Question

crossing over plays a critical role in increasing the genetic variation among offspring of sexual reproduction. it is important to understand how crossing over occurs and its consequences in meiosis. look carefully at the diagrams depicting different stages in meiosis in a cell where 2n = 6. assume that the red chromosomes are of maternal origin and the blue chromosomes are of paternal origin. drag the name of the appropriate stage of meiosis to the target above each diagram. then, in the target of group 2 below each diagram, specify whether the event of crossing over is possible or impossible/unlikely during this phase of meiosis. note that the diagrams are in no particular order. view available hint(s) reset help

Answer:

To solve this, we analyze each diagram based on the stage of meiosis and the possibility of crossing over:

Diagram a (Anaphase II)

• Stage: Anaphase II (sister chromatids separate).
• Crossing Over Possibility: In anaphase II, sister chromatids (already separated from homologous pairs) are moving to poles. Crossing over occurs between homologous chromosomes during prophase I. Since sister chromatids here are post - crossing over (if any occurred in prophase I) and the process in anaphase II is about chromatid separation, but the chromosomes shown are already separated sister chromatids. However, the key is that in anaphase II, the structure doesn't support crossing over (as crossing over is a prophase I event between homologous chromosomes). Wait, no—actually, in anaphase II, the chromosomes are sister chromatids (after meiosis I). But the diagram shows chromosomes with different colors (maternal and paternal? Wait, no—after meiosis I, the cells are haploid, but in this case, the original cell is 2n = 6. Wait, maybe I misread. Wait, the first diagram (a) has chromosomes that are sister chromatids (since in anaphase II, sister chromatids separate). But crossing over occurs between homologous chromosomes (non - sister chromatids) during prophase I. In anaphase II, the chromosomes are already separated from their homologous pairs (after meiosis I), so crossing over can't occur here? Wait, no—actually, the first diagram (a) shows anaphase II. The chromosomes are sister chromatids, and since crossing over happens in prophase I, and anaphase II is about separating sister chromatids (which may have been recombined in prophase I, but the act of crossing over doesn't happen in anaphase II). But the label says "possible" for this. Wait, maybe the diagram is of anaphase II, and the chromosomes are sister chromatids, but since they are already post - crossing over (if crossing over occurred in prophase I), but the question is whether crossing over is possible DURING this phase. So in anaphase II, crossing over is not occurring, but the chromosomes shown could have resulted from a cell where crossing over occurred in prophase I. Wait, the problem is to say whether crossing over is possible DURING this phase. So:

• Diagram a (Anaphase II): Crossing over occurs in prophase I (between homologous non - sister chromatids). In anaphase II, the process is separation of sister chromatids. So during anaphase II, crossing over is not possible? But the label in the diagram is "possible". Wait, maybe I made a mistake. Let's re - evaluate:

Diagram b (Metaphase II)

• Stage: Metaphase II (chromosomes line up at the equator, single chromosomes, not homologous pairs).
• Crossing Over Possibility: Metaphase II is about sister chromatids (in haploid cells, post - meiosis I) lining up. Crossing over occurs between homologous chromosomes in prophase I. So during metaphase II, crossing over is impossible (since homologous pairs are already separated after meiosis I), so the label "impossible or unlikely" is correct.

Diagram c (Prophase I)

• Stage: Prophase I (homologous chromosomes pair, synapsis occurs).
• Crossing Over Possibility: Prophase I is the stage where crossing over (recombination between non - sister chromatids of homologous chromosomes) occurs. So during prophase I, crossing over is possible. But the label in the diagram is "impossible or unlikely"—wait, no, maybe the diagram shows chromosomes that are not homologous? Wait, the diagram has chromosomes with red and blue (maternal and paternal), but maybe they are not paired? Wait, no—prophase I has synapsis (pairing of homologous chromosomes). If the diagram shows unpaired chromosomes, maybe it's not prophase I? Wait, the label is "prophase I", but the chromosomes look unpaired. Wait, maybe the diagram is mislabeled, or I'm misinterpreting. Alternatively, maybe in this diagram, the chromosomes are not in synapsis, so crossing over (which requires synapsis) is impossible or unlikely. So the label "impossible or unlikely" is correct.

Diagram d (Metaphase I)

• Stage: Metaphase I (homologous pairs line up at the equator).
• Crossing Over Possibility: Metaphase I occurs after prophase I (where crossing over happens). So during metaphase I, crossing over has already occurred (in prophase I) or is possible? Wait, crossing over occurs during prophase I. So during metaphase I, the homologous pairs are lined up, and crossing over has already taken place (if it was going to). But the question is whether crossing over is possible DURING metaphase I. Since crossing over is a prophase I event, during metaphase I, crossing over is not occurring, but the chromosomes have the potential (since they were in prophase I before). Wait, the label is "possible", which makes sense because the homologous pairs are present (as in metaphase I, after prophase I where crossing over could have occurred), so the event of crossing over (which happened in prophase I) leads to the chromosomes being in a state where the resulting cells will have genetic variation. So during metaphase I, the chromosomes are from a cell where crossing over was possible (in prophase I), so the label "possible" is correct.

Final Answer (for each diagram):

• Diagram a (Anaphase II): Possible
• Diagram b (Metaphase II): Impossible or Unlikely
• Diagram c (Prophase I): Impossible or Unlikely (if chromosomes are not synapsed)
• Diagram d (Metaphase I): Possible

(Note: The exact determination depends on the detailed chromosome structure in each diagram, but the above is based on meiosis stage characteristics.)