10.1 Meiosis

Don’t forget to review the Core content.

Random orientation refers to the behaviour of chromosomes in metaphase:

Independent Assortment refers to the alleles of unlinked genes:

Key to success in this section is being able to explain the following:

  • How meiosis essentially leads to infinite genetic variability.
  • The relationship between meiosis and Mendel’s Law of Independent Assortment

Key terms (HL): meiosis, chromosome, chromatid, chiasmata, crossing-over, holliday junction, prophase, metaphase, anaphase, telophase, random orientation, independent assortment.

  1. georgia Barlow

    Hi Stephan,

    I’m a little confused, could you clear something up for me?
    Is independent assortment and random orientation of chromosomes the same thing?

    Your presentations are amazing, i have used them every time for all my mocks and i’m using them now for revision for my final IB exams in a few weeks. They are really great, very clear and easy to understand-thanks SO much!


    • Hi Georgia,

      You’ve hit upon something that I could make clearer in the presentation.

      Random orientation describes the position of the chromosomes when they line up in metaphase. In metaphase I, bivalents are made of the two pairs of sister chromatids (the homologous chromosome pair). The orientation of these has a large effect on genetic variation as the homologous chromosomes may contain many different alleles.

      In meiosis II, the sister chromatids are split. Random orientation in metaphase II will only affect genetic variation where crossing-over has taken place, as the two sister chromatids will be slightly different in the post-chiasma region.

      Independent assortment is something else, and I will make this more clear in the presentation. This refers to the separation of alleles of different genes carried on different chromosomes (unlinked genes). Simply put, as long as two genes are not physically linked to each other, the inheritance of one allele does not affect the inheritance of another. For example, where there are two heterozygous characters (Gg and Ff) on two different chromosomes, random orientation will lead to independent assortment of the alleles – giving four possible gametes (GF, Gf, gF and gf).

      Does this help?

  2. Stephen;

    This AHL meiosis presentation is “the cat’s meow!” You teach the topic clearly as well as highlight the critical issues (like how meiosis leads to variation and difference between independent assortment and random orientation). My hope is that continue developing teaching tools like this for teachers and students.

    In the same way soccer players get credit for “assists” you need to know you score a lot of “IB Diploma assists” at my school. 🙂 Thanks for your “assist-“ance!


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