Where does this evidence come from? – crossing over takes place at a four-stranded stage and not at two stranded stages of chromosomes

The correct option is "2:2:2:2 organization of ascospores in Neurospora".

To understand why this is the correct answer, let us look at the process of meiosis and ascospore formation in the fungus Neurospora crassa.

Neurospora is an ideal model organism for genetic studies because the products of a single meiosis are held together in a sac-like structure called an ascus. These products, called ascospores, are arranged in a linear order that directly reflects the steps of division and segregation during meiosis. This is known as tetrad analysis.

Let us compare what happens if crossing over occurs at the two-stranded stage versus the four-stranded stage:

1. If crossing over occurred at the two-stranded stage (prior to chromosome replication):
Both chromatids of a chromosome would be involved in the exchange. After subsequent replication and meiotic divisions, all four resulting chromatids would be recombinant. For a gene locus with alleles A and a, the segregation would result in an ascus containing 4 spores of one type and 4 spores of another type in a basic parental configuration, yielding a 4:4 arrangement of ascospores (e.g., AAAAaaaa), but without showing any recombinant patterns among the sister chromatid pairs within a tetrad division.

2. If crossing over occurs at the four-stranded stage (after chromosome replication, when each homologous chromosome consists of two sister chromatids):
Crossing over involves only two of the four chromatids (one from each homologous chromosome), leaving the other two chromatids as non-recombinant (parental type).
When these chromosomes segregate through meiosis I and meiosis II, followed by a post-meiotic mitotic division, they form eight linear ascospores. The resulting sequence of alleles in the ascus exhibits a pattern showing that recombinant and parental alleles alternate. Specifically, it yields a 2:2:2:2 arrangement (such as AA:aa:AA:aa or aa:AA:aa:AA) of the ascospores inside the ascus.

Therefore, the observation of the 2:2:2:2 organization of ascospores in Neurospora provides direct, cytological and genetic evidence that crossing over takes place at the four-stranded stage rather than the two-stranded stage.