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A Punnett square is a type of mathematical tool that geneticists use to predict offspring ratios and show the allelic combinations of gametes. Geneticists make a Punnett square to determine the probability of a person inheriting a specific trait. It is a graph that takes the genotypes of the parents to see all of the possible genotype combinations for their children, as well as the odds of these genotypes happening.
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Next, put one parent's genotype across the top in the two right-hand side squares. Put the other parent's genotype in the two lower squares on the left-hand side. It doesn't matter which parent's genotype is placed where in the grid.
Now, the remaining two squares in the middle column and the remaining two squares in the right column should be filled in. For example, if the top two squares are “G" and the left-hand side squares are “Y", the squares filled in in this step would all be “YG". The YG provides geneticists with the predicted frequency of all possible genotypes that two parent's may pass down to their child.
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Importance of Punnett Squares
Some feel that this tool is just a fun academic game, but it can be used to determine important genetic information. For example, if a set of parents both carry a gene associated with a genetically inherited disease, they can use this tool to determine how much of a chance their offspring will have in terms of inheriting it.
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Organisms only being controlled by one gene with two alleles is very rare. Just about all phenotypes in the real world require complex interactions between more than one set of genes to be produced. A good example of how traits are inherited are how a child inherits their eye color from their parents. A child will carry a gene from each parent, or two copies of each gene. If one parent has green eyes and the other has brown eyes, the child can carry two copies of of the green-eye gene, two copies of the brown-eye gene, or one copy of the green-eye gene and one copy of the brown-eye gene. This is just a simple explanation of eye color genetics and how it is inherited. In order to fully understand this complicated process, one would have to study the polygenic system.
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Morris, P. J. (2000). Figuring Out How Genes Combine. Retrieved on January 28, 2010 from Athro Limited: http://www.athro.com/evo/gen/punnett.html
Palomar College. (2009). Probability of Inheritance. Retrieved on January 28, 2010 from Palomar College: http://anthro.palomar.edu/mendel/mendel_2.htm
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Punnett Square: Genetica9 – Wikimedia Commons