J. Lindsten

Chromosome breakage in humans exposed to methyl mercury through fish consumption. Preliminary communication.

Chromosome analysis was performed on cells from lymphocyte cultures from nine subjects with increased levels of mercury in their red blood cells and in four healthy controls. The elevated mercury levels were likely tal have originated from dietary fish with high levels of methyl mercury. A statistically significant rank correlation was found between the frequency of cells with chromosome breaks and mercury concentration. The biological significance of these findings is at present unknown.

Cytogenetic Aspects of Human Male Meiosis

During the first half of the present century human meiotic chromosomes were studied mainly in order to obtain information on the chromosome number of man. Different authors came to different conclusions regarding this number due to the fact that the only technique available for such studies was the use of histological sections from fixed testicular material, which does not give adequate spreading of the chromosomes (see, e.g., reviews by Kjessler81 and Hamerton55). In 1952 Mittwoch,103 using squash preparations, reported that there were 24 bivalents in primary spermatocytes from a subject with Down’s syndrome. These results were at that time believed to support the opinion that the normal chromosome number of man was 48, and mongoloid patients were therefore believed to have a normal chromosome constitution. The definite demonstration that there are 23 bivalents in normal human primary spermatocytes came in 1956, when Ford and Hamerton41 introduced hypotonic pretreatment of the testicular material before squashing. The mean chiasma count was found to be 55.9 (range 50–63), and on the basis of this result the total length of the human genome was estimated to be 27.9 morgans. The finding of 23 bivalents constituted an immediate confirmation of the observation made by Tjio and Levan143 on somatic cells that man has 46 chromosomes.

Presumptive direct insertion within chromosome 2 in man.

Most congenital structural autosomal aberrations reported in man are two point rearrangements, mainly reciprocal translocations. Some presumptive inversions (see Parrington & Edwards, 1971 ; Wahrman et al. 1972) and a presumptive chromatid interchange (Caspersson et al. 1971a) have also been found. Lejeune & Berger (1965) have suggested that some translocations may be three-point rearrangements (insertions) and may be responsible for the fact that an identical-looking chromosome rearrangement may be found in phenotypically normal as well as phenotypically abnormal members of the same family. Morphologically identical but genetically different chromosomes may be formed by crossing-over between an inserted segment and the normally located segment. Lejeune & Berger (1965) coined the term ‘aneusomie de recombinaison’ for such an event, and their hypothesis has got support also from findings by de Grouchy et al. (1966). The published cases were all translocations. I n the present paper we report on a family in which an intrachromosomal insertion seems to be the most likely explanation of the cytogenetic and clinical findings.

Timing of Sex Chromosome Replication in Somatic and Germ-Line Cells of the Mouse and the Rat

Timing differences of the G2 periods were found among bone marrow cells labelled with 3H-thymidine in vivo , cultured kidney cells labelled in vitro<

An explanation of the apparently increased incidence of moderate mental retardation in Turner's syndrome.

The distributions of scores achieved by 44 persons with Turners syndrome on two tests of intelligence are analyzed. Bimodal trends in these distributions are attributed to the effect on the Full Scale IQ of lower scores on specific, nonverbal subtests. Therefore, the diagnosis of Turners syndrome does not imply an increased risk of a general if moderate intellectual retardation but does indicate a need for careful evaluation of the range of abilities in affected persons.

RELEVANCE OF MEIOTIC STUDIES IN HUMAN CHROMOSOME IDENTIFICATION

Identification of normal mitotic chromosomes is an important and interesting cytological exercise although at present void of genetic interest, because no autosomal linkage group has been established in man. Analysis of mitotic karyotypes can be performed with different techniques that have varying degrees of resolution. None of these techniques is final per se and optimal results should he obtained only by their combined use. Analytical methods at our disposal are essentially: 1) Visual morphological analysis aided by use of estimates of metrical characters such as absolute and relative total length of chromosome arms, arm ratios, centromeric indexes, and similar characters. The possibility of automating all or part of this type of analysis is being actively considered by several groups. Combined use of automated scanning procedures and computers (Ruddle, 1964) should help to eliminate a major part of the human effort in this field. 2) Analysis of differentials in the chronology and patterns of DNA replication at the level of whole chromosomes and/or parts of chromosomes. Asynchrony of DNA replication is usually extreme in the case of the sex chromosomes in most species (FIGURE 1 ) . In addition, critical use of replication patterns is undoubtedly a major aid in identification of autosomal pairs (Schmid, 1963; German, 1964). 3) Analysis of the ultrastructure of whole chromosomes or of sections of them by the use of electron microscopy. The results obtained by this method up to now have been rather disappointing when used for the identification of single chromosomes (Barnicot & Huxley, 1961). However it should he more useful in its application to problems of chromosome topology within the cell. Electron microscope autoradiography should result in greatly improved resolution in the type of analysis mentioned in (2). 4) Various types of analyses that we could call generically “biophysical.” Among these, quoting at random: differential sensitivity of chromosome structures to physical or chemical agents which modify and/or interfere with their components; chemical composition and other characteristics of isolated chromosomes; and immunological properties of chromosomes. There is ample scope for the ingenuity of the experimental cell biologist in devising a wide range of similar approaches. Identification of abnormal chromosomes requires the same battery of technical procedures as used in the identification of normal chromosomes, but with the additional difficulties posed by the problems of phenotype-

An early behavioral description of a person with Turner's syndrome

A person with Turners syndrome was described by Dr. Charles Pears in thePhilosophical Transactions of the Royal Society, London, in 1805. The description included behavioral traits of mild temperament, absence of heteroxesual interests, and concern about social stigmatization. These traits are the object of current behavior genetic studies of persons with Turners syndrome.