Applied genetics is the manipulation; of the hereditary characteristics
of an organism to improve or create specific traits in offspring. Controlled
breeding involves manipulating the hereditary characteristics of offspring
by selection parents with specific phenotypic traits. This process enables
a breeder to develop new strains of a species or to maintain existing strains.
Applying controlled breeding to plants and animals has traditionally been
based upon three techniques: mass selection, inbreeding, and hybridization.
Mass Selection and Inbreeding
Mass selection is the process of choosing a few individuals.
Inbreeding is based on the assumption that individuals that possess similar
phenotypes also will possess genotypes. When phenotypically similar individuals
are bred, offspring are more likely to be similar to their parents. In addition, offspring are likely to possess
the same desired phenotype. Unfortunately, inbreeding can eventually produce
weaker organisms because it increases the incidence of harmful homozygous
recessive traits. In hybridization two different but related species or
varieties of plants or animals are crossed. The products of this type of
crossbreeding are called hybrids.
Hybridization
Hybrids
possess a different genotype and usually a phenotype different from that
of either parent. When two different but closely linked species are crossed,
the chromosome incompatibility during meiosis causes most of their hybrid
offspring to be sterile. Some hybrid individuals grow faster and larger
and ar healthier than either parent. Such an individual is said to display
hybrid vigor. Hybridization can reverse the damaging effects of inbreeding
and results in hybrid vigor in the offspring. This occurs because hybridization
increases the number of heterozygous genes in an organism, thus reducing
the likelihood that a harmful, recessive allele will be expressed. Other
techniques in applied genetics manipulate the genes themselves. Among these
techniques are induction of mutations, induction of polyploidy, and use
of genetic engineering and gene splicing. Mutations occur at a very low
rate in nature. However, in 1927 the American geneticist H J Muller (1890-1967)
found that he could induce a much greater rate of mutation in Drosophila
by treating the flies with X-rays.
Mutations in Plants
Breeders can also
introduce desirable traits into a species
by inducing polyploidy, a conditioning which cells contain multiple, complete
sets of chromosomes. Although polyploidy is rare and usually lethal in
animals, it often occurs naturally in plants. Plant
breeders artificially induce polyploidy by administering colchicine, a
chemical that prohibits the formation of the cell plate during cell division.
Colchicine is usually applied by placing the roots of the plant in a colchicine
solution. Chromosomes in the cells of the plant go through the phases of
cell division, but the cell plate does not form. Therefore, two complete
sets of chromosomes exist. When the plant is removed from the colchicine
solution the cells will again form cell plates during mitosis. Thus all
resulting cells will contain an extra set of chromosomes. Genetic engineering,
in which scientists directly manipulate genes.
