Question

in the following image, which number(s) identify homologous chromosomes?

options:
1 only
2 only
1 and 2
3 only
3 and 4

Explanation:

Step1: Recall Homologous Chromosomes

Homologous chromosomes are pairs of chromosomes (one from each parent) with similar size, shape, and gene loci. Before replication (in 1), the red and purple chromosomes are homologous. After replication (in 2), each chromosome is a sister chromatid pair, but the original pairs (1's chromosomes) remain homologous, and 2 shows the replicated homologous pairs. Wait, no—wait, the number 1 labels the pair of non - replicated homologous chromosomes (red and purple), and number 2 labels the pair of replicated homologous chromosomes? Wait, no, let's re - examine. The first set (labeled 1) has two non - replicated chromosomes (red and purple), which are homologous. After replication, each chromosome has two sister chromatids, and the set labeled 2 has the two replicated homologous pairs? Wait, no, the question is which number(s) identify homologous chromosomes. Homologous chromosomes are a pair (one maternal, one paternal) with same genes. In the image, 1 is the pair of non - replicated homologous chromosomes (red and purple). After replication, 2 shows the two replicated chromosomes (each with sister chromatids), but the homologous relationship is still between the red - derived and purple - derived chromosomes. Wait, but the options: the correct option is 1 and 2? Wait, no, let's think again. Homologous chromosomes are pairs. Before replication (1), the two chromosomes (red and purple) are homologous. After replication (2), each chromosome has two sister chromatids, but the two "chromosomes" (now with sister chromatids) are still homologous. Wait, but the options include "1 and 2". Let's check the options:

• 1 only: No, because after replication (2), the homologous pairs still exist (the red - replicated and purple - replicated are homologous).
• 2 only: No, because 1 also has homologous chromosomes.
• 1 and 2: Yes, because 1 has non - replicated homologous chromosomes, and 2 has replicated homologous chromosomes (each chromosome is a sister chromatid pair, but the two chromosomes in 2 are homologous to each other, same as in 1).
• 3 only: 3 is a sister chromatid pair (replicated chromosome), not homologous (homologous are pairs of different chromosomes, not sister chromatids).
• 3 and 4: 3 and 4 are sister chromatid pairs of different homologous chromosomes, but they are not homologous to each other (3 is red - sister chromatids, 4 is purple - sister chromatids; 3 and 4 are homologous? Wait, no, 3 is a single replicated chromosome (sister chromatids), 4 is another. Wait, no, 3 is the sister chromatids of the red chromosome, 4 is sister chromatids of the purple chromosome. So 3 and 4 are homologous? Wait, no, I think I made a mistake. Homologous chromosomes are pairs of chromosomes (one from each parent). Sister chromatids are identical copies of the same chromosome. So in 1: two homologous chromosomes (red and purple, non - replicated). In 2: two homologous chromosomes (each with sister chromatids, red - replicated and purple - replicated). In 3: sister chromatids of red chromosome (not homologous, just sister chromatids). In 4: sister chromatids of purple chromosome (not homologous, just sister chromatids). So homologous chromosomes are in 1 (non - replicated pair) and 2 (replicated pair). So the correct option is 1 and 2.

Answer:

1 and 2 (the option labeled "1 and 2")