Sources of Genetic Variation

The question asks why mutations are considered the ultimate source of all genetic variation. We need to identify the unique property of mutations compared to other sources.

Meiosis (crossing over + independent assortment) and fertilisation rearrange and recombine existing alleles into new combinations. They are excellent at shuffling variation but cannot produce alleles that don't already exist.

Mutations change the actual DNA nucleotide sequence, producing alleles with new base sequences. This is the only way a genuinely new allele — one that has never existed before — can arise.

The correct answer is that mutations are the only source that creates entirely new alleles that did not previously exist in the gene pool. All other sources of variation rely on rearranging alleles that mutations originally produced.

We need to find the correct reason why mutations are the ultimate source of all genetic variation.

This is incorrect. The reason mutations are the ultimate source has nothing to do with their frequency. In fact, mutations are relatively rare events. Frequency is irrelevant to this question.

This is factually wrong. Most mutations are neutral, some are harmful, and only a small number are beneficial. Mutations are certainly not always beneficial.

This is incorrect. Crossing over, independent assortment, and fertilisation all produce new genotype combinations that result in different phenotypes. Environmental factors also affect phenotype.

The remaining option — mutations create entirely new alleles that did not previously exist — is correct. This is the key distinction: only mutations add new genetic information to the gene pool.

The question describes homologous chromosomes failing to separate properly during meiosis. We need to name this event and identify its consequence.

Non-disjunction is the failure of homologous chromosomes (or sister chromatids) to separate correctly during meiosis. This is a type of chromosomal mutation.

When homologous chromosomes fail to separate, one resulting gamete receives two copies of the chromosome and another receives none. If the gamete with two copies is fertilised by a normal gamete, the zygote has three copies — a trisomy. For example, trisomy 21 causes Down syndrome.

The correct answer names non-disjunction and correctly states that a gamete may be produced with two copies of a chromosome instead of one.

We need to identify the correct name and consequence of chromosomes failing to separate during meiosis.

Crossing over involves the exchange of DNA segments between homologous chromosomes in prophase I — it does not describe a failure to separate. This option describes a normal, beneficial process, not an error.

Independent assortment is the random distribution of homologous chromosomes to daughter cells — also a normal process during meiosis I. It does not refer to a failure of separation.

A substitution mutation is a change at the level of a single DNA base — it has nothing to do with entire chromosomes failing to separate. This is a gene mutation, not a chromosomal event.

Non-disjunction is the correct term for chromosomes failing to separate, and the consequence is a gamete with two copies of a chromosome — matching the first option perfectly.