How To Use a Punnett Square to Do a Monohybrid Cross
The Punnett square was created by an English geneticist.A Punnett square is a simple method for determining the theoretical ratios of genotypes and phenotypes that would occur in the offspring of a cross between two parents.A mono hybrid cross is when you only look at the genetic outcomes for a single gene.
Step 1: Understand the genetics.
A genetic code is the heritable code of an individual.The alleles on the two chromosomes that make up an individual's genotype come from their parent.There is a specific form of a gene.One allele can code for blonde hair, while another can do the same for brown hair.Every person has two chromosomes and two alleles that make them different, so they are represented with two letters.Capital and lowercase letters represent dominant and recessive alleles.It doesn't matter what letter you choose, it makes sense to you.The first letter of the allele is usually used.The B and b genes are used to represent the dominant and recessive genes for brown and blonde hair.
Step 2: Draw a grid.
The Punnett square seems to be just a divided square.If you want to divide your square into four smaller squares, you need to draw two lines through the center of the square.For two letters, leave enough room in each box.Leave room at the top and the left side of the square.
Step 3: The top of the square should be labeled with one parent's name.
You would put a B over the top left-hand square if the mother has brown hair and a Bb.It doesn't matter where you put the parents' genes.Each column has one letter above it.
Step 4: The other parent should be labeled with the left side of the square.
If the father also has brown hair, you would put one B to the left of the upper left-hand square and the other B in the lower left hand square.
Step 5: The boxes are used to match the alleles.
The alleles will fit into the two boxes below it or to the right of it.If the allele B is in the horizontal top- left corner, write it on the two left boxes below.If the allele B is in the vertical top left corner, you have to write it in two boxes to the right.Continue to fill in the boxes until there are at least two alleles from each parent.The convention says to always write the capital dominant allele first.The parents of our hair color example are brown-haired.You would have to know their specific genetic makeup.If the parent is blonde, you know their genotype.
Step 6: Take the number of each genotype.
There are only three possible combinations of genes for a mono hybrid cross.The brown hair and blonde hair all have the same allele in their genes.There are two different alleles for the gene in brown hair.You can only get one or two genotypes from some crosses.The Punnett square would show the possibilities as twoBB and twoB.If you cross two parents with the same genotype, you will have all of them.If you cross two parents with different genes, all of them will be the same.You will have two homozygotes if you cross a Heterozygous with a parent.If you cross two parents with the same name, you will have two different types ofzygotes.
Step 7: The ratio is calculated.
The counts from the previous step can be used to determine the ratio.A hair or eye color is a phenotype.The dominant phenotype will be displayed if the trait shows complete dominance.The ratio would be 4:0 if there were four dominant brown-haired offspring and zero blonde offspring.Half of the offspring will have brown hair, while the other half will be Heterozygous.