P1                   x                   P2 
 

                                                  F1         x         F1 

 
                                                             F2 

The P's above represent the parents for the first cross. The "x" represents that they are being mated. The "F1" represents the offspring produced by these parents. The F2 are the offspring  only when two F1 are crossed. 

 If your parents are represented by the P's than you would be some of the F1. Are your children going to be the F2?  No!  Not unless you marry your brother or sister. Remember F2 are the offspring produced when two F1 are crossed. 

How to use the Punnett Square. 

This method of determining  likely genotypes and phenotypes of offspring is a non- mathematical method. Read the pages indicated by D-3. The first step is to draw a Punnett box with the correct number of rows and columns. The number of rows and columns is determined by calculating the number of genotypically different types of gametes. The rows would represent the gametes (sperms or eggs) of one parent and the columns would represent the gametes (sperms or eggs)  of the other parent. 

The number of genotypically different types of gametes can be determined by the following formula: 
 
     number of different types of gametes = 2ⁿ ;   where n = the number of heterozygous genes involved in the cross. 

1.     Rr = 1 set of heterozygous genes 
2.     rr  = 0 set of heterozygous genes 
3.     RrLl = 2 sets of heterozygous genes 
4.     rrLl = 1 set of heterozygous genes 
5.     RrLlCc = 3 sets of heterozygous genes 

Note: The first problems on this web page (monohybrid) will consist of only one set of genes; therefore,  only 1 and 2 will be the case. Punnett squares involving monohybrid problems will contain 1 or 2 row and 1 or 2 columns. 
 

   Example:     P1 =   Rr                 x            rr  =  P2  

 
Rr will form how many different types of gametes?    2ⁿ     = 2¹     = 2 
 
rr will form how many different types of gametes?     2ⁿ    = 2⁰       = 1 
 
 

 
Since parent 1 (P1) is Rr and parent 2 (P2) is rr, how many columns and rows will be in the Punnett square? 

   P1 = Rr = 2 (see above) therefore 2 columns; 
   P2 = rr = 1 (see above) therefore 1 row 

 Draw 
 

The next step is to determine the different types of gametes for each parent. Study behavioral objective 7. Remember, the gametes are haploid; therefore, there will only be one allele (letter) for each gene. 

Let's review diploid/haploid and somatic/sex cells (gametes). Cells that make up an organism are called somatic cells. Cells which undergo meiosis forming sex cells which may be fused to form a zygote are gametes. In humans the  body cells (somatic cells) are diploid. Diploid refers to the condition that the chromosomes are in pairs; therefore, the genes must also be in pairs (2 alleles for each gene). The gametes are haploid. Haploid refers to the condition that there is only one of each homologous pair present; therefore, there will only be 1 allele for each gene. 

Example Rr = one gene, two alleles and represents a diploid condition. 
              R   = one gene, one allele and represents a haploid condition. 
 

Now let us go back to the earlier problem.   Rr    x    rr 
 
If parent 1 (P₁) is Rr and parent 2 (P2) is rr, how many columns and rows will be in the Punnett square? 

   P1 = Rr = 2 (see above) therefore 2 columns;  P2 = rr = 1 (see above) therefore 1 row 

What are the different types of gametes (sperm or eggs) for each parent? 

                                     P1 = "R" or  "r"    and P2 = only  "r" 

1. Place these in the correct space of the Punnett square. 
 
 

Click here to check answer.  click 
 

2. During fertilization one sperm will combine with one egg to form a diploid zygote. The zygotes are represented by the "cells" of the square. Fill in the "cells" of the square. (one allele from each parent) 
 

 Click here to check answer.  click 
 
 
 
 
 
 
 
 
 
 
 
 

   Ratio -  1 Rr : 1 rr (genotype ratio) or   1 red : 1 white (phenotype ratio) 

   Frequency -   50 % of the offspring will be  heterozygous and 50 % will be homozygous recessive;  50 % of the offspring will be red and 50% will be white 
 
   Fraction -        1/2 of the offspring will be heterozygous and 1/2 will be homozygous recessive 
 

Example: Using the cross above calculate the ratio, frequency and fraction of offspring which may be produced by these two parents. 

         (P1)   Rr              X                   (P2)    rr 

The ratio would be 1 Rr to 1 rr (this would be called a genotype ratio because is lists the genotype.  The phenotype ratio would be 1 red to 1 white. 

The frequency would be 50% Rr and 50% rr (genotype) and the phenotype frequency would be 50% red and 50% white. 

The fractions would 1/2 Rr and 1/2 rr. 
 

One more time! 

3.    Example:    P1 = Ll        and       P2   =  Ll 
  

                  (LL, Ll = long stem and ll = short stem) 

       What would be the phenotypes of the parents? 

Click here to check answer.  press 
 
 
 
 
 
 
 
 

Example:      Cross these two parents -        P1 = Ll     X       P2   =     Ll 

This is a monohybrid problem because only one set of genes is involved, the parents are given as P1 and P2, and the F1 are the offspring formed "cells in square" . 

4. How many different types of gametes will be produced by each parent? (columns and rows) 
 

Click here to check answer.  click 
 
 
 
 
 
 
 
 
 
 

5. What are the different types of gametes for each parent? 

Click here to check answer.  click 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

6. Draw the Punnett square with the "cells" filled in. 
 
 

 Click here to check answer.  click 
 
 
 
 
 
 
 
 
 

7. What would be the genotype ratio?    phenotype ratio? 
 
Click here to check answer.  click 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

8. What percent of the offspring  would you expect to have long stems?   short stems? 
 
Click here to check answer.  click 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

This is the end of this web page. You must answer the following question correctly to go  to the next web page involving  monohybrid problems. 

9. Which set up would be the correct Punnett square for the following cross? 

                       Ss       x         Ss 

 a) 

 b) 

 c) 

 
 
 
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