Meiosis and the random fertilisation of gametes during sexual reproduction…

The idea

Meiosis and fertilisation do not make new alleles; they make new combinations of the alleles already present, and three processes do the work. In metaphase I, homologous chromosome pairs line up on the equator and the orientation of each pair is independent of every other pair, so independent segregation sends a random mixture of maternal and paternal chromosomes into each daughter cell; with 23 pairs that alone gives 2^23 = 8,388,608 combinations. In prophase I, homologous chromosomes form bivalents and crossing over exchanges sections of chromatid at the chiasmata, producing chromosomes that carry new combinations of alleles of linked genes. Then random fertilisation means that any one of the genetically different male gametes can fuse with any one of the female gametes, multiplying the variation again.

What earns the marks

• Independent segregation, or random assortment, of homologous chromosomes.
• Crossing over between homologous chromosomes, or within a bivalent.
• Random fertilisation, or random fusion, of gametes.
• Stating that these processes produce new combinations of alleles, or of maternal and paternal chromosomes.
• Distinguishing these processes from mutation, which is the source of new alleles.

Common mistakes and how to put them right

• Writing random mating when the marking point is the random fertilisation of gametes.

• Saying meiosis creates new alleles, when it only recombines alleles that already exist.

• Describing crossing over as occurring between sister chromatids rather than between homologous chromosomes.

• Naming the processes without saying what they produce, so the new combinations of alleles mark is lost.

• Placing independent segregation in mitosis, or in meiosis II.

Exam tips

• Clearly list the three named processes and explicitly state their outcome to ensure all aspects of the mechanism are covered.
• Write homologous every time you mention the chromosome pairs, because that word is needed for precision.
• If the question has already given you mutation, do not repeat it; it is asking for the other processes.
• Show the calculation 2^23 = 8,388,608 if asked to explain the number of combinations.