A pair of genes, several traits
Pleiotropy (from Greek, pleion = more numerous and tropes = affinity) is the phenomenon in which a pair of allele genes conditions the appearance of several traits in the same organism.
Pleiotropy shows that the Mendelian idea that each gene affects only one trait is not always valid. For example, certain mice are born with thickened ribs, narrowed trachea, diminished lungs, and blocked nostrils, which will inevitably lead to death. All of these characteristics are due to the action of only one pair of genes, hence a case of pleiotropy.
Chromosomal mutations and aberrations
Mutation is a change in genetic material. There are two types of mutation, gene and chromosomal.
THE gene mutation It is a change in the gene due to changes in the frequency of DNA nitrogen bases. THE chromosomal mutation (chromosomal aberration) is a change in the number or structure of chromosomes.
In 1941, researchers Beadle and Tatum, experimenting with a type of bread mold, the Neurospora spobserved that DNA self-duplication did not always occur perfectly. The mold grew in a culture medium containing sugar and various inorganic salts. Their spores were subjected to X-rays and some of them later produced molds with new characteristics. For example, some lost their ability to make lysine and could survive only when that amino acid was added to the culture medium. This inability was related to the lack of an enzyme needed for lysine synthesis. They then concluded that X-rays would have damaged the formation of that specific type of enzyme.
Since the production of an enzyme depends on information encoded in DNA, their conclusion was known as the "one gene - one enzyme" relationship. Today, one speaks more precisely of the relation "one gene - one polypeptide chain".
X-ray induced genetic modification is known as mutation. Mutations may result from a change in nucleotide sequence, or from breaks and changes in position of DNA molecule fragments. Therefore, mutations are the numerical and structural alterations of chromosomes that persist through self-duplication and are transmitted to daughter cells. There are also errors that occur in RNA at the time of transcription or translation and affect only the cell itself.
Mutations are produced by mutagenic agents, which mainly comprise various types of radiation, including ultraviolet rays, x-rays and substances that interfere with DNA self-duplication or mRNA transcription, leading to errors in nucleotide sequences.
Mutagenic agents are factors that can increase the frequency of mutations. In 1920, Hermann J. Muller discovered that by subjecting Drosophila to X-ray, the frequency of mutations increased by a hundredfold over the unexposed population. The increase in mutation rate can be obtained by employing numerous physical and chemical agents.
The list of mutagenic substances has increased a lot in the last years, being well known the mustard gas, O nitrous acid, a bromouracil, O formaldehyde, a nicotine. Several types of cancer can be produced by changes in nucleic acids; therefore the same mutagenic agents may also be carcinogenic. But the most important among them is radiation. When a cell receives radiation, molecules can be broken or altered in their structures. When the changes are too large, they can interfere with cell metabolism and division, and the cell dies.
When it survives radiation, the modifications are duplicated and transmitted to the cells of succeeding generations.
Among the physical agents, the best known are radiation as well as x-ray. Heat also increases the incidence of mutations: in humans, their frequency in blast furnace workers at steel mills, who spend a long time in places high temperature, is higher than in the general population.
Chemicals such as mustard gas and nitrous acid (HNO)2) may also increase the frequency of mutations. Aerosols, food colorings, and some components of cigarette smoke can alter a cell's genetic heritage and can lead to the development of various forms of mutations and cancer.
All living beings are subjected daily to several of these agents. However, mutations remain as infrequent events. The relative stability of genetic material is due to the existence of a group of repair enzymes, who permanently "patrol" DNA molecules for changes in their nucleotide sequence. Most of the time, these changes are detected and fixed.