Question

Which of the following is the condition of an extra chromosome in addition to its homologous pair?
A) Trisomy
B) Monosomy
C) Polyploidy
D)Nullisomy

Answer 1

Every species has a characteristic chromosome number. In most of its members this number normally remains constant. In a number of instances alterations do occur in the monoploid or diploid chromosome number. A zygote, that receives a gamete with genome mutation, will have genetic imbalance compared to normal zygotes.

Complete answer:
The chromosome number, present in the zygotic and somatic cells of animals, and also in the zygotic and sporophytic cells of plants, is known as diploid number (2n). The number, present in spores, gametes and gametophytic cells is known as haploid number (n). Haploid number is just half of the diploid number.

A set of chromosomes containing only one member of each homologous pair is called the basic chromosome set or basic genome. Organisms having only a single basic set in the somatic cells are called monoploids, and those having more than one basic set are called polyploids. Similarly, organisms with haploid sets of chromosomes in the somatic cells are called haploids and those having diploid sets are called diploids.

Ploidy changes:- Changes in the monoploid number are generally called genome mutations, and changes in the diploid number are called ploidy changes. They are occasional or incidental alterations in the diploid number of chromosomes. Essentially, they involve the gain or loss of either an individual chromosome or monoploid set of chromosomes.
Ploidy changes may occur during mitosis and meiosis. The major cause of ploidy changes is nondisjunction. Nondisjunction is either the non-separation of homologous chromosomes during the first meiotic anaphase, or the non-separation of sister chromatids during mitotic anaphase. It results in two daughter cells, one with an extra chromosome and the other with a missing chromosome.

Ploidy changes are two types, euploidy and aneuploidy. Euploidy is the condition in which the somatic chromosome number is an exact multiple of the basic (monoploid) number of the species and each chromosome is represented only once in each set. It involves the duplication or loss of one or more monoploid sets of chromosomes. Euploidy includes monoploidy, haploidy and polyploidy.
Polyploidy: It is the condition in which somatic cells contain more than two basic genomes (monoploid sets or basic chromosome sets). It includes triploidy(3x), tetraploidy(4x), pentaploidy(5x) etc. They are quite common among plants, but very rare among animals.

Aneuploidy- It is the condition in which individual chromosomes or chromosome pairs are added to or lost from the somatic (diploid) chromosome complement (2n). So, it does not represent an exact multiple of the monoploid value. Aneuploid organisms will always have incomplete genomes. Aneuploidy usually results from mitotic or meiotic nondisjunction. Aneuploidy is of two kinds, namely hyperploidy and hyperploidy. Hypoploidy involves the loss of chromosomes. It includes monosomy, double monosomy and nullisomy.

Monosomy:-In this, one chromosome is lost from the diploid set (2n-1).
Nullisomy:-It is the loss of a homologous pair of chromosomes from a somatic set (2n-2).
Hyperploidy involves the addition of extra chromosomes, which includes trisomy, double trisomy, tetrasomy, pentasomy etc.
Trisomy:-It is the addition of an extra chromosome to a diploid set (2n+1).

So, here the correct answer is (A) Trisomy.

Note: Polyploidy serves as a mechanism of speciation and evolution. It can be produced artificially by using colchicine, which is a chemical that prevents the formation of a mitotic spindle. Aneuploidy first observed in drosophila by Bridges (1916). Aneuploids are detected in Datura, Zea mays, Tobacco, Oats etc.