Punnett Grids

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•  Construction of Punnett grids for predicting the outcome of monohybrid genetic crosses 

A monohybrid cross determines the allele combinations for potential offspring for one gene only

Monohybrid crosses can be calculated according to the following steps:

  • Step 1:  Designate letters to represent alleles (dominant = capital letter ; recessive = lower case ; co-dominant = superscript)
  • Step 2:  Write down the genotype and phenotype of the prospective parents (this is the P generation)
  • Step 3:  Write down the genotype of the parental gametes (these will be haploid and thus consist of a single allele each)
  • Step 4:  Draw a grid with maternal gametes along the top and paternal gametes along the left (this is a Punnett grid)
  • Step 5:  Complete the Punnett grid to determine potential genotypes and phenotypes of offspring (this is the F1 generation)

Overview of a Monohybrid Cross

punnett grid

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•  Comparison of predicted and actual outcomes of genetic crosses using real data

The genotypic and phenotypic ratios calculated via Punnett grids are only probabilities and may not always reflect actual trends

  • E.g. When flipping a coin there is a 50% chance of landing on heads – this doesn’t mean you will land on heads 50% of the time

When comparing predicted outcomes to actual data, larger data sets are more likely to yield positive correlations

  • Gregor Mendel performed over 5,000 crosses as part of his pea plant experiment
  • However many statisticians believe Mendel’s results are too close to the exact ratios predicted to be genuine

Mendel’s Pea Plant Experimental Data

Mendel's data

Mendel crossed different varieties of pea plants and recorded the characteristics of resultant offspring

  • Initially, he crossed purebred dominant and purebred recessive plants in order to produce heterozygotes (F1 generation)
  • He then self-pollinated the heterozygotes to produce an F2 generation and counted the dominant and recessive phenotypes
  • The expected ratio of dominant : recessive phenotypes was 3 : 1 – this ratio was supported by the experimental data