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Match the alleles together using the boxes as guides. Count the number of each genotype. Calculate the phenotype ratio.
Each allele will fit into the two boxes below it or to the right of it, depending on its placement. For example, if the allele B is in the horizontal top-left corner, write B in the two left boxes below. If the allele B is in the vertical top left corner, you must write B in the two boxes to the right. Continue to fill in the boxes with alleles until each box has two alleles, one from each parent. The convention is to always write the capital dominant allele first followed by the lowercase recessive allele. For our hair color example two brown-haired parents could either be BB or Bb. You would have to know their specific genotype. However, if the parent is blonde you know their genotype is the recessive bb. For a monohybrid cross, there are only three possible genotypes: BB, Bb, and bb. BB (brown hair) and bb (blonde hair) are homozygous for a gene meaning that they have two identical alleles for one gene. Bb (brown hair) are heterozygous meaning that they have two different alleles for the gene. Some crosses may only give you one or two genotypes. For our example, using the cross BB x Bb, the Punnett square would show the possibilities as two BB and two Bb. If you cross two homozygous parents with the same genotype (BB x BB or bb x bb) all of the genotypes will be homozygous (BB or bb). If you cross two homozygous parents with different genotypes, BB x bb, all of the genotypes will be Bb. If you cross a heterozygous with a homozygous parent (BB x Bb or bb x Bb), you will have two homozygotes (BB or bb) and two heterozygotes (Bb). If you cross two heterozygous parents, Bb x Bb, you will have two homozygotes (1 BB and 1 bb) and two heterozygotes (Bb) Using the counts from the previous step, you can determine the phenotype ratio. A phenotype is the physical characteristic of the gene, such as hair or eye color. Assuming the trait shows complete dominance, the heterozygous genotype (the cross showing dissimilar pairs of genes for a hereditary characteristic) will display the dominant phenotype. In the BB x Bb cross, the phenotypes will be four dominant brown-haired offspring (2 BB and 2 Bb) to zero recessive blonde-haired offspring (bb) so the ratio would be 4:0. 100% of the offspring will have brown hair, but 50% of them will be homozygous while the other 50% will be heterozygous.