1. A disease cause by a very small genetic change. Most cases result from the deleting of three bases in a gene for a protein called cystic fibrosis transmembrane changes which in a gene's DNA sequence can change proteins by altering their amino acid sequences which may affect one's phenotype.
2. It causes a mutation in the number of chromosomes, either extra chromosomes or missing chromosomes. Nondisjunction during meiosis can cause chromosomal disorders , such as a trisomy 21 (down syndrome) , Tuners syndrome , and Klinefelter's syndrome. It is a monosomy of the X sex chromosome.