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| The DNA molecule specifies the genetic code or set of instructions needed to produce biological structures. DNA acts along with a related molecule, RNA, to translate these instructions into proteins. The DNA is contained along structures within the cell called chromosomes. Chromosomes come in pairs. A segment of DNA that codes for a certain product is called a gene. The different forms of genes present at a locus are called alleles. The DNA molecule has the ability to make copies of itself, allowing transmission of genetic information from cell to cell and from generation to generation. Meiosis is the process of sex cell formation that results in one of each chromosome pair being transmitted from parent to offspring. An individual receives half of his or her alleles from each parent. The two alleles together specify the genetic constitution of an individual—the genotype. The physical manifestation of the genotype is known as the phenotype. The relationship between genotype and phenotype depends on whether an allele is dominant, recessive, or codominant. In complex physical traits, the phenotype is the result of the combined effect of genetics and environment. The ultimate source of all genetic variation is mutation—a random change in the genetic code. Some mutations are neutral in effect; others are helpful or harmful. The effect of any mutation often depends on the specific environmental conditions. Mutations for any given allele are relatively rare events, but given the large number of loci in many organisms, it is highly probable that each individual has at least one mutant allele. |