1.5 Punnett Squares – 3:1 Ratio

The specific ratios seen in the monohybrid cross can be described using a Punnett square, named after R.C. Punnett who devised this approach.

Given the genotypes of any two parents, we can predict all of the possible genotypes of the offspring. Furthermore, if we also know the dominance relationships for all alleles, we can predict the phenotypes of the offspring. This provides a convenient method for calculating the expected genotypic and phenotypic ratios from a cross.

A Punnett square is a matrix in which all of the possible gametes produced by one parent are listed along one axis, and the gametes from the other parent are listed along the other axis. Each possible combination of gametes is listed at the intersection of each row and column, since we know through the process of meiosis that the alleles on each chromosome separate to form the gametes.

The F1 cross would be drawn as in Figure 1.5.1. As you can see, in a Monohybrid cross, the offspring ratios will be 3:1 of dominant phenotype (purple): recessive phenotype (white). Punnett squares can also be used to calculate the frequency of offspring. The frequency of each offspring is the frequency of the male gametes multiplied by the frequency of the female gamete.

A Punnett square showing cross of big A small a by big A small a
Figure 1.5.1 A Punnett Square Showing a Monohybrid Cross. The purple boxes represent the purple colour of the dominant (A) allele, while the white box represents the recessive (aa) allele homozygote.


Two monohybrid crosses - first one is homozygote dominant by homozygote recessive and second is heterozygote by heterozygote
Figure 1.5.2 General Example of a Monohybrid Cross Showing Expected Progeny and Ratios in the F1 and F2 Generations.

View the video, Monohybrid Cross Examples – GCSE Biology (9-10), by Mr Exham Biology (2018), and watch for some worked examples on Monohybrid crosses.

Media Attributions


Betts, J. G., Young, K. A., Wise, J. A., Johnson, E., Poe, B., Kruse, D. H., Korol, O., Johnson, J. E., Womble, M., & DeSaix, P. (2013, Apr 25). Figure 28.25 Random segregation [digital image]. Anatomy and Physiology. CNX OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/28-7-patterns-of-inheritance

Canham, L. (2017). Figure 9. A Punnett square showing a monohybrid cross [diagram]. In Locke, J., Harrington, M., Canham, L. and Min Ku Kang (Eds.), Open Genetics Lectures, Fall 2017 (Chapter 16, p. 9). Dataverse/ BCcampus. http://solr.bccampus.ca:8001/bcc/file/7a7b00f9-fb56-4c49-81a9-cfa3ad80e6d8/1/OpenGeneticsLectures_Fall2017.pdf

Mr Exham Biology. (2018, Dec 31). Monohybrid cross examples – GCSE Biology (9-10) [Video file]. YouTube. https://www.youtube.com/watch?v=Xld3-Fr9oUU


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Open Genetics by Natasha Ramroop Singh, Thompson Rivers University is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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