406
T. G. HSU
demonstrated that the X X Y constitution also results in sterile males.
More recently, Thuline and Nor by (1961) found a similar situation in
the cat. Whether this phenomenon is a generalized one for all mammals
remains to be seen.
It is rather startling to learn that Klinefelter's syndrome is not the
limit for additional sex chromosomes occurring in human populations.
Ferguson-Smith, Johnston and Handmaker (1960a) and later Barr and
Carr (1960) found cases of X X X Y individuals who have double sex
chromatin in the nuclei. Even an X X X X Y genotype has been found
(Fraccaro, Kaijser and Lindsten, 1960b). There is also a case of X X Y Y
anomaly (Muldal and Ockey, 1960). All these genotypes can be regarded as variations on the theme of the Klinefelter's syndrome.
The origin of X X Y individuals is popularly postulated as the result
of a union between a normal gamete and one which has a nondisjunction of the chromosome in question. It is rather difficult to conceive
an alternate possibility for the nondisjunction hypothesis. But it is
equally perplexing to use this mechanism to explain the X X X Y and
X X X X Y genotypes. T o reach an X X X Y constitution, both sperm and
egg should have had a nondisjunction in their history; or if one gamete
is a normal haploid, two successive nondisjunctions must have occurred
in the other. It appears somewhat curious that cell division in man is so
full of mistakes. If the anomaly is only confined to the sex pair, the
high frequency of abnormal individuals might be attributed to the
peculiar association between the X and the Y during meiosis. But as
we shall see later, autosomes likewise have a high frequency of nondisjunction. Neither is nondisjunction limited to the germ line. The
abundance of mosaics is good evidence to show that mitosis in somatic
tissues, too, may frequently go wrong. In Klinefelter's syndrome, for
example, X X Y / X X , X X Y / X Y and X X Y / X O mosaics have been
reported (Ford, Polani, Briggs and Bishop, 1959c; Hay ward, 1960;
Bergman et al., 1960; Nowakowski et al., 1960).
B. THE XO GENOTYPE
Each somatic cell of individuals with Turner's syndrome contains a
total of only 45 chromosomes, one X being missing (Ford, Jones,
Polani, de Almeida and Briggs, 1959b; Fraccaro, Kaijser and Lindsten,
1959; Tjio, Puck and Robinson, 1959). Thus Turner's syndrome is the
expression of the X O genotype. The negativeness of the sex chromatin
in such female individuals is explained by the singularity of the X
element, the same condition existing in normal males. In contrast to
the sex-determining mechanisms known in Drosophila in which the X O
genotype is expressed phenotypically as sterile male, the Turner's
T. G. HSU
demonstrated that the X X Y constitution also results in sterile males.
More recently, Thuline and Nor by (1961) found a similar situation in
the cat. Whether this phenomenon is a generalized one for all mammals
remains to be seen.
It is rather startling to learn that Klinefelter's syndrome is not the
limit for additional sex chromosomes occurring in human populations.
Ferguson-Smith, Johnston and Handmaker (1960a) and later Barr and
Carr (1960) found cases of X X X Y individuals who have double sex
chromatin in the nuclei. Even an X X X X Y genotype has been found
(Fraccaro, Kaijser and Lindsten, 1960b). There is also a case of X X Y Y
anomaly (Muldal and Ockey, 1960). All these genotypes can be regarded as variations on the theme of the Klinefelter's syndrome.
The origin of X X Y individuals is popularly postulated as the result
of a union between a normal gamete and one which has a nondisjunction of the chromosome in question. It is rather difficult to conceive
an alternate possibility for the nondisjunction hypothesis. But it is
equally perplexing to use this mechanism to explain the X X X Y and
X X X X Y genotypes. T o reach an X X X Y constitution, both sperm and
egg should have had a nondisjunction in their history; or if one gamete
is a normal haploid, two successive nondisjunctions must have occurred
in the other. It appears somewhat curious that cell division in man is so
full of mistakes. If the anomaly is only confined to the sex pair, the
high frequency of abnormal individuals might be attributed to the
peculiar association between the X and the Y during meiosis. But as
we shall see later, autosomes likewise have a high frequency of nondisjunction. Neither is nondisjunction limited to the germ line. The
abundance of mosaics is good evidence to show that mitosis in somatic
tissues, too, may frequently go wrong. In Klinefelter's syndrome, for
example, X X Y / X X , X X Y / X Y and X X Y / X O mosaics have been
reported (Ford, Polani, Briggs and Bishop, 1959c; Hay ward, 1960;
Bergman et al., 1960; Nowakowski et al., 1960).
B. THE XO GENOTYPE
Each somatic cell of individuals with Turner's syndrome contains a
total of only 45 chromosomes, one X being missing (Ford, Jones,
Polani, de Almeida and Briggs, 1959b; Fraccaro, Kaijser and Lindsten,
1959; Tjio, Puck and Robinson, 1959). Thus Turner's syndrome is the
expression of the X O genotype. The negativeness of the sex chromatin
in such female individuals is explained by the singularity of the X
element, the same condition existing in normal males. In contrast to
the sex-determining mechanisms known in Drosophila in which the X O
genotype is expressed phenotypically as sterile male, the Turner's