410
T . C .
H S U
chromosome save one centromere. The only true monosomic is still
the X O genotype. One of the reasons for the scarcity of monosomy is
probably due to the expression without inhibition of the lethal genes
in the monosomic chromosomes. Perhaps this also accounts for the
scarcity of quasidiploids {2n-\~ 1 — 1).
F . T R A N S L O C A T I O N S
The case reported by Turpin et al. (1959) just mentioned was interpreted as the result of a translocation between chromosome 22 and an
acrocentric in group D (13-15). The first case of a translocation in
man was found by Polani, Briggs, Ford, Clarke and Berg (1960) in
a mongoloid girl who displayed 46 chromosomes instead of the usual
47. Idiogram analysis showed that there were only 5 chromosomes
belonging to group D , 4 to group G, and 17 to group C. The extra
chromosome in the last group probably represented a fusion between
chromosomes 14 (?) and 21. Since then many cases of mongolism have
been found to bear similar translocations (e.g. Hamerton, Cowie,
Giannelli, Briggs and Polani, 1961).
Also of interest to the study of translocation is the family reported
by Moorhead, Mellman and Wenar (1961). According to these authors,
the father had a normal male karyotype, but the mother had 45 chromosomes, missing one of the smallest acrocentrics (probably 22) and one of
the longer acrocentrics (group D ) . An extra chromosome belonging to
the group C was found. The situation was very similar to the case
reported by Turpin et al. (1959), i.e. a translocation between 22 and
13 or 14. The couple studied by Moorhead and associates had six
children, four of whom had the same karyotype as the mother. The
fifth member of the sibship was a normal child, and the sixth a mongoloid with 47 chromosomes. The first four children were mentally
retarded and had speech defects. The mother, though with the same
chromosome constitution, was normal. The inheritance of the same
translocated chromosome through the mother is not difficult to explain,
nor is the fifth child who was normal. The mongoloid child was
interpreted as the result of nondisjunction of chromosome 21, independent of the translocation.
Theoretically, there should be endless possibilities of translocations,
but so far only cases of fusion of 2 entire chromosomes have been found.
Reciprocal translocations, involving only parts of chromosomes, are
difficult to detect unless the exchanged parts are highly unequal in
length. Furthermore, such translocation heterozygotes are expected
to be phenotypically normal. The present trend of cytogenetic investigations of man is to seek phenotypic anomalies and then to analyze
T . C .
H S U
chromosome save one centromere. The only true monosomic is still
the X O genotype. One of the reasons for the scarcity of monosomy is
probably due to the expression without inhibition of the lethal genes
in the monosomic chromosomes. Perhaps this also accounts for the
scarcity of quasidiploids {2n-\~ 1 — 1).
F . T R A N S L O C A T I O N S
The case reported by Turpin et al. (1959) just mentioned was interpreted as the result of a translocation between chromosome 22 and an
acrocentric in group D (13-15). The first case of a translocation in
man was found by Polani, Briggs, Ford, Clarke and Berg (1960) in
a mongoloid girl who displayed 46 chromosomes instead of the usual
47. Idiogram analysis showed that there were only 5 chromosomes
belonging to group D , 4 to group G, and 17 to group C. The extra
chromosome in the last group probably represented a fusion between
chromosomes 14 (?) and 21. Since then many cases of mongolism have
been found to bear similar translocations (e.g. Hamerton, Cowie,
Giannelli, Briggs and Polani, 1961).
Also of interest to the study of translocation is the family reported
by Moorhead, Mellman and Wenar (1961). According to these authors,
the father had a normal male karyotype, but the mother had 45 chromosomes, missing one of the smallest acrocentrics (probably 22) and one of
the longer acrocentrics (group D ) . An extra chromosome belonging to
the group C was found. The situation was very similar to the case
reported by Turpin et al. (1959), i.e. a translocation between 22 and
13 or 14. The couple studied by Moorhead and associates had six
children, four of whom had the same karyotype as the mother. The
fifth member of the sibship was a normal child, and the sixth a mongoloid with 47 chromosomes. The first four children were mentally
retarded and had speech defects. The mother, though with the same
chromosome constitution, was normal. The inheritance of the same
translocated chromosome through the mother is not difficult to explain,
nor is the fifth child who was normal. The mongoloid child was
interpreted as the result of nondisjunction of chromosome 21, independent of the translocation.
Theoretically, there should be endless possibilities of translocations,
but so far only cases of fusion of 2 entire chromosomes have been found.
Reciprocal translocations, involving only parts of chromosomes, are
difficult to detect unless the exchanged parts are highly unequal in
length. Furthermore, such translocation heterozygotes are expected
to be phenotypically normal. The present trend of cytogenetic investigations of man is to seek phenotypic anomalies and then to analyze