Principles of Genetics and Breeding

1.3.1 Introduction

The chromosomes are darkly stained small bodies present in the nucleus and determine the mechanism of inheritance.

• In 1902, W.S. Sutton and T. Boveri suggested that chromosomes were the physical structures which act as messenger of heredity.
• After specific staining, they can be clearly seen during mitotic cell division particularly during the metaphase.
• In all eukaryotic organisms including fishes, the DNA molecules in the nucleus are combined into proteins, mainly histones, to make chromosomes.
• The number of chromosomes in cells of different species is variable. In general, all the somatic cells of all the individuals of same species possess the same number of chromosomes.
• Both the size and shape of chromosomes differ in different species of fish. Chromosomes appear in the nucleus during cell division.
• The make up of a chromosome is considered to be highly coiled DNA. Every species of organism contains a characteristic number of chromosomes in each of its cells. For example, each cell in every human being has 46 chromosomes consists of 23 different pairs. Of the 23 pairs, one pair are chromosomes that determine sex, called sex chromosomes.
• The process of meiosis that occurs during the formation of eggs and sperm involves reduction of the number of chromosomes to one half, and each gamete receives half as many chromosomes as those contained in the cells of the parent.

• Genes are located on structures called “chromosomes”, which are located in the nucleus of every cell. Although there are some exceptions, chromosomes typically occur as pairs in animals, and for practical fish culture management, fish can be considered to be diploids (chromosomes occur in pairs).
• Some rare species of fish are tetraploids (chromosomes occur in sets of four),notably the salmonids and catostomids; however, for practical breeding work these species behave as if they were diploids, so they can be considered to be diploids.
• Because chromosomes occur in pairs, each gene also occurs as a pair. There are some exceptions in fish with morphologically distinct sex chromosomes, but few fish have such chromosomes. One chromosome of each pair comes from the fish’s mother, while the other comes from its father; this means one gene from each pair comes from the mother, while the other comes from the father. The two chromosomes that form a pair are called “homologues”.
• In many organisms there is a special pair of chromosomes which defines the sex of their carriers. For example in the XX-XY system present in humans and some fish, females have a pair of identical sex chromosomes (the X chromosomes) while males have one X chromosome and a reduced size Y chromosome. The other chromosome pairs are called autosomes. In many shellfish there are no identifiable sex chromosomes and, in the case of certain molluscs such as oysters, individuals may even change their sex during their lives.
• The chromosome number in the cells of the body is normally made up of two sets, one of maternal (female) and one of paternal (male) origin,and is termed as diploid and is denoted by 2n .
• A single set of chromosomes is found in a mature egg or in a spermatozoa is known as a haploid set and is denoted by n.
• A haploid set of chromosome is called genome.
• The fertilization process restores the diploid number of a species. Thus, a somatic cell possess two haploid sets of chromosomes. Each chromosome of a haploid set has an identical partner or homologue in the other haploid set. The two together constitute a homologous pair and are seen to pair up during the first prophase of meiosis.
• When more than two sets of chromosomes occur in cells, the condition is termed polyploidy. (Polyploidy involves the presence of extra sets of chromosomes. Many polyploidy are of sterile because their multiple sets of chromosomes seggregate irregularly in meiosis. However polyploides produced by chromosome doubling in inter specific hybrids may be fertile if their constitutent genomes seggregate independently).