Question

3. What is crossing over? When does it occur during meiosis? Why did you have difficulty modeling this element of meiosis using your “chromosomes?”

Answer 1

Crossing over happens in Meiosis 1 only. In Prophase 1 a cells chromatin (chromatin = chromosomes that have not condensed yet) condense and pair up forming homologous chromosomes (paired = XX (2 chromosomes together)). When this happens segments/ alleles of the chromosomes pairing up swap over. This creates genetic diversity as each chromosome is different, it has parts from its pair. This leaves every chromosome unique and individual.

I hope this helps, sorry some of the vocab is rather technical. By the way I would suggest watching the 'Crash Course: Meiosis' on YouTube, this really helped me when I was learning this topic. :D 

Answer 2

Crossing over can be expressed in three points:
1. Crossing over is when two chromosomes exchange their genetic materials in a process called synapsis. 2. It occurs in meiosis 1 during prophase 1. 3. One example: Your chromosomes may end up crossing over with wrong chromosomes due to mutation in one of the chromosome or both.