Homologous pairs of chromosomes are lined up independently of other such pairs during _____.
During metaphase I of meiosis. Homologous chromosome pairs (tetrads) line up at the metaphase plate, and each pair orients randomly and independently of every other pair. This independent assortment shuffles maternal and paternal chromosomes, creating genetic variation in the resulting gametes.
The answer
The blank is filled by metaphase I of meiosis. This is the stage where independent assortment physically happens. During prophase I, each chromosome pairs with its homolog to form a tetrad (bivalent). Then in metaphase I, these homologous pairs line up along the middle of the cell, the metaphase plate. The crucial point is that the orientation of each pair, which homolog faces which pole, is random and independent of every other pair. Whether the maternal or paternal chromosome of pair 1 points "up" has no effect on how pair 2, pair 3, or any other pair orients.
Why this creates variation
Because each pair orients independently, the chromosomes get sorted into daughter cells in many different combinations. This is the physical basis of Mendel's Law of Independent Assortment. The number of possible gamete combinations from independent assortment alone is 2^n, where n is the number of homologous pairs. In humans, n = 23, so 2^23 is more than 8 million possible chromosome combinations, before crossing over adds even more variety. This is a major reason siblings (except identical twins) look different despite sharing the same parents.
Why not the other stages
It is worth ruling out the tempting distractors:
- Metaphase II is wrong. By metaphase II the homologous pairs have already been separated during anaphase I. Metaphase II lines up individual chromosomes (each with two sister chromatids) much like mitosis, so there are no homologous pairs left to assort. Independent assortment is already finished.
- Anaphase I is when the homologs actually separate and move to opposite poles. The independent lining up that sets the combinations is decided one step earlier, at metaphase I. Anaphase carries out the result rather than establishing it.
- Prophase I is when pairing (synapsis) and crossing over occur, but the pairs are not yet aligned at the plate, so independent orientation has not happened.
- Mitosis / metaphase of mitosis is wrong because in mitosis, homologous chromosomes do not pair up at all. Single chromosomes line up individually and produce two genetically identical cells, so no independent assortment occurs.
The bigger picture
Meiosis generates diversity through two independent mechanisms: crossing over in prophase I (which recombines alleles within a chromosome) and independent assortment in metaphase I (which recombines whole chromosomes). Together they ensure that essentially every gamete is genetically unique. Keeping the stages straight, metaphase I aligns pairs, anaphase I separates them, and metaphase II handles individual chromosomes, is the key to answering meiosis questions correctly.
- 1
Prophase I
Homologous chromosomes pair up (synapsis) forming tetrads; crossing over exchanges DNA between homologs.
- 2
Metaphase I
Homologous pairs line up at the metaphase plate and orient RANDOMLY and independently of other pairs. This is independent assortment.
- 3
Anaphase I
Homologous chromosomes separate and move to opposite poles, carrying out the combinations set at metaphase I.
- 4
Metaphase II
Individual chromosomes (no longer paired) line up single-file, like mitosis. No homologous pairs remain to assort.
- 5
Result
Four genetically unique haploid gametes, with 2^n possible chromosome combinations (over 8 million in humans).
Frequently asked
What is independent assortment in meiosis?
Independent assortment is the random orientation of each homologous chromosome pair at metaphase I, so maternal and paternal chromosomes are sorted into gametes in independent combinations. It is one of the main sources of genetic variation and is the basis of Mendel's Law of Independent Assortment.
During which phase of meiosis does independent assortment occur?
It occurs during metaphase I, when homologous pairs align at the metaphase plate and each pair orients randomly and independently of the others. The actual separation of those pairs then happens in anaphase I.
How does independent assortment increase genetic variation?
Because each homologous pair can face either pole independently, chromosomes combine in many ways in the gametes. This produces 2^n possible combinations, where n is the number of chromosome pairs, generating millions of genetically different gametes even before crossing over is counted.
How many gamete combinations are possible from independent assortment?
The number is 2^n, where n is the haploid number of chromosome pairs. In humans n equals 23, giving 2^23, which is more than 8 million possible chromosome combinations from independent assortment alone.
What is the difference between metaphase I and metaphase II?
In metaphase I, homologous pairs (tetrads) line up together and assort independently. In metaphase II, the homologs are already separated, so individual chromosomes line up single-file much like mitosis. Independent assortment happens only in metaphase I.