In trisomic individuals one of the pairs of homologues will have an additional member. Trisomic individuals exhibit a trivalent together with the normal bivalents during meiotic metaphase. These are designated as 2n+1. There could be a double trisomy in which case two of the bivalents will have one additional member.

At the metaphase of meiosis I, there will be two trivalents. The number of possible trisomy in individuals equals the number of haploid set. For eg. in barely where the haploid number is 7, seven trisomies are possible.

In plants, trisomy has been studied extensively in Datura, maize, tomato and Nicotiana. In animals, trisomy has been reported in Drosophila and others including human beings.

Blackslee and his associates have described a series of trisomy in the Jimson weed (Datura stramonium) belonging to the family Solanaceae. The base chromosome number of this species is 12(n=12) and 12 primary trisomies are possible. In this case instead of 12 bivalents at the metaphase of meiosis I, there will be 12 trivalents. Each one of this is associated with a distinct fruit character and can be separated from the others.

ADVERTISEMENTS:

Blackslee has named these trisomies (as represented by the fruit character) as follows – Rolled, Glossy, Buckling, Elongate, Echinus, Cocklebur, Microcarpic, Reduced, Poin- settia, Spinach, Globe and Hex. Secondary and teritary trosomics have also been reported from Datura.

In Drosophila, trisomy has been reported for the 4th chromosome; the fly can survive this and breed also.

Trisomy has also been reported in human beings leading to many abnormali­ties, both in the physical features as well as in mental faculties. Down’s syn­drome (mongoloid idiocy) was first described by Langdon Down in 1866. Affected individuals are short in stature, have slanting eyes, broad skulls, and stubby hands.

They have low mental faculties. It was Lejuene (1959) who first associated this disorder with a chromosomal abnormality. Cyto- logical investigations have revealed that in Down’s syndrome, chromosome 21 is produced due to primary non disjunction occurring at meiosis in the mother.

ADVERTISEMENTS:

The two chromatids do not separate and as a result the egg has both the chromatids of 21, while the first polar body receives none. The incidence of Down’s syndrome occurs quite frequently (1:700) among European people, (see later for details)

Other trisomic abnormalities in human beings include the Patau syndrome (Patau 1960), Edward’s syndrome (Edward J.H, 1960) and Kleinfeter’s syn­drome (H.F. Kleinfelter 1942). The first two are autosomal trisomies while the third one is a trisomy for the sex chromosomes.

Tetrasomy: In tetrasomy, a particular bivalent has two extra members; in other words there will be four homologous chromosomes in one pair, Sym­bolically tetrasomy is represented as 2n+2. Tetrasomics have been indentified in wheat (E.R. Sears).