Biology · Introductory biology · Worked example
Count chromosomes through meiosis
A cell from an organism with 2n = 8 enters meiosis. How many chromosomes and chromatids does it have after DNA replication, after meiosis I and after meiosis II? Ignoring crossing over, how many combinations of chromosomes can its gametes carry?
After replication
The cell still has 8 chromosomes, but each now has two sister chromatids.
After meiosis I
The homologs of each pair separate, so each of the two cells has 4 chromosomes, still of two chromatids each: 8 chromatids per cell.
After meiosis II
The sister chromatids separate, so each of the four cells has 4 single chromosomes: n = 4.
Count the combinations
Four homologous pairs each line up one of two ways in metaphase I, independently of the others, so independent assortment alone gives 2 × 2 × 2 × 2 kinds of gamete.
Result
After replication: 8 chromosomes and 16 chromatids. After meiosis I: two cells, each with 4 chromosomes and 8 chromatids. After meiosis II: four cells, each with 4 chromosomes (n = 4). Independent assortment alone gives 2⁴ = 16 kinds of gamete.
Your turn
A fruit fly gamete has 4 chromosomes. How many are in its body cells, and how many chromosome combinations can its gametes carry, ignoring crossing over?
Show the answer and explanation
8 in its body cells, and 16 combinations.
The gamete is haploid, so n = 4 and 2n = 8. Four pairs assort independently: 2 × 2 × 2 × 2 = 16.
Keep exploring
The cell opens in Chromosomes & meiosis with two pairs, already replicated; step through meiosis I and II and count the chromosomes and chromatids at each stage.
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