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Translocation heterozygotes:
Consequences for duplication / deletion products, balanced translocations, and speciation

    Consider two chromosome pairs 1& 2, a small acrocentric and a large metacentric,respectively [top left]. A reciprocal translocation exchanges a portion of their long arms, producing two translocated chromosomes, 1T and 2T (top, centre). After duplication of chromatids at meiosis, alignment of the centromeres of either chromosome requires formation of a cross-like pairing configuration such that the translocated segments as well as the non-translocated portions align properly (top, right).

    At segregation of Meiosis I, the tetrad can be segregated in either of two ways. In adjacent segregation (bottom, left), segregation of the adjacent centromeres to the same pole yield two duplication / deletion products, 1N 2T and 1T 2N, the first with an extra bit of Chromosome 1 and missing a bit of Chromosome 2, and the second missing part of 1 and having an extra bit of 2.
These duplication / deletion gametes will produce less viable offspring, for example in the heritable form of Down Syndrome that involves a reciprocal translocation between the ends of Chromosomes 14 & 21.

    In opposite segregation (bottom, right), segregation of centromeres from across the tetrad produces one product with two untranslocated (normal) chromosomes (1N  2N), and another with a balanced translocation, that is, two chromosomes
1T 2T that between them include all loci on both chromosomes in their translocated arrangement.
 
   Fertilization of a balanced translocation gamete by a normal gametes will produce a phenotypically normal individual, with meiosis complicated as above. Fertilization by another gamete with the same translocation stabilizes the chromosome rearrangement: all gametes will be viable. Balanced reciprocal translocation is thus one means of creating the novel chromosome configurations that are often seen in closely-related species. Finally, the expression of genes may be influenced by the surrounding genes, so that a translocated gene will have a different effect on the phenotype: this position effect may also be important in species differences.


Figure after ©2002 by Griffiths et al.; all text material ©2009 by Steven M. Carr