The structure of chromosomes and the genes they carry is very consistent otherwise there would not be continuity in inheritance. However, changes to chromosomes do occasionally occur. Sometimes individual genes change to give new alleles and there may be larger alterations in structure. Chromosomes may break and pieces may be lost (deletion), pieces may break off and be rejoined in the reverse order (inversion), or a portion of one chromosome may be transferred to another chromosome (translocation). The way such changes can take place and the pairing of altered chromosomes is illustrated in Figure 6.1. Inversions and translocations in which the positions of chromosome portions are altered, but in which the gene complement remains the same, can still have a heritable effect because the chemical compounds produced by genes to influence development interact with those of their neighbours. Linkages are also altered. These changes, whether they involve individual genes or whole chromosomes, introduce new heritable characters known as mutations. Many drastic chromosome mutations are lethal and the cells containing them cannot survive, but smaller changes often give the individuals carrying them an opportunity to compete with more normal progeny and sometimes to gain ascendency and to develop as new forms better adapted to the environment.
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