Random mutation provides new material for natural selection. Changes in the genetic code are passed on to offspring when random reading and copying errors occur during meiosis. Mutations also occur through exposure to sequence-changing, mutation-inducing agents ("mutagens") like radiation and chemicals. Most mutations cause spontaneous abortions, are harmful or are simply neutral. Beneficial mutations are rare, but when they occur in the gametes and can be passed on to offspring they can make a large evolutionary impact.
Point mutations, which are nucleotide deletions, substitutions, or additions, are the most common kind of change in genetic code. Mutations can involve whole or partial chromosomes as well. These errors occur during meiosis when eggs or sperms receive too many or too few chromosomes and then get fertilized. The result is often miscarriage. Trisomy 21, or Down Syndrome, is one of the few conditions that is viable. It occurs in one out of a thousand live births. Individuals have three instead of two copies of Chromosome 21 and develop a unique, recognizable face and head shape, are normally smaller in stature, and have varying degrees of developmental and health problems.
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