10.2 Dihybrid Crosses and Gene Linkage (AHL)


Dihybrid crosses:

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

Dihybrid Crosses from ZeroBio

Dihybrid cross problem set, from Arizona


Gene Linkage, Crossing Over and Recombination:

Alleles that don’t assort independently, from LifeWire

Crossing Over animation:

Recombination frequencies activity, from ThinkQuest

Holliday junctions and crossing over, from Wisconsin


Key terms: genotypic, phenotypic, ratio, dihybrid, linked, unlinked, autosomal, sex, chromosomes, genes, autosomes, crossing-over, non-sister, chromatids, meiosis, prophase I, alleles, recombination, linkage group.

  1. Stephanie Chisholm

    Hi Stephen,

    You ppt’s are amazing and I use them in my classes all the time. I was wondering if this new one was available on slideshare? Thanks for all your hard work!

  2. Cristina Guevara

    Hi Stephen,

    I’ve been using your PPt’s (giving credits to you obviously) and found them really useful. I like the links and how they make connections with ‘real life’. Thanks for sharing such amazing resources.

    A comment about the above presentation: On slide 27, in the diagram at the bottom I think the first ‘ssnn’ was swapped with the second ‘SsNn’.

    Thanks again,


    • Hi Cristina,

      Thanks for the comment – and especially for spotting the error. I will fix it an re-upload it as soon as time allows.

      I’m glad the resources can be of use.

      Good luck!


  3. Thanks Steve again…excellent work. I think error on slide 14, for 3.1 ratio, shld be SsYY

  4. hi stephen,

    those slides are really helpful.thx a lot..

  5. Hi Stephen,

    Thank you for what is an excellent resource.

    I had a question about Topic 10.2, slides 21 – 25. Maybe my class and I have interpreted the information incorrectly, but we are not sure why there would be a 9:1 phenotypic ratio in the test cross. Are we missing something, or is this a mistake?

    Thanks once again,

    Steve Potts
    DP Biology, UWC Maastricht.

    • Hi Steve,

      I’m not sure I get the question. In this cross, we know that a smooth green pea is test crossed. The test cross must be ssyy. However, there is an even likelihood of the smooth green pea being either SSyy or ssyy. We end up with a 3:1 phenotypic ratio.

      I hope this helps,


  6. Hello Stephen,
    Thank you for making your resources available to the public. It is appreciated on multiple levels!
    I think I’m missing something about slide #14, 4:3:1, could you elaborate on why “ssYy” (yellow rough) is together with SSYy and SsYy (yellow smooth)?
    Thank you,

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