit an antibody response against itself because even though the expression of the D antigen is weaker, it is still present and recognized as 'self' by the immune system. The term 'positional effects' refers to changes in an antigen's expression due to its location on the cell membrane, and 'transmissible genes' could refer to genes that can be passed from parent to offspring, both of which are not directly related to the production of anti-D antibodies in a D positive person.