Inheritance (Theoretical Genetics)


Monohybrid Cross, from the College of the Siskiyous (more animations here)

Try out this Dragon Breeding game, but remember dragons aren’t real, so don’t use it in the exam.

Monohybrid cross practice problems:

Problem set (online) from The Biology Project

More practice (first few sets only) from CLC

Pedigree Charts

Great interactive pedigree charts from ZeroBio (more resources here) (Flash Version)

You do not need to know about the Chi Squared test for the IB exams, but it can be useful in investigations of phenotype ratios, so here is how to do it. There is a Chi-Squared calculator in our IBDP Bio Excel StatBook.

What’s it like to be a geneticist? Here’s a profile on the working life from the Guardian.

  1. Wahida Mostafa

    Is there a essential biology revision guide for the genetics unit?

  2. Dr. Sreekala Pillai

    dear steve, i always looked to your website for ready refrence for my teaching . i would be greatful if you could send me the ppts on theoretical genetics by email. i am a bio teacher at podar international school, mumbai

  3. Sayoka Takemura

    I can’t seem to download the presentation for Theoretical Genetics in SlideShare. Is this intentional?

    • Hi Sayoka – yes it is. I’m in the process of updating and improving the presentations, replacing them with editable pptx files. However, to be sent the file you will need to make a donation to one of the charities on my list:

      It takes a lot of work to put these together, so I want it to generate some good.

      Good luck!


  4. Stephen;

    The page from Essential Biology 4.3 may need some changes on the sickle cell page. The trait is describe as “codominant” yet the term “carrier” is used. I think carrier is only for traits which can be hidden, like recessive, sex linked hemophilia. In that case a female may carry the allele and be a “carrier” but if a female is heterozygous for sickle cell then she has “some” sickle cell shaped erythrocytes.

    If this is the case, then maybe the individual homozygous for normal erythrocytes is “normal” and the heterozygous individual is “affected” while the individual homozygous for the sickle cell condition would be “anemic.”

    In any case, that is the changed notation I used with my students. Kudos to Peter Baek in on of my HL classes who asked, “If this is a codominant trait, how can a heterozygous individual “carry” the disease without showing the phenotype?”

    Jay Reimer
    GSIS, Korea

  5. Thanks for picking up on this Jay – and to Peter too!

    This is a funny example where “carrier” does get used (though it seems with ” “)- perhaps not technically correctly – in a lot of sources. I suppose people are thinking from the point of view of having or not-having the disease.

    Perhaps a better set of terms than I have used would be “normal”, “sickle cell trait” and “sickle cell disease”. It might even be simpler just to say heterozygous and then explain what that means. Sickle cell anemia is another name for the disease, so I would personally avoid saying anemic for the heterozygous individual.

    I hope this helps,


  6. I am looking at slide 23. I don’t think we can determine by looking just at the tree whether the PKU gene is dominant or recessive. It could be possible that both affected individuals were heterozygous and passed on the non-PKU allele to their offspring. I could replace PKU with Huntington’s disease, a dominant trait, and still produce the same tree.

    Is my reasoning flawed? Am I missing something?

  7. Stephen,

    Got the ppts off of dropbox – they are, as always, exceptional. Your efforts are insightful and appreciated and your biology4good initiative is commendable and inspiring. Well done!


    • Thank-you very much, Rick!

      I’m going to replace the Theoretical Genetics later in the week with an update.

      Good luck,


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