Meiosis and Variation

The cells produced by meiosis are haploid (n) — they contain half the number of chromosomes of the original diploid (2n) cell.

They are also genetically non-identical — each cell differs genetically from the parent cell and from the other cells produced, because meiosis introduces genetic variation.

A gene locus is the fixed position (location) of a gene on a chromosome. The same gene is always found at the same locus on both members of a homologous pair.

An allele is one of the different versions (forms) of a gene that can occupy that locus — for example, the allele for a tall plant versus the allele for a short plant.

Linkage is the situation in which two or more genes are located on the same chromosome, so they tend to be inherited together rather than independently (unless separated by crossing over).

Before meiosis begins, the DNA is replicated once, so each chromosome consists of two identical sister chromatids.

The cell then divides twice with no further replication. In meiosis I the homologous chromosomes are separated, halving the chromosome number so that the two cells produced are haploid.

In meiosis II the sister chromatids of each chromosome are separated, doubling the number of cells from two to four, each still haploid. The single replication followed by two divisions is what gives four cells, each with half the original chromosome number.

During metaphase I, the homologous chromosomes (as bivalents) line up at the equator of the cell.

The orientation of each homologous pair is random — which chromosome of a pair faces which pole is a matter of chance — and the orientation of one pair is independent of every other pair.

When the homologues are separated in anaphase I, each gamete therefore receives a random mixture of maternal and paternal chromosomes.

Because the chromosomes carry different alleles, different mixtures give gametes with different combinations of alleles, producing genetic variation.

For an organism with n homologous pairs, the number of different chromosome combinations from independent assortment alone is 2ⁿ — for example, in humans (n = 23) this gives 2²³ ≈ 8.4 million genetically different gametes.

Crossing over takes place in prophase I, when the homologous chromosomes pair up closely to form a bivalent. Each chromosome already consists of two sister chromatids.

The chromatids of the two homologues become entangled, and non-sister chromatids — one chromatid from each homologue — make contact at points called chiasmata.

At each chiasma, the chromatids break and rejoin, so equivalent lengths of DNA, and therefore the alleles they carry, are exchanged between the non-sister chromatids.

This produces recombinant chromatids carrying new combinations of alleles that were not present on either original chromosome.

Because each gamete may receive a recombinant chromatid, crossing over increases the variety of allele combinations among the gametes, adding to the genetic variation already created by independent assortment — and it is the main way that linked genes can be separated.

Meiosis generates genetic variation in gametes through two distinct processes.

The first is independent assortment. During metaphase I, the homologous pairs line up at the equator with a random orientation, and the orientation of each pair is independent of all the others. When the homologues separate in anaphase I, each resulting cell receives a random combination of maternal and paternal chromosomes. For an organism with n homologous pairs this alone gives 2ⁿ different chromosome combinations.

The second is crossing over. During prophase I, homologous chromosomes pair to form bivalents, and non-sister chromatids exchange sections of DNA at chiasmata. This swaps alleles between the chromatids, creating recombinant chromatids with new allele combinations that did not exist on the parental chromosomes.

Because the homologous chromosomes carry different alleles, both processes shuffle which alleles end up together in a gamete. The combined effect of independent assortment and crossing over means that the four gametes from a single meiosis — and the many gametes an organism makes — are nearly all genetically non-identical, providing the variation on which natural selection can act.

Independent assortment acts on whole chromosomes. In metaphase I the homologous pairs line up at the equator with a random orientation, and the orientation of each pair is independent of every other pair. When the homologues separate in anaphase I, each gamete receives a random combination of whole maternal and paternal chromosomes. For an organism with n homologous pairs this alone gives 2ⁿ different chromosome combinations.

Crossing over, by contrast, acts within chromosomes. In prophase I — before the chromosomes assort — homologous chromosomes pair to form bivalents, and non-sister chromatids exchange sections of DNA at chiasmata. This swaps alleles between the chromatids, producing recombinant chromatids that carry combinations of alleles not present on either original parental chromosome.

