Question

3. A) Describe the reasons that we discussed explaining why meiosis is important to evolution. B) One way that meiosis can go wrong is by nondisjunction. Describe what nondisjunction is and describe a condition that is caused by nondisjunction.

Answer 1

A).

Law of segregation occurs during anaphase-II of meiosis and ends by the completion of cytokinesis. Law of independent assortment occurs during metaphase of meiosis.

1.Law of segregation: One of 2 principles of genetic heredity first formulated by Gregor Mendel, elucidated on his genetic experiments with pea plants.

He finally described that the homologous chromosomes segregate during meiosis and are distributed to different gametes, thereby the offspring acquires one chromosome from each parent. The chromosomal segregation occurs during anaphase -II of meiosis. The segregated chromosomes reunite randomly during the fertilization process.

2. Law of independent assortment: According to Mendel’s law of independent assortment, each member of a pair of genes on homologous chromosomes tends to be randomly distributed into gametes independently of how other chromosomes are distributed. It results in giving all possible combinations in equal frequency.

These mechanisms in meiosis allow the formation of reproductive cells with the new genotypes, thus, favouring the process of evolution to occur.

B).

During meiosis, the homologous chromosomes align during the prophase. These chromosomes crossover at some point and rejoin to form recombinant chromatids, and move towards the opposite poles during anaphase.

Sometimes, an error occurs in meiosis during the meiosis – I or II, i.e. during the separation of homologous chromosomes. These errors result in excess or fewer chromosomes in the daughter cell, and this is called “non-disjunction.” The two reasons for nondisjunction are,

• Failure of the spindle checkpoint, which is necessary for the segregation of chromosomes
• Increasing age of the cells. The nondisjunction is most likely to occur on older cells than young cells