In this article we will discuss about the classification and origin of aneuploidy in chromosomes.
Classification of Aneuploidy:
Broadly, aneuploids may be divided into the following two classes (a) hypoploid and (b) hyperploid. In a hypoploid, the chromosome number is lower than the euploid number, whereas in a hyperploid it is greater than the euploid number. A detailed classification of aneuploidy is shown in Table 16.1, and discussed later in this chapter.
A peculiar type of aneuploidy is monosomictrisomic or quasi-diploid (2n-1+1, AABCCC) where the chromosome number of the individual is diploid, but the individual is, in fact, aneuploid since it is monosomic for one chromosome (the chromosome denoted by B in the notation AABCCC) and trisomic for the other chromosome (chromosome designated as C) at the same time.
Origin of Aneuploids:
Aneuploids originate from aneuploid gametes. Sometimes abnormal mitosis may also produce aneuploid sectors. Following are the main modes of the origin of aneuploids.
(1) During mitosis or meiosis, abnormalities, e.g., lagging chromosomes result in the formation of nuclei/cells with hypoploid chromosome numbers.
(2) Hypo- and hyper-ploid nuclei may be formed due to chromosome/chromatid nondisjunction during mitotic or meiotic cell division. Such gametes would, on union with normal (n) gametes, give rise to aneuploid progeny.
(3) In the polyploids, especially those with an odd number of chromosome sets, e.g., triploid, pentaploid etc., univalents are often observed during meiosis. The anaphase distribution of the univalents is irregular as a result of which, aneuploid gametes are produced.
(4) Aneuploid cells may be produced due to multipolar mitoses where distribution of chromosomes in the daughter cells is irregular. Such aneuploid cells may possess varying numbers of chromosomes and may occur in the same tissue forming chromosome mosaicism. Such type of aneuploidy is called as multiform-aneuploidy.
(5) Aneuploid gametes may be produced due to nondisjunction during postmeiotic divisions, such as during the formation of microgametophytes and megagametophytes.