The two processes multiply each other's effect because they act at different levels and at different stages. Crossing over first creates chromosomes that are themselves new mixtures of maternal and paternal alleles; independent assortment then shuffles these already-recombined chromosomes randomly into the gametes. So instead of assorting only the original parental chromosomes (the 2ⁿ combinations), the cell assorts a far larger pool of genetically unique chromosomes, and crossing over can occur at many different points along each chromosome.

Acting together, therefore, the number of genetically different gametes is vastly greater than the 2ⁿ value that independent assortment alone would produce — effectively making every gamete genetically unique, which is the raw material for natural selection.

Both meiosis and mitosis are forms of nuclear division in which the DNA is replicated once beforehand, both involve the separation of chromosomes by spindle fibres, and both pass on copies of the genetic material to daughter cells.

They differ in several important ways. Mitosis is a single division that produces two daughter cells, whereas meiosis is two successive divisions (meiosis I and II) that produce four cells.

The cells produced by mitosis are diploid (2n) — the same chromosome number as the parent — while those produced by meiosis are haploid (n), the chromosome number having been halved when homologous chromosomes separated in meiosis I.

Crucially, mitosis produces cells that are genetically identical to the parent and to each other, because there is no homologous pairing. Meiosis produces cells that are genetically non-identical, because independent assortment (random orientation at metaphase I) and crossing over (exchange of alleles between non-sister chromatids in prophase I) introduce variation.

Their roles reflect these outcomes: mitosis is used for growth, repair and asexual reproduction, where identical cells are needed, while meiosis produces the gametes needed for sexual reproduction, where variation is advantageous.

(a) The number of homologous pairs is $n = 14 \div 2 = 7$.

The number of different chromosome combinations from independent assortment is $2^{n} = 2^{7}$.

$2^{7} = 128$, so the plant can produce 128 genetically different gametes from independent assortment alone.

(b) This figure of 128 counts only the different combinations of whole maternal and paternal chromosomes produced by independent assortment.

The actual number is far greater because crossing over also occurs. During prophase I, non-sister chromatids exchange alleles at chiasmata, producing recombinant chromatids with combinations of alleles that do not exist on the original chromosomes.

Crossing over can happen at many different points along each chromosome, creating an enormous additional number of allele combinations. Because this variation is generated within chromosomes — on top of the 2ⁿ ways of assorting whole chromosomes — the true number of genetically distinct gametes is vastly larger than 128, effectively unlimited.

Genetic variation in a sexually reproducing species comes from three sources, two acting during meiosis and one at fertilisation.

Independent assortment occurs in metaphase I, when the homologous pairs line up at the equator in a random orientation, independently of one another. When the homologues separate, each gamete receives a random mix of maternal and paternal chromosomes. For an organism with n pairs this alone gives 2ⁿ chromosome combinations (2²³ ≈ 8.4 million in humans).

Crossing over occurs in prophase I, when non-sister chromatids of a bivalent exchange alleles at chiasmata, producing recombinant chromatids with new allele combinations. This shuffles alleles within chromosomes and can separate linked genes, multiplying the variation beyond the 2ⁿ figure.

Random fertilisation then acts after meiosis: which sperm fuses with which egg is a matter of chance, so any one of the millions of genetically different male gametes can combine with any one of the millions of female gametes. In humans this gives roughly $2^{23} \times 2^{23} \approx 7 \times 10^{13}$ possible zygotes from one couple, before crossing over is even considered.

This variation is biologically important because it produces genetically different individuals within a population. If the environment changes, some individuals are more likely to possess advantageous alleles; these survive, reproduce and pass the alleles on (natural selection). Genetic variation is therefore the raw material for evolution and helps a species adapt and avoid being wiped out by a single change in conditions.

It is important to distinguish the sources: independent assortment and crossing over create variation among the gametes (within meiosis), whereas random fertilisation creates variation among the zygotes (after meiosis).