Variations and Mutations
In the cells of plants and animals that reproduce sexually, at least two genes are present for every trait. In these organisms one set of chromosomes is received from the male parent. The other set of chromosomes is received from the female parent. The offspring may show some of its mother’s traits and some of its father’s traits. It passes on a combination of these traits to its offspring. Each of these offspring then passes on another combination to its own offspring. These different combinations of traits come from different combination of chromosomes and genes, causing a variety within each kind of organism. Another way organisms may become different is by mutations, which are random changes in genes and chromosomes. Mutations occur naturally in the cells of organisms. Usually, mutations produce no noticeable changes in organisms. Some mutations, however, can lower an organism’s chances of surviving and producing the young. A mutation is generally recessive. It may be passed on from generation to generation. Mutations happen all the time. They may have been happening since life first began on the earth. Probably, all of us saw animals (birds) with white fur (feathers) and red eyes. They are albinos. Albinos have a mutation in their genes that prevents them from having normal body colors. This mutation can be harmful to the living organisms since it will not blend in with their surroundings. Therefore, the albino animal (bird) may be more visible to its enemies. Other mutations can be an advantage for an organism. For example, some mosquitoes sprayed by DDT probably had mutations that enabled them to resist the DDT poison. These mosquitoes lived and passed their DDT resistance on to their offspring.
The process of selecting individuals with desirable traits to produce the next generation is called controlled breeding.In recent decades breeders have applied the knowledge of genetics to controlled breeding. In this way, they have been able to adapt plants and animals to many human needs in a much shorter period of time than was previously possible.
Modern breeders use combinations of several methods to develop plants and animals with desired traits. The most important methods of controlled breeding are mass selection, hybridization, and inbreeding. Each of these traditional methods has been made more effective through the application of genetic principles.
The process of raising a great many plants and animals and selecting the best in each generation for further breeding is called mass selection.
Frequently breeders want to establish pure lines, or populations of plants or animals made up of genetically similar individuals. Such lines usually breed true for certain traits, which means that offspring are almost identical to their parents in these traits. The pure lines also serve as known quantities in breeding experiments.
Pure lines are established by following mass selection with inbreeding,a method that involves mating genetically similar individuals. With animals, close relatives such as brothers or sisters are mated over several generations. With plants, inbred varieties are produced by self-pollination. After many generations, inbreeding produces individuals that are homozygous for most traits. Some of these traits are desirable. However, others may be undesirable, caused by homozygous recessive genes that are not expressed in a heterozygous individual.
Hybridizationis a method of crossing two different species, breeds, varieties, or pure lines. When inbred varieties are crossed, the resulting hybrids may show every possible combination of traits of the parent species. Thus, some hybrid offspring inherit the best traits of each parent and are larger, hardier, and more productive than either parent. When the offspring is superior to both parents for a specific trait, this improvement is known as hybrid vigor.
Hybridization has its drawbacks, however. In some cases offspring inherit the worst traits of each parent. Other hybrids, such as mules, are sterile. Even in hybrids that can reproduce, hybrid vigor may disappear if hybrids are crossed over many generations.
Controlled breeding is just one means of developing organisms adapted to human needs. Biologists have also learned to produce identical copies of desirable organisms. In the last 40 years, biologists have found new ways to change the genetic make up of an organism or its offspring by artificial means.
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