La Griffe has nice post on the Black-White familial mental retardation gap. As Steve Sailer notes, the MR gap is what led Jensen to begin investigating the race-IQ connection. Jensen makes a point of this gap in his infamous Harvard Educational Review paper:
Socioeconomic Level and Incidence of Mental Retardation. Since in no category of socioeconomic status (SES) are a majority of children found to be retarded in the technical sense of having an IQ below 75, it would be hard to claim that the degree of environmental deprivation typically associated with lower-class status could be responsible for this degree of mental retardation. An IQ less than 75 reflects more than a lack of cultural amenities. Heber (1968) has estimated on the basis of existing evidence that IQs below 75 have a much hither incidence among Negro than among white children at every level of socioeconomic status, as shown in Table 3. In the two highest SES categories the estimated proportions of Negro and white children with IQs below 75, are in the ratio of 13.6 to 1. If environmental factors were mainly responsible for producing such differences, one should expect a lesser Negro-white discrepancy at the upper SES levels. Other lines of evidence also show this not to be the case. A genetic hypothesis, on the other hand, would predict this effect, since the higher SES Negro offspring would be regressing to a lower population mean than their white counterparts in SES, and consequently a larger proportion of the lower tail of the distribution of genotypes for Negroes would fall below the value that generally results in phenotypic IQs below 75
Anyways, Statsquatch referred me to a paper  which provides ethnoracial rates of Mild MR for the state of Florida:
With regards to the paper, some of the authors’ comments are interesting:
Addressing all of these potential pathways related to maternal education may still not be enough to eliminate the large racial disparities found in mental retardation and among mild mental retardation placements in particular. Compared to White children, the prevalence of mild and moderate/severe mental retardation among Black children was 4.5 and 2.1 times higher. These racial disparities have persisted, even after controlling for sociodemographic factors (Yeargin-Allsopp et al., 1995). To fully address this problem, we may need to consider intergenerational risk factors, which involve the mother’s own developmental history. Maternal intergenerational factors clearly play a role in low birthweight (Emanuel, 1986; Emanuel, Filakti, Alberman, & Evans,1992), and it is likely that other aspects of development, including cognitive development, also have an intergenerational component (Chapman & Scott, 2001). Intergenerational factors may explain, in part, why race differences in mental retardation placements and risk factors associated with mental retardation, such as low birthweight, have persisted, even after controlling for maternal factors, such as age, education, SES, and prenatal care (G. Alexander, Kogan, Himes, Mor, & Goldenberg, 1999; Din-Dzietham & Hertz-Picciotto, 1998; Foster, Wu, Bracken, Semenya, & Thomas, 2000; Migone, Emanuel, Mueller, Daling, & Little, 1991; Starﬁeld et al., 1991)
I have no doubt that a significant portion of the intergenerational factors are environmental or are due to g x e interactions. I did my time assisting with inner city Special Ed classes and have had the opportunity to meet quite of few of those parents; needless to say, some of the proclivities of the parents weren’t a recipe for normal intrauterine development. I also have no doubt that a significant portion is genetic but not directly related to the genetics of IQ. That is, a large portion of severe and moderate MR gap is due to medical genetic differences . That said, given that a standard distribution of intelligence would necessitate that some individuals fall in the MR category based on intellectual differences alone, and given that there is a B-W g gap, it’s hard to see how some of the MR gap could not be a function of IQ differences. And, of course, to the extent that it is, we are confronted by Jensen’s point.
 Jensen, 1969. How much can we boost IQ and scholastic achievement?
 Chapman, et al., 2008. Public Health Approach to the Study of Mental Retardation
 Plomin and Spinath, 2004. Intelligence: Genetics, Genes, and Genomics
A second issue concerns the relationship between the normal and abnormal. For example, to what extent is mild mental retardation (MMR) genetically distinct from the rest of the distribution of intelligence? Surprisingly, no twin or adoption studies of MMR have been reported until recently (see the next paragraph). More than 200 rare single-gene disorders include mental retardation, often severe retardation, as a symptom (Zechner et al., 2001), and many chromosomal causes of mental retardation are also known (Plomin, DeFries, et al., 2001), including microdeletions of bits of chromosomes (Baker et al., 2002; Knight et al., 1999). In general, many of the single-gene mutations tend to be spontaneous in the affected individual as are most of the chromosomal anomalies. That is, these DNA causes of severe mental retardation are not usually inherited. Although no twin studies of severe mental retardation have been reported, an interesting sibling study shows no familial resemblance. In a study of over 17,000 children, 0.5% were moderately to severely retarded (Nichols, 1984). As shown in Figure 4 (dotted line), siblings of these retarded children were not retarded. The siblings’ average IQ was 103, with a range of 85 to 125. In other words, moderate to severe mental retardation showed no familial resemblance, a finding implying that mental retardation is not heritable. In contrast, siblings of mildly retarded children (1.2% of the sample) tend to have lower than average IQ scores (see Figure 4, solid line). The average IQ for these siblings of mildly retarded children was only 85. Similar findings—that MMR is familial but moderate and severe retardation are not familial—also emerged from the largest family study of MMR, which considered 80,000 relatives of 289 mentally retarded individuals (Reed & Reed, 1965).