• In general, organisms need a balanced number of chromosomes: equal numbers of each chromosome.  This condition is called “euploid”.But,  the Chromosomal alterations leads to formation of trisomic,monosomy            condition etc..,

• Having an extra chromosome (trisomic) or missing a chromosome (monosomic) is usually lethal.  The chromosomes in this case are unbalanced, and not equal.  This condition is called “aneuploid”.
• .Most diploids don’t survive as haploids, because they are usually heterozygous   for recessive lethal alleles.  Similarly, making an organism homozygous at most genes (through repeated matings between close relatives) is usually lethal. 

• most of trisomies ,monosomies leads to formation a number of physical malformations and svere symtoms of neurological conditions like  anenchaly,hydrocephaly gross analnomied of brain  etc..,in the fetus
• Thus Chromosomal alterations often result in spontaneous abortion.

     Example

            Incompatible genetic constitution :

         Trisomy condition like        47,xyy ,                 +47,xy ,21+

                              45,x0, Catcry syndrome,hirschoff syndrome etc

                          92,xxyy

                                  ,47,xy,18+ Edward syndrome

                                  47,xy,13+ pataus syndrome