Passage of information from parents to offspring

· Genetic information is passed from parents to offspring through gametes during sexual reproduction.
· Meiosis produces haploid gametes from diploid cells.
· Fertilisation restores the diploid number by the random fusion of gametes.
· Meiosis and fertilisation create genetic variation, so offspring resemble parents but are not genetically identical.

Haploid, diploid and homologous chromosomes

· Haploid (n) = cells contain one set of chromosomes; gametes are haploid.
· Diploid (2n) = cells contain two sets of chromosomes, one set inherited from each parent.
· Homologous chromosomes = a pair of chromosomes with the same genes at the same loci, but they may carry different alleles.
· In diploid cells, chromosomes occur as homologous pairs.
· After fertilisation: haploid gamete + haploid gamete → diploid zygote.

This image shows that homologous chromosomes form pairs with the same genes in the same positions. They may contain different alleles, which is important for genetic variation during meiosis. Source

Why meiosis is needed

· Reduction division is needed because gametes must have half the chromosome number.
· Without meiosis, chromosome number would double every generation after fertilisation.
· Meiosis reduces 2n → n, so fertilisation restores n + n → 2n.
· This maintains the constant chromosome number of a species across generations.
· Meiosis also produces genetically different gametes, increasing variation.

Overview of meiosis

· Meiosis has two nuclear divisions: meiosis I and meiosis II.
· DNA replicates before meiosis, producing chromosomes made of two sister chromatids joined at a centromere.
· Meiosis I separates homologous chromosomes.
· Meiosis II separates sister chromatids.
· Final result: four haploid cells, usually genetically different.

This diagram summarises the main events of meiosis. It shows how a diploid cell undergoes two divisions to produce haploid daughter cells. Source

Stages of meiosis I

· Prophase I: homologous chromosomes pair up; crossing over occurs between non-sister chromatids; nuclear envelope breaks down; spindle forms.
· Metaphase I: homologous pairs line up at the equator; random orientation of pairs occurs.
· Anaphase I: homologous chromosomes are pulled to opposite poles; centromeres do not divide.
· Telophase I: chromosomes reach poles; nuclear envelopes may reform; cytokinesis may occur.
· Meiosis I is the reduction division because it separates homologous chromosomes and halves chromosome number.

Stages of meiosis II

· Prophase II: chromosomes condense again; nuclear envelope breaks down; new spindle forms.
· Metaphase II: chromosomes line up singly at the equator.
· Anaphase II: centromeres divide and sister chromatids separate.
· Telophase II: chromosomes arrive at poles; nuclei reform; cytokinesis produces four haploid cells.
· Meiosis II is similar to mitosis, but starts with haploid cells.

This diagram helps identify the key stages of meiosis. It is useful for comparing chromosome behaviour in meiosis I and meiosis II. Source

Genetic variation during meiosis

· Crossing over occurs in prophase I when non-sister chromatids of homologous chromosomes exchange equivalent DNA sections.
· Crossing over forms new combinations of alleles on chromosomes.
· Chiasmata are the visible points where chromatids have crossed over.
· Random orientation occurs in metaphase I, when each homologous pair lines up independently at the equator.
· This produces different combinations of maternal and paternal chromosomes in gametes.
· Independent assortment means homologous chromosome pairs are distributed independently of other pairs.
· Random orientation of sister chromatids in meiosis II can also contribute to genetically different gametes.

This image shows crossing over between non-sister chromatids of homologous chromosomes. It explains how recombination creates new allele combinations in gametes. Source

Random fertilisation

· Fertilisation = fusion of two haploid gametes to form a diploid zygote.
· Fertilisation is random, because any genetically different sperm can fuse with any genetically different egg.
· Random fertilisation increases the number of possible genetically different offspring.
· Variation is produced by crossing over, random orientation / independent assortment, and random fusion of gametes.

These diagrams show how meiosis produces gametes with different genetic combinations. They are useful for linking chromosome behaviour to genetic variation in offspring. Source

Interpreting meiosis diagrams and photomicrographs

· Identify prophase I by paired homologous chromosomes and possible chiasmata.
· Identify metaphase I by homologous pairs lined up at the equator.
· Identify anaphase I by homologous chromosomes separating while sister chromatids remain joined.
· Identify metaphase II by chromosomes lined up singly at the equator.
· Identify anaphase II by sister chromatids separating as centromeres divide.
· In exam answers, describe what chromosomes are doing, not just the stage name.

Common exam mistakes to avoid

· Do not say meiosis produces genetically identical cells; it produces genetically different haploid cells.
· Do not confuse homologous chromosomes with sister chromatids.
· Do not say centromeres divide in anaphase I; centromeres divide in anaphase II.
· Do not say crossing over happens in mitosis; it happens in prophase I of meiosis.
· Do not forget that fertilisation restores the diploid number.