Tamar Schaap

Chiasmata and the breeding system in wild populations of diploid wheats

Seven populations of the selfer Triticum longissimum (= Aegilops longissima) and five populations of the closely related outbreeder T. speltoides (= Ae. speltoides) were scored for chiasma frequencies in pollen mother cells. The populations of the selfer have significantly higher frequencies of chiasmata than the outbreeding populations. This difference becomes even clearer when interstitial chiasmata alone are compared. It is argued that an optimal degree of effective recombination is achieved by the balance between outbreeding and interstitial chiasmata. — There are wider differences between the selfing populations than between the outbreeding populations, but the differences between families (within populations) are small in both species. Variation between plants within families seems to be lower in the selfer, but nevertheless high enough to be inexplicable on the basis of selfing alone. — Small populations subject to hardship conditions show a higher frequency of chiasmata than others.

Mucolipidosis type IV: ganglioside sialidase deficiency.

Summary A solubilized sialidase is partially deficient in cultured fibroblasts derived from skin biopsies of four mucolipidosis IV patients. Fibroblasts from two obligate heterozygotes also have sialidase activity lower than normal controls. Membrane-bound sialidase activity is not affected in this disease. Based on previous and present findings, we propose that this solubilized activity is probably lysosomal origin. Sialidase activity in mucolipidosis IV cells is normal when neuraminlactose is used as substrate. Mixing cell homogenates from mucolipidosis IV patients and from controls results in the expected combined sialidase activity, indicating the absence of an internal inhibitor in the deficient cells. It is therefore suggested that the mutation in mucolipidosis IV specifically affects a lysosomal ganglioside sialidase, while the remaining non-lysosomal sialidases partially mask this deficiency.

Incidence of mucopolysaccharidoses in Israel: is Hunter disease a "Jewish disease"?

SummaryHunter disease in Israel occurs among Ashkenazi, Oriental, and Sephardic Jews and is by far more frequent than Hurler disease. None of the other mucopolysaccharidoses has been diagnosed in Ashkenazi Jews. The possibility of Hunter disease being a “Jewish” disease is discussed.

Dual information in DNA and the evolution of the genetic code

Abstract Some of the molecules acting on DNA do so at specific points along the DNA macromolecule and consequently require recognition sites. It seems likely that at least some of the recognition sites are situated intragenically. It is suggested that the information of the recognition sites is carried by a code using the four nucleotides to form words which differ in length and structure from the conventional codons. The same recognition site may therefore correspond to different amino acid sequences. On the other hand, the redundancy of the genetic code permits the correspondence between the same amino acid sequence and different recognition sites. The presence or absence of a recognition site in a particular nucleotide sequence may have an adaptive value. The establishment of homologous and neutral mutations, usually attributed to random processes, can be accordingly understood as the result of natural selection acting at the level of the DNA rather than that of the polypeptide.

Hunter syndrome in Jews in Israel: further evidence for prenatal selection favoring the Hunter allele

SummaryAmong all the Jewish families with Hunter patients in Israel, 10 were Ashkenazi or Moroccan in origin. In those families, there was a paucity of new mutations. In addition, a significant deviation of the segregation ratio between the Hunter gene and the normal allele was demonstrated among the offspring of heterozygous mothers or siblings of affected children in these families. These results confirm and extend our previous observations suggesting selection in favor of the X chromosome carrying the Hunter allele among Ashkenazi and Moroccan Jews.

Cytological evidence of defective template in the fragile X chromosome

In cells of fragile X patients, the changed X segment may appear as a poorly staining region or a gap, or as a deletion, involving one or both chromatids. To find out whether the fragile site represents ah incompletely replicated DNA sequence, as has been suggested recently, we analyzed the four chromatids of methotrexate-induced endoreduplicated fragile X chromosomes. Our main observations were: (1) a deleted chromatid was never internal to a poorly staining one; (2) an endoreduplicated X chromosome with a fragile site never included a normal chromatid. These results can be explained by assuming that DNA at the fragile site, when replicated in the presence of methotrexate, may undergo defective replication and give rise to improperly packaged chromatin, appearing as a chromatid with a poorly staining region or a gap in the following metaphase. The same DNA may fail to function as a template in the following S-phase and give rise to a chromatid with a single-stranded segment, appearing as a deleted chromatid in the following metaphase.

