Types of Variation (A-level Biology)
Types of Variation
Non-Disjunction
Non-disjunction mutations involve a problem in the separation of chromosomes during meiosis. This can result in the loss of an entire chromosome in one daughter cell, and a gain of a chromosome in another daughter cell.
- Improper separation occurs during anaphase and telophase. Non-disjunction results from improper separation of chromosomes during anaphase and telophase of meiosis.
- Down’s Syndrome occurs when there is an extra chromosome 21. Trisomy 21 (Down’s syndrome) results from non-disjunction during meiosis. The sex cells of one parent can have three copies of chromosome 21, resulting in an offspring with three copies of chromosome 21 which causes Down’s syndrome.
- Turner’s Syndrome occurs when there is only 1 complete X chromosome. Turner’s Syndrome is another condition caused by chromosomal mutation. This condition affects females. Turner’s Syndrome is known as ‘monosomy’, where the second copy of the X chromosome is incomplete or missing.
Non-Disjunction in Mitosis
Non-disjunction, as well as other chromosomal mutations, can occur in both mitosis and meiosis.
- Mutations in mitosis are generally not very harmful.
- In most cases, the cell can undergo apoptosis after detecting genetic abnormalities.
- In some cases, these chromosomal mutations can result in defective cells such as cancer cells (e.g. Philadelphia chromosome)
- Mutations arising in mitosis are not heritable by off-spring.
- Mutations in meiosis can be inherited by off-spring.
- Meiosis, as you will see in this chapter, is the mechanism of division for gametes (sperm and egg).
- Gametes are responsible for transferring genetic information to offspring.
- Mutations in gametic chromosomes can result in those mutations being inherited by an offspring.
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