Genetic Origin of Traits in Diploid Organisms
The heritable unit that may influence a trait is called a gene. A gene is a portion of a chromosome. An important reference point along a chromosome, which is a very long and compacted string of DNA and proteins, is the centromere; the distance from a gene to the centromere is referred to as the gene's locus or map location.
The nucleus of a diploid cell contains two of each chromosome, with homologous (mostly identical) pairs of chromosomes having the same genes at the same loci.
Different phenotypic traits are caused by different forms of genes, or alleles, which arise by mutation in a single individual and are passed on to successive generations.
Read more about this topic: Phenotypic Trait
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—Policy statement, 1944, of the Youth League of the African National Congress. pt. 2, ch. 4, Fatima Meer, Higher than Hope (1988)