Example 1
A species has 2n=16. Find the chromosome count in a normal gamete and zygote.
A gamete has n=8. Fusion of two normal gametes restores 16 in the zygote.
Year 12 · SACE Stage 2
Meiosis, fertilisation and variation
Track homologues, chromatids and chromosome sets.
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Method and worked examples
Diploid cells have two sets of chromosomes, including homologous pairs. After one DNA replication, meiosis has two divisions. Homologous chromosomes pair and can exchange corresponding segments in prophase I. Homologues separate in anaphase I, reducing chromosome-set number. Sister chromatids separate during meiosis II. The usual result is four haploid products rather than two diploid daughters.
Crossing over creates new combinations along chromosomes. Independent assortment reflects the different possible orientations of homologous pairs at metaphase I. Ignoring crossing over, n independently assorting chromosome pairs can produce 2n chromosome combinations in gametes. Fertilisation combines two haploid gametes to restore diploidy and adds further variation through random combinations. These processes rearrange existing alleles; mutation is the ultimate source of new alleles. Mitosis usually preserves chromosome sets rather than halving them.
Example 1
Example 2
There are three homologous pairs. Ignoring crossing over, how many assortment combinations are possible?
23=8 combinations of maternal and paternal chromosomes can enter a gamete.
Watch out: It usually rearranges existing alleles between homologues; mutations create new sequence variants.