Faculty Bibliography

To investigate whether recurrent mutation has contributed to the high frequency of the beta E-globin gene in Southeast Asia, we used the haplotypes at three polymorphic restriction sites within and to the 3' side of the beta-globin gene to predict the framework of 23 beta E-globin genes. These haplotypes suggested that beta E-globin genes are present in two different beta-globin gene frameworks. DNA sequence determination of one gene representing each framework demonstrated that the same mutation (GAG leads to AAG at codon 26) was present in both frameworks. Moreover, the frameworks differed at three nucleotide positions known to be polymorphic in Mediterraneans. These polymorphic sites are located 70 nucleotides to the 5' side of the beta E mutation and 382 and 1032 nucleotides to the 3' side of it. The existence of the beta E mutation in these two beta-globin gene frameworks can be explained by (i) recurrent mutation giving rise to beta E-globin, (ii) a double crossing-over event, or (iii) two single crossing-over events. Mathematical analysis suggests that the first alternative, recurrent mutation of G leads to A at the first nucleotide of codon 26, is most likely.

For a linked marker locus to be useful for genetic counseling, the counselee must be heterozygous for both disease and marker loci and his or her linkage phase must be known. It is shown that when the phenotypes of the counselee's previous children for the disease and marker loci are known, the linkage phase can often be inferred with a high probability, and thus it is possible to conduct genetic counseling. To evaluate the utility of linked marker genes for genetic counseling, the accuracy of prediction of the risk for a prospective child with a given marker gene to develop the genetic disease and the proportion of families in which a particular marker locus can be used for genetic counseling are studied for X-linked recessive, autosomal dominant, and autosomal recessive diseases. In the case of X-linked genetic diseases, information from children is very useful for determining the linkage phase of the counselee and predicting the genetic disease. In the case of autosomal dominant diseases, not all children are informative, but if the number of children is large, the phenotypes of children are often more informative than the information from grandparents. In the case of autosomal recessive diseases, information from grandparents is usually useless, since they show a normal phenotype for the disease locus. If we use information on the phenotypes of children, however, the linkage phase of the counselee and the risk of a prospective child can be inferred with a high probability. The proportion of informative families depends on the dominance relationship and frequencies of marker alleles, and the number of children. In general, codominant markers are more useful than are dominant markers, and a locus with high heterozygosity is more useful than is a locus with low heterozygosity.

Five generations of a family with autosomal-dominant cone-rod dystrophy (CRD) with complete penetrance have been previously studied extensively clinically. The young members of this family were reevaluated, and blood from 73 available family members was studied with 17 biochemical and serological markers. A total of 25 relatives was found to be affected. Linkage between the gene for CRD in this family and the markers studied could not be established by maximum likelihood analysis.

Maximum likelihood analysis for linkage between autosomal dominant aniridia and 12 biochemical and serological markers in a single large family showed a probable linkage between autosomal dominant aniridia and the enzyme acid phosphatase-1. The presence of an autosomal dominant aniridia gene linked to acid phosphatase-1 on chromosome arm 2p and the existence of an aniridia syndrome resulting from deletion of band 13 of the short arm of chromosome 11 establishes a chromosome basis for genetic heterogeneity of aniridia phenotypes.

Using the steady-state distribution of recessive lethal gene the probability of finding the elevated frequency of Tay-Sachs (TSD) gene among Ashkenazic Jews is computed. For various estimated values of mutation rate and population size, this probability is found to be statistically significant. This probabiltiy, in fact, becomes even smaller if a steady influx of foreign genes into the Ashkenazic Jewish populations is considered. It is suggested that heterozygote advantage together with random genetic drift should be considered as the most probable mechansim for the elevation of TSD gene frequency among the Ashkenazic Jews.

Genetic differentiation among the 22 Dhangar castes of Maharashtra, India, is studied using data on several polymorphic serological and biochemical loci employing Nei's distance measures. The intercaste genetic distances and the coefficient of gene diversity among these caste groups are found to be rather small. The relationship between gene identity and geographic distance is also studied empirically from the gene frequency data. All these analyses indicate that genetic differentiation among the Dhangar castes is at its very early stage only. It is also suggested that these caste groups probably originated from a common stock and are in the process of differentiation by fission with very little intercaste migration in the recent past.