What happens during meiosis that ultimately results in a defect characterized by the deletion of chromosomes?
You must be referring here to the phenomenon of non-disjunction that occasionally takes place during meiosis. Under normal conditions, meiosis proceeds in 2 parts: Meiosis1: Double stranded homologous chromosomes separate, giving rise to 2 haploid cells with duplicate chromosomes. Meiosis 2: The 2 chromatids then separate, giving rise to another 2 cells. Occasionally, non-disjunction occurs. It can take place in either Meiosis 1 or 2. Non disjunction 1: Homologous chromosomes fail to segregate. Non disjunction 2: Sister chromatids fail to segregate. In both cases, the process will give rise to cells that are characterized by either trosomy (3 chromosomes for the affected pair, so what you refer to as “chromosome addition”) or monosomy (1 chromosome per affected pair, so what you refer to as “chromosome deletion”). Trisomic cells can sometimes give rise to viable offspring (e.g cells trisomic for chromosome 21 give rise to individuals with Down’s Syndrome). Monosomies are usually lethal