Cytogenetics of Aneuploids by Gurdev S. Khush

By Gurdev S. Khush

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The products of misdivision may be two telocentrics, two isochromosomes, one for each arm, one telocentric and one isochromosome, or four telocentrics. Logical sources, then, of secondary trisomics are the genotypes with univalent chromosomes. Monosomic individuals always have one univalent chromosome at metaphase I, and it frequently misdivides at anaphase I. 7% of the microsporocytes of mono-5A of hexaploid wheat, and over half of these misdivisions were of the type that produced isochromosomes.

Monotelodisomics and telotrisomics may also yield secondary trisomics by conversion of the telocentric chromosome into the isochromosome. Thus, Lesley and Lesley (1941) obtained a secondary trisomic of tomato in the progeny of a plant deficient for one arm of a chromosome; and a secondary trisomic of maize (In + 5S-5S) was generated at a high frequency in the progeny of a telotrisomic for 5S (Rhoades, 1938, 1940). 42%. When used 30 2. 22%. The secondary chromosome 5S-5S did not transmit through the male but the telocentric -5S did.

Such designations as 2n + 1-18, which implies that the extra chromosome comprised the · 1 arm of the 1-2 chromosome and • 18 arm of the 17-18 chromosome, were therefore arbitrary. Without precise knowledge of the points of breakage and reunion of the parental translocation and in the absence of pachytene analysis, the content of the tertiary chromosome could not be known exactly. If the chromosomes had not been broken and united in their centromeres, the supposed 1-18 chromosome might have consisted of more than the arm of one and less than the arm of the other interchanged chromosome.

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