A Punnett square is a square diagram used to predict the genotypes of an offspring. The genotype is the actual genetic makeup of an organism, usually written in alleles. Humans have two alleles for every autosomal gene in our body. A typical monohybrid cross follows one gene and can be easily observed in a 2 x 2 Punnett square. If you wanted to track two genes together a 4 x 4 Punnett square will come in handy. But, instead of filling in 16 boxes, there is a quicker way to find out the genotypes using the forked line method.
Steps
Part 1
Part 1 of 3:
Setting Up the Problem
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1Highlight clues in the problem. It is important to decipher what the problem is asking. Make sure to highlight what the different alleles produce because it can get confusing.
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Write down the genotypes for both parents.
- Heterozygous for a gene means that one allele is dominant and one allele is recessive.
- Homozygous means that both alleles are either dominant or recessive.
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Draw two 2x2 Punnett squares. First, draw a large square and divide it into 4 equal squares. Repeat this step one more time. You will be crossing each gene from each parent separately. The gene for size and the gene for color will go in their own 2x2 Punnett square.
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Label the Punnet square. The mother’s genotype, and the sides of the Punnett square with the father’s genotype. Remember that an individuals genotype for a dihybrid cross has 4 alleles. These 4 alleles make up two different genes.Advertisement
Part 2
Part 2 of 3:
Breaking it Down
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Perform a Punnet square cross two times. Take the mother first allele write it into the two boxes below. Do this step again for the mother’s second allele. Then take the father’s first allele and write it into the two boxes to its right. Do this step again for the father’s second allele. Once one punnet square is full do it again for the other gene. It is important to write the dominant allele first in the box when crossing the two alleles. “Rr” is preferred over “rR”
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2List down the genotypes for each cross. The two letters in each box are the genotypes.
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3Calculate how frequent each genotype shows up in both Punnett square. Answers should be written in fraction form. The numerator for each genotype should be somewhere from 1-4 and the denominator should be 4 for each genotype.
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List down the new three genotype frequencies you found from the first genetic cross (Punnett square #1). Make sure to write these numbers above one another, leaving about 2 or 3 inches of space between them.
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Draw three arrows after each of the three frequencies. One arrow should point slightly up, one horizontal, and the other slightly down. This is where the phrase forked line method is derived from. It should look like a fork at this point. A total of 9 arrows should be drawn.
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List down the other 3 possible genotypic frequencies for the second Punnett square on each end of an arrow. There should be a total of 9 genotypic frequencies written, but they are only three different frequencies.
- Make sure this step looks the same after each of the 3 separate genotypic frequencies.
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Draw a horizontal arrow after the genotypic frequencies you wrote down. There should be 9 arrows.
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Combine the genotype from the first column and second column and write them after the last arrow. Do this step 9 times until all arrows have a 4 letter genotype.Advertisement
Part 3
Part 3 of 3:
Calculating Your Genotypic Frequencies Phenotypic Ratios
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Write the genotypic frequency for each of the 4 letter genotype. To find this multiply the frequency in the first column to the frequency in the second column.
- A calculator can be used or just multiply the numerator separately and the denominator separately and turn it into a fraction. The denominator should be out of 16 because (4 * 4= 16).
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Group the genotypic ratios of the genotypes that produce the same looking offspring. This is to find the phenotypic ratios. There are 9 different genotypes with 4 different physical combinations possible. There should be 4 different groups of phenotypic ratios.
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Find the sum of the genotypic ratios that produce the same phenotype.
- The number should be in fraction form with the denominator of 16.
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Write down the phenotypic ratio in standard form for the entire cross. The phenotypic ratios for this cross are 9/16, 3/16, 3/16, 1/16. To simplify these numbers, you would write 9:3:3:1.Advertisement
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Tips
- If two heterozygous parents are crossing then the ratio will always be 9:3:3:1.Thanks
- A homozygous dominant parent crossed with a homozygous recessive parent then the all the offspring will be heterozygous.Thanks
- These crosses only works for genes that follow independent assortment.Thanks
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