Crossing over and independent assortment
Two sources of variation in meiosis. In crossing over, homologous chromosomes swap pieces in prophase I. In independent assortment, each pair lines up and separates at random.
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Analogy
Shuffling and then cutting a deck of cards.
Example
23 chromosome pairs.2^23 = 8,388,608.Crossing over addsmany more.
In practice
With 23 pairs, independent assortment alone gives 2 to the power of 23, over 8 million, different combinations.
Practice
Flashcards
- When does crossing over occur?In prophase I of meiosis.
Interview questions
- Describe the ways meiosis produces genetic variation.
- Crossing over in prophase I
- Independent assortment of homologous pairs
- Random fertilisation adds more variation
Questions people ask
What is a chiasma?
A chiasma is the place where two homologous chromosomes are joined while crossing over. Under the microscope it appears as an X-shaped link. It shows where pieces have been swapped, and it helps hold the pair together so that they separate correctly in anaphase I.
Why is genetic variation useful?
Variation means individuals in a population differ, so when conditions change, such as a new disease, some are likely to survive. Without variation, a single threat could wipe out everyone. Meiosis and sexual reproduction constantly shuffle alleles, giving natural selection more to work with.