One of the most extensively studied trisomic series is that produced and studied by T. Tsuchiya
(who died in May, 1992) in barley. Trisomics are also known in Homo sapiens
(human beings). Trisomy for certain chromosomes causes definite morphological abnormalities in human beings.
Mongolism (Down's syndrome)
is one such feature, which
is common in children and is characterized by mental retardation, a short body, swollen tongue and eyelid folds resembling those of Mongolian races (for details, see Human Genetics
). Other cases of trisomy are also known in a number of different plant and animal species.
Production of trisomics.
Trisomics may originate spontaneously due to production of n
+ 1 type of gametes (Fig. 20.4) due to rare non-disjunction of a bivalent. However more often trisomics are produced artificially either by selfing triploids (produced by crossing diploids and autotetraploids) or by crossing these Iriploids as females with diploids as male (3x
In either case trisomics are obtained in large number and can be identified through phenotypic effects of individual chromosomes.