Genetic variation from meiosis and fertilisationAQA A-Level Biology: Revision notes

Meiosis produces gametes that are genetically different from each other. There are two sources of variation during meiosis: independent segregation of homologous chromosomes and crossing over between homologous chromosomes. Random fertilisation adds further variation within a species.

This variation is important because sexually produced offspring are different from their parents and from each other.

In meiosis I, each pair of homologous chromosomes lines up at the cell equator and the two chromosomes are pulled to opposite poles. Which chromosome of one pair goes to which pole does not affect how the other pairs separate, so the pairs segregate independently.

This produces gametes with different combinations of maternal and paternal chromosomes. Each gamete gets one chromosome of each pair, but a different mixture of the two parental origins.

During meiosis I, homologous chromosomes pair up and their non-sister chromatids twist around each other. Sections of the chromatids break and are exchanged, so alleles are swapped between the maternal and paternal chromatids.

The chromatids that result carry new combinations of alleles. For a plant with $A$ and $B$ on one chromosome and $a$ and $b$ on the other, crossing over can produce chromatids with $Ab$ and $aB$ as well as the original $AB$ and $ab$. This gives further genetic variation among the daughter cells.

Random fertilisation of haploid gametes further increases variation: any male gamete can fuse with any female gamete, so the zygote gets a combination of alleles that is almost certainly different from every other zygote. This increases genetic variation within a species, because offspring of the same parents differ from each other and from their parents.

With $n$ pairs of homologous chromosomes and no crossing over, the number of possible chromosome combinations in a gamete is

$2^n$

because each pair can separate in 2 ways, independently of the rest.

Random fertilisation of two gametes gives

$(2^n)^2 = 2^{2n}$

Worked example: a species with $n = 3$ gives $2^3 = 8$ gamete combinations. Random fertilisation gives $8^2 = 64$ zygote combinations. For humans, $n = 23$, so $2^{23} = 8\,388\,608$ gamete combinations and $2^{46}$ (about $7.0 \times 10^{13}$) zygote combinations. Crossing over makes the true numbers even larger.