The role of recombination in the evolvement of the fragile X mutation.

SummaryThe frequency of recombination in the regions adjacent to the fragile X locus was studied in two groups of carriers: daughters of transmitting males and transmitters of maternally inherited fragile X chromosomes. Approximately one-half of the offspring of the former and one quarter of the off-spring of the latter are recombinant. Recombinants and parentals are equally distributed among affected and normal off-spring in the two groups. These results indicate that crossing-over at or around the fragile X locus occurs in every meiosis in doughters of transmitting males, although the recombinant chromatids do not necessarily carry the fragile X mutation. Hence, crossing-over is unequivocally associated with, but is not the direct cause of, the transition from the primary genetic lesion to the final mutation.

Werdnig‐Hoffmann disease on Reunion Island: a founder effect?

Sirs, A preponderance of Werdnig-Hoffman (W-H) disease has been described in several populations: Pearn (1 973) found a gene frequency of 0.000625 in North East England; Fried & Mundel (1977) reported a gene frequency of 0.05 in a small isolate the Karaite community in Israel; Pascalet-Guidon et al. (1984) found a gene frequency of 0.0281 in another isolate a European group on Reunion Island. A high frequency of an autosomal recessive lethal gene can result from any of several genetic effects and processes, e.g. genetic drift, founder effect, selection favoring the heterozygote, gametic selection favoring the deleterious allele, etc. Pearn (1973) favors the hypothesis of heterozygote advantage for the W-H allele in North East England. Pascalet-Guidon et al. (1984) favor the founder effect hypothesis for the European population of Reunion Island, because 19 affected children in 13 sibships can be traced back to a single ancestral couple in the 17th century. The aim of this communication is to use the Reunion Island pedigree in an attempt to prove that the preponderance of W-H disease in that population cannot be attributed to the founder effect alone. According to Pascalet-Guidon et al. (1 984), the actual European population of Reunion Island includes 100,000 descendants of the original settlers in the 17th century. Of these, 90,000 are “Gros Blancs” and 10,000 “Petits Blancs”, the separation between them having occurred in the 18th century. All the W-H patients found by Pascalet-Guidon and co-workers are Petits Blancs. The pedigree includes one couple, Julien R. and Perrine C. , who immigrated to Reunion Island in 1642, and ten generations of their descendants. The last generation includes 13 sibships, each including at least one W-H patient. The preceding generation includes the parents of these sibships, 22 of whom can be traced back to the same ancestral couple. The a priari probabilities of these 22 parents of being carriers were calculated by the classical method of path coefficients (Li 1955) and are represented in Table 1. These probabilities can be used to estimate the expected total number of descendants in the same generation. For instance, a heterozygote whose probability for heterozygosity is 1/512, theoretically represents a cohort of 512 individuals, 511 of whom are non-heterozygous. According to the pedigree, none of the 22 obligatory heterozygotes is the product of a consanguineous marriage. Furthermore, no W-H patients are recorded among the sibs of these 22 carriers. Therefore, it can be assumed that these 511 non-hetero-

Confidentiality in counseling for X-linked conditions

Genetic counseling in a synthetic fragile‐X family is used to illustrate some dilemmas which may arise from the genetic counselors wish to respect the patients confidence. What is the counselors obligation towards family members who are unaware that they are carriers? Should the counselor try to avoid disclosing information concerning such family members to the patient, and if so ‐ how? May the future father of an affected fetus be prevented from participating in the reproductive decisions concerning that fetus, out of respect for the mothers wishes?