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

Gametogenesis - segregation and independent assortment
Gametogenesis is the production of gametes. In gametogenesis, the diploid (2n) number of
chromosomes in somatic cells is reduced to the haploid (n) number. Reduction in the number of
chromosomes is accomplished through meiosis. Two meiotic cell divisions are required (Figure 4). The
chromosome number is reduced during the first meiotic division. The second meiotic division results in
mature eggs or sperm. Gametogenesis has two important consequences, segregation and
independent assortment, that are known as Mendel's principles. The two products of the first
meiotic division each contain a single chromosome from every homologous pair found in the parent cell.
Homologous chromosomes and their respective genes are said to segregate during gametogenesis
because pairs of homologous chromosomes separate into different daughter cells. Independent
assortment means that non-homologous chromosomes of maternal and paternal origin segregate
randomly so that each gamete receives a mixture of maternal and paternal chromosomes. Because of
segregation and independent assortment, the chromosomes in a single germ cell are a complete haploid
set and are random mixtures of maternal and paternal chromosomes.
Crossing over
During crossing over (also called recombination), pieces of chromosomes are exchanged
between homologous chromosomes (Figure 5). Crossing over is an important source of genetic variation
because genes of maternal and paternal origin come to reside on the same chromosome. Chromosomes
in an individual's gametes may differ substantially from chromosomes in the individual's somatic cells
because of recombination.
Fertilization
The genetic composition of an organism is determined at fertilization when the egg and sperm
unite. At fertilization, the diploid (2n) condition is normally restored and the cytoplasmic DNA of the egg
becomes the cytoplasmic DNA of the offspring. An abnormal chromosome number after fertilization may
result from: 1) union of an unreduced germ cell (2n) with a normal germ cell (n) resulting in a triploid (3n),
and 2) hybridization between two species, one with haploid number n and the other with haploid number
n', producing an offspring with n+n' chromosomes.
GENETICS OF INDIVIDUAL ORGANISMS
Genetics at the molecular and cellular level is the foundation of genetics in individual organisms.
Similarly, the genetics of individual fish is the foundation for the genetics of broodstocks and populations.
Phenotype and genotype
Every individual has both a phenotype and a genotype. The genotype is the specific set of
genes carried by the individual. The phenotype is the set of characteristics (e.g., morphological,
physiological, behavioral) expressed by the individual. The phenotype is produced by the genotype in
combination with the environment.
Description of the genotype
There are two representatives of every gene (called alleles) in a normal diploid
individual because alleles of the same gene occupy the same place (locus) on both homologous
chromosomes. The genotype is the set of alleles an organism carries at one or more loci in an organism.
Consider a fictitious gene with two alleles denoted A and a. Three genotypes are possible in a
diploid individual: AA, Aa, and aa. Individuals with two copies of the same allele (i.e., those with AA or aa)
are homozygous while those with different alleles (Aa) are heterozygous. In a triploid individual the
number of potential genotypes is larger than in a diploid because all possible combinations of the two
alleles taken three at a time (e.g., aaa, aaA, aAA, etc.) must be counted.
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