Aconsiderable proportion ofheritable disease inman isa consequence ofthepresence ofconstitutional chromosome anomalies. We needtodistinguish between constitutional anomalies whicharepresent in thegermcell orwhicharise during theearly cleavage divisions ofazygote, sothatall, orasubstantial proportion, ofthecells carry theabnormality, and acquired somatic abnormalities whichareacquired later indevelopment, orlife, andwhichtherefore affect asmaller proportion ofcells. Acquired chromosomeabnormalities maybeimportant intheaetiology ofcertain leukaemias, lymphomas, andbrain tumours, butthese arenottobeconsidered underthecategory ofheritable changes inthenewborn. (A)Types andconsequences Individuals whohaveabnormal chromosome complements fall intotwobroadcategories: (a)those whohaveanabnormal number ofchromosomes and (b)thosewhohaveabnormalities ofchromosome structure. (1)ANEUPLOIDY
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H.J. Evans (1977) studied this question.