8122

• UCREFRP Catalog Number: 8122
• Author: Kapuscinski, A. R. and L. D. Jacobson.
• Title: Genetic Guidelines for Fisheries Management.
• USFW Year: 1987.
• USFW - Doc Type: St. Paul, Minnesota.
• Copyright Material: NO

Chromosomes - number
The number of chromosomes in somatic cells (cells that are not eggs or sperm) differs widely
among species but is relatively constant among individuals of the same species. The number of
chromosomes in a cell is normally an even number (denoted 2n) because both parents contribute an
equal number of chromosomes to their progeny. The number of chromosomes contributed by each
parent through either the egg or sperm is called the haploid number and is denoted by n. Normal
individuals of most species are said to be diploid because they have 2n chromosomes in each somatic
cell.
The chromosomes in a haploid cell (egg or sperm) differ from one another in size, shape, banding
patterns, and in the genes they carry. In a cell of a diploid individual, pairs of homologous
chromosomes are distinguishable. Homologous chromosomes originate from the gametes (eggs and
sperm) of different parents but have the same general shape and carry the same genes.
Variation in the ploidy of individuals is common but is usually associated with reduced viability and
fertility. Individuals with a single haploid complement of chromosomes are haploid or monoploid; those
with more than two haploid complements are polyploid. There are various types of polyploids.
Triploids, for example, have 3n chromosomes and tetraploids have 4n chromosomes.
Triploid individuals are usually sterile because the triploid genome cannot be split into two parts
with equal numbers of chromosomes; inviable gametes result. Tetraploid genomes can be split into two
equal parts (each with 2n chromosomes) and viable tetraploid species of fish are common. Tetraploidy is
thought to have played an important part in salmonid evolution (Turner 1984).
Aneuploidy is another kind of variation in chromosome number where there are extra or too few
copies of a single chromosome. Individuals with three copies of a particular chromosome are said to be
trisomic; those with four copies are said to be tetrasomic. Aneuploidy in animals usually results in
deformity, sterility, and loss of viability.
Sex chromosomes
Sex chromosomes are the major determinants of sex in fish. Chromosomes that are not
principal determinants of sex are called autosomes. Most fish have a pair of sex chromosomes.
Chromosome aberrations
Chromosome abnormalities occur when chromosomes break and reunite at the broken ends.
Multiple breaks may occur on the same chromosome, resulting in deficiencies (loss of chromosome
segments), inversions (inverted segments of chromosomes), and duplications (presence of multiple
copies of a segment). Multiple breaks on different chromosomes may result in a translocation (a segment
of a chromosome inserted into the body of another, nonhomologous chromosome). Duplications and
deficiencies of chromosome segments are usually lethal. Translocations and inversions, as in sockeye
salmon (Thorgaard 1978), are not necessarily detrimental.
Chromosomes during growth, gametogenesis, and fertilization
Chromosomes are the vehicles of genes. Consequently, the behavior of chromosomes during
the life cycle of an organism is very important.
Growth
Cells must divide to replace senescent cells or increase in number. Cell division for growth
involves duplication (rather than splitting) of whole chromosome sets because daughter cells are diploid
and genetically identical to each other and their precursor.
Mitosis is the division of the cell nucleus. Cytokinesis is the division of the actual cell,
including the cytoplasm. Chromosome duplication is completed before mitosis begins. Figure 4
shows the sequential stages of mitosis: interphase, prophase, metaphase, anaphase, and
telophase.
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