The diagnostic [cytological or genetic] features used to identify the chromosomal alterations are:
a) Deletions:
• Cytologically, deletions lead to shorter chromosomes. In banded chromosomes, some bands are missed. It also leads to an unpaired loop, during meiosis in heterozygous condition.
• Genetically, when deletions are homozygous, they are lethal. In heterozygous position, pseudodominance can be seen. Sometimes a mutant phenotype is seen in heterozygotes. They can be detected using hybridization signals.
b) Duplications:
• Cytologically, duplications lead to longer chromosomes. In the heterozygous condition, it can lead to unique pairing structures. These structures might be simple unpaired loops or complicated twisted loop structures.
• Genetically, duplications can lead to asymmetric pairing and unequal crossing over events. Some duplications can produce mutant phenotypes. They can be detected using hybridization signals.
c) Inversions:
• Cytologically, inversions can be detected in banded chromosomes. In heterozygous condition, the twisted inversion loop is seen during homologous pairing. Pericentric inversions can result in a change in the position of the centromere.
• Genetically, when recombination occurs within the inversion, no viable products are formed in the heterozygous condition. The flanking genes show a reduction in the recombination frequency.
d) Reciprocal translocations:
• Cytologically, these translocations may be detected in banding chromosomes. The size of the chromosomes and the position of the centromere can also change due to these translocations.
• Genetically, new linkages are established. In the heterozygous condition, the unique cross structure is seen during meiotic pairing. Semisterility is seen with 50% of gametes being viable.