We will consider four types of common aneuploid conditions in diploid individuals: nullisomy, monosomy, trisomy, and tetrasomy.
Nullisomy is the loss of both members of a homologous pair of chromosomes. It is represented as 2n − 2, where n refers to the haploid number of chromosomes. Thus, among humans, who normally possess 2n = 46 chromosomes, a nullisomic zygote has 44 chromosomes.
Monosomy is the loss of a single chromosome, represented as 2n − 1. A human monosomic zygote has 45 chromosomes.
Trisomy is the gain of a single chromosome, represented as 2n + 1. A human trisomic zygote has 47 chromosomes. The gain of a chromosome means that there are three homologous copies of one chromosome. Most cases of Down syndrome, discussed later in this chapter, result from trisomy of chromosome 21.
Tetrasomy is the gain of two homologous chromosomes, represented as 2n + 2. A human tetrasomic zygote has 48 chromosomes. Tetrasomy is not the gain of any two extra chromosomes, but rather the gain of two homologous chromosomes, so that there are four homologous copies of a particular chromosome.
More than one aneuploid mutation may occur in the same individual organism. An individual that has an extra copy of each of two different (nonhomologous) chromosomes is referred to as being double trisomic and is represented as 2n + 1 + 1. Similarly, a double monosomic individual has two fewer nonhomologous chromosomes (2n − 1 − 1), and a double tetrasomic individual has two extra pairs of homologous chromosomes (2n + 2 + 2).