The Principles of Clinical Cytogenetics - Extra Materials - Springer

198 Kathleen Kaiser-Rogers and Kathleen Rao
Fig. 22. Models for meiotic pairing during which partial pairing is observed between the insertion chromosome
and its homolog.
insertion will have a liveborn child with an unbalanced karyotype (155). This risk might differ greatly
for individual insertions depending on the size of the inserted segment and the viability of the partial
trisomies and monosomies produced by the abnormal recombinant chromosomes.
Interchromosomal Insertions
Interchromosomal insertions involve the movement of material from one chromosome to another.
As discussed earlier, the inserted segment can be either direct or inverted relative to its original
position in the chromosome. The incidence of interchromosomal insertions is estimated to be approximately
1/80,000. Approximately 85% are inherited, usually from a carrier mother, and no fertility
differences were noted between the two sexes (156).
For relatively small inserted segments, it seems most likely that the homologs involved in the
rearrangement will pair independently (159). The inserted segment and its homologous region on the
normal chromosome can loop out, allowing full pairing of the uninvolved segments of the bivalents
(see Fig. 22). Independent 2:2 segregation of the homologs in these two bivalents can result in the
formation of four gamete types, two of which have a normal or balanced chromosome complement
and two of which have an unbalanced complement, one duplicated and one deleted for the inserted
segment. The theoretical risk, in this situation, would be 50% for producing a conceptus with an
unbalanced karyotype. The risk for having a liveborn abnormal child would depend on the viability
of the partial trisomy or partial monosomy of the inserted segment involved.
In the case of very long inserted segments, a quadrivalent containing an insertion loop might be
formed, allowing complete pairing of the chromosomes involved in the rearrangement (160). If no
crossover occurs within the insertion loop, the consequences are the same as described earlier for
nonpaired bivalents. If a crossover occurs within the insertion loop, however, recombinant chromosomes
that would lead to the production of gametes with duplications and deletions might be formed.
Once again, the risk for having a liveborn abnormal child will depend on the viability of the partial
trisomies and monosomies produced.
Regardless of whether complete pairing is achieved between the chromosomes involved in an
interchromosomal insertion or whether recombination takes place, compared to carriers of other chromosome
rearrangements, an insertion carrier’s risk of having an abnormal liveborn child is among