Nullisomy

A diploid cell or individual missing both copies of the same chromosome. A cell or individual with one chromosomal type missing, with a chromosome number such as n-1 or 2n-2.

Nullisomy is a genome mutation where a pair of homologous chromosomes that would normally be present is missing. Thus, in nullisomy, two chromosomes are missing, and the chromosomal composition is represented by 2N-2. Individuals with nullisomy are referred to as nullisomics.

Nullisomy is caused by non-disjunction, during meiosis that causes two of the gametes to have no chromosomal material, leaving the other two gametes to have double the amount of chromosomal material (disomic). Due to the lack of genetic information, the nullisomic gametes are rendered unviable for fertilization.

Nullisomy is caused by a nondisjunction during cell division, particularly meiosis. Nondisjunction occurs when the homologous chromosomes (or sister chromatids) failed to separate. The outcome would be a gamete lacking a pair of homologous chromosome and another gamete gaining that pair. The gamete lacking that pair would give rise to a nullisomic or a nullisome whereas the gamate gaining that pair would give rise to a disomic. A disomic individual is one exhibiting disomy, i.e. an aneuploidy characterized by having one or more chromosomes present twice. Nullisomes, particularly higher animals, would not be able to survive. In plants, nullisomy may produce viable polyploid plants.

Although nullisomy is a lethal condition in diploids, an organism such as bread wheat, which behaves meiotically like a diploid although it is a hexaploid, can tolerate nullisomy. The four homoeologous chromosomes apparently compensate for a missing pair of homologs.