Use a Punnett Square to Do a Monohybrid Cross

The Punnett square was invented by an English geneticist by the name of Reginald Punnett in the early 20th century.[1] A Punnett square is a simple method for determining the theoretical ratios of gene expression that would occur in the children of a cross between two parents. A monohybrid cross is when you are only looking at the genetic outcomes for a single gene.[2]

Steps

Setting Up the Punnett Square

  1. Understand genes and genotypes. A genotype is the heritable, genetic code of an individual.[3] An individual’s genotype comes from the alleles on the two chromosomes inherited from their parent. An allele is the specific form of a gene.[4] For example, a gene codes for hair color but one allele could code for blonde hair, while another codes for brown hair.
    • Every individual has two chromosomes with two alleles that make their genotype so it is represented with two letters.
    • Capital letters indicate dominant alleles while lowercase letters represent recessive alleles.[1]
    • It doesn’t matter what letter you choose to represent your gene of interest, so choose something that makes sense to you. Usually, the first letter of the dominant allele is used.
    • For example, B represents the dominant gene for brown hair and b represents the recessive gene for blonde hair.
  2. Draw a 2 x 2 grid. As its name suggests a Punnett square is just a divided square. Draw your square and divide it into four smaller squares by drawing two lines (one horizontal and one vertical) through the center of the square.
    • Leave enough room in each box for two letters.
    • Also, leave room at the top and left side of the square.
  3. Label the top of the square with one parent genotype. For example, let’s say the mother has brown hair and a genotype of Bb, then you would put a B over the top left-hand square and a b over the top right-hand square.
    • It doesn’t matter where you put each parent’s genotype.
    • Only one letter goes above each box.
  4. Label the left side of the square with the other parent genotype. For example, if the father also has brown hair, but his genotype is BB, you would put one B to the left of the upper left-hand square, and another B to the left of the lower left-hand square.

Performing the Cross

  1. Match the alleles together using the boxes as guides. 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.
  2. Count the number of each genotype. 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.[5] 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)
  3. Calculate the phenotype ratio. 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.[6] 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.

Tips

  • Check to make sure how the trait you are crossing is expressed. For example, if you are asked to give a phenotype ratio using a Punnett square, your ratio will be different if the trait shows incomplete dominance or co-dominance than if your trait shows complete dominance.

Related Articles

Sources and Citations

  1. ↑ 1.0 1.1 http://anthro.palomar.edu/mendel/mendel_2.htm
  2. ↑ http://biology.about.com/od/geneticsglossary/g/monohybridcross.htm
  3. ↑ http://www.pged.org/personal-genetics-101/what-is-genotype-what-is-phenotype/
  4. ↑ http://www.nature.com/scitable/definition/allele-48
  5. ↑ http://www.phy.ornl.gov/csep/mu/node6.html
  6. ↑ http://www.pged.org/personal-genetics-101/what-is-genotype-what-is-phenotype/
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