12. G E N E T I C C Y T O L O G Y
411
the chromosomes, a rather sensible method. Nevertheless, this method
screens out all phenotypically normal persons. If the family studied by
Moorhead et al. (1961) had not had so many mentally defective
children, the karyotype of the mother would not have been studied.
If the family described by Fraccaro, Kaijser and Lindsten (1960a) had
not had an abnormal mongoloid child with 46 chromosomes, the abnormal karyotype of his father would likewise be unnoticed. O n the
other hand, if a translocation does involve some loss of genie material
and does express some abnormality phenotypically, but the swapped
pieces are similar in length, cytological abnormality cannot be detected
by the present method of idiogram analysis. Based on these considerations, it is conceivable that a large number of translocation heterozygotes may exist in human populations but will not be discovered
until techniques of studying pachytene chromosomes can be improved.
Fusion between acrocentrics or telocentrics into a biarmed chromosome, the so-called Robertsonian effect, is a common procedure in
evolution. In cell populations in vitro, formation of metacentrics from
telocentrics have been witnessed repeatedly (Hsu, Billen and Levan,
1961). Terminal associations between acrocentric elements in man have
been noticed by many experienced cytologists, and a special note was
reported by Ferguson-Smith and Handmaker (1961). The terminal
association is probably a relic of the association with the nucleolus
during interphase. Since all five pairs of the acrocentrics (13-15, 21
and 22) have been seen to bear satellites or nucleolar organizers at one
time or another, the probability of their short arms being proximal to
another is better than a random chance, hence more translocations
among them. Fig. 4 diagrammatically represents some changes that
might occur between two acrocentrics that bear satellites. According
to Ohno et al. (1961), the nucleolus organizers, or secondary constrictions, are liable to break. Since these chromosomes are more often
associated than not, breakages at the vicinity of the constrictions may
result in a switch of satellites (Fig. 4a) or even a fusion between long
arms (Fig. 4b and c ) .
G. P O L Y P L O I D Y
Probably most remarkable among human chromosome anomalies
was that of a triploid child (Book and Santesson, 1960). This is the first
polyploid individual known in mammals other than a few cases found
in embryonic rodents (Austin, 1960). Cells from skin cultures of the
male child each contained 69 chromosomes, forming three complete
haploid sets with an X X Y sex chromosome constitution. More data
on this case was presented by Book (1961). Apparently the patient
411
the chromosomes, a rather sensible method. Nevertheless, this method
screens out all phenotypically normal persons. If the family studied by
Moorhead et al. (1961) had not had so many mentally defective
children, the karyotype of the mother would not have been studied.
If the family described by Fraccaro, Kaijser and Lindsten (1960a) had
not had an abnormal mongoloid child with 46 chromosomes, the abnormal karyotype of his father would likewise be unnoticed. O n the
other hand, if a translocation does involve some loss of genie material
and does express some abnormality phenotypically, but the swapped
pieces are similar in length, cytological abnormality cannot be detected
by the present method of idiogram analysis. Based on these considerations, it is conceivable that a large number of translocation heterozygotes may exist in human populations but will not be discovered
until techniques of studying pachytene chromosomes can be improved.
Fusion between acrocentrics or telocentrics into a biarmed chromosome, the so-called Robertsonian effect, is a common procedure in
evolution. In cell populations in vitro, formation of metacentrics from
telocentrics have been witnessed repeatedly (Hsu, Billen and Levan,
1961). Terminal associations between acrocentric elements in man have
been noticed by many experienced cytologists, and a special note was
reported by Ferguson-Smith and Handmaker (1961). The terminal
association is probably a relic of the association with the nucleolus
during interphase. Since all five pairs of the acrocentrics (13-15, 21
and 22) have been seen to bear satellites or nucleolar organizers at one
time or another, the probability of their short arms being proximal to
another is better than a random chance, hence more translocations
among them. Fig. 4 diagrammatically represents some changes that
might occur between two acrocentrics that bear satellites. According
to Ohno et al. (1961), the nucleolus organizers, or secondary constrictions, are liable to break. Since these chromosomes are more often
associated than not, breakages at the vicinity of the constrictions may
result in a switch of satellites (Fig. 4a) or even a fusion between long
arms (Fig. 4b and c ) .
G. P O L Y P L O I D Y
Probably most remarkable among human chromosome anomalies
was that of a triploid child (Book and Santesson, 1960). This is the first
polyploid individual known in mammals other than a few cases found
in embryonic rodents (Austin, 1960). Cells from skin cultures of the
male child each contained 69 chromosomes, forming three complete
haploid sets with an X X Y sex chromosome constitution. More data
on this case was presented by Book (1961). Apparently the patient