Final answer:
The mutation type leading to trisomy 21 (Down Syndrome) is nondisjunction, where chromosomes don't separate correctly during meiosis, causing an extra chromosome. Nondisjunction also results in Turner's Syndrome (monosomy XO). Mutations are essential for evolution as they provide genetic diversity.
Step-by-step explanation:
The type of mutation that results in trisomy 21, or Down Syndrome, is known as nondisjunction. This occurs when chromosomes do not separate properly during meiosis, leading to an egg or sperm cell with an abnormal number of chromosomes. When such a gamete fuses with a normal gamete, the resulting zygote has an extra chromosome, leading to trisomy.
For instance, in the case of Down Syndrome, the individual has three copies of chromosome 21 instead of the usual two. On the other hand, Turner's Syndrome, or monosomy XO, involves a missing X chromosome in females, also due to nondisjunction.
Mutations like nondisjunction are critical for evolution because they introduce genetic variety into populations. This diversity is the raw material upon which natural selection acts, enabling populations to adapt over time to changing environments.