Constructing Punnett Grids
- A monohybrid trait is one that is controlled by only one gene
- Generally, we consider that such a gene has two alleles
- Either: one allele is dominant and the other is recessive
- Or: the alleles are co-dominant
- A monohybrid cross starts with pure-breeding parents (homozygous), each displaying a different phenotype
- The purpose of a Punnett grid is to predict the probability of a certain offspring displaying a certain genotype or phenotype
- In the case where multiple offspring are produced, Punnett grids can predict the numbers of offspring that will display a certain genotype or phenotype after a cross
Steps in constructing a Punnett Grid
- Write down the parental phenotypes and genotypes
- Write down all the possible gamete genotypes that each parent could produce for sexual reproduction
- A useful convention is to write the gamete genotypes inside a circle to denote them as gametes (haploid cells)
- Place each parental genotype against one axis of a Punnett grid (2 x 2 table)
- In the boxes of the Punnett grid, combine the gametes into the possible genotypes of the offspring
- This gives the offspring of the F1 generation (1st filial generation)
- List the phenotype and genotype ratios for the offspring
Worked example
Sweet peas grow pods that are either green or yellow. The allele for green, G, is dominant to the allele for yellow, g.Construct a Punnett grid to predict the outcome when crossing green and yellow pure-bred plants to show the F1 generation offspring.Using plants from the F1 generation, construct a second Punnett grid to show the outcomes of the F2 generation.
Step 1: Write down the parental phenotype and genotypes
Green coloured pods Yellow coloured pods
GG gg
Step 2: Write down all the possible gamete genotypes that each parent could produce
Step 3: Place each parental genotype against one axis of a Punnett grid (2 x 2 table)
Step 4: Combine the gametes in each box of the Punnett grid
Genotypes of the F1 cross between homozygous green (GG) and homozygous yellow (gg) pea plants. All offspring (100%) have the genotype Gg and the phenotype is green.
Step 5: Take two heterozygous offspring from the F1 generation and cross them
Step 6: Combine the gametes in each box of the Punnett grid
Punnett grid showing the results of the F2 generationPhenotype ratio is 3:1 green:yellow, Genotype ratio is 1 GG: 2 Gg: 1 gg