Punnett Squares

     Punnett squares are tools that can help determine the possibilities of allele* combinations for a genetic cross. That means that by using Punnett squares, you can discover both the physical appearance as well as the genes of an offspring. For example:

• the parents are both heterozygous* (Rr, Rr) 
• right handed people are dominant to left handed people (R = right handed, r = left handed)

The possibility of having a right handed offspring is 75%. The possibility of having a left handed offspring is 25%. The genotype of the offspring is 50% heterozygous (Rr), 25% homozygous* dominant (RR), and 25% homozygous recessive (rr). There are many types of Punnet squares, but the most commonly used are monohybrids (4 squares), dihybrids (16 squares), and trihybrids (64 squares). A monohybrid Punnett square looks like this:

     The working for the problem above was done on a Punnett square identical to this one. The value of each square is 25%. A dominant allele stands out and basically covers up a recessive allele. The only time a recessive allele is seen is when there are two. 

     Two other terms used with Punnett square are codominance and incomplete dominance. Codominance is when two dominant alleles are arguing and they both are seen. That is why some people have one green eye and one blue eye. Incomplete dominance is when two dominant alleles are arguing and decide to combine. A red flower and a white flower make a pink flower. It is just like mixing paints.

                                                                                                                                      

*heterozygous - two different alleles
*homozygous - two of the same allele
*allele - different forms of a gene

I got all my information from my online textbook at http://www.pearsonsuccessnet.com/.