A Monohybrid Cross with Complete Dominance

Punnett Square Sample Problem 1:
In rose flowers, red is dominant over white. What are the possible offspring when a homozygous red rose is crossed with a homozygous white rose?
Six steps are needed to solve this problem:

1. Write genotype for each parent.
  • Use a capital case letter of the dominant trait to represent a dominant gene.
    R - for the dominant trait red.
  • Use a small case letter of the dominant trait to represent a recessive gene.
    r - for the recessive white.
  • Both parents are homozygous, meaning the two genes for the trait are the same.
     
  • The genotypes for the cross are -
RR X rr
 
 
2. Write the possible gametes of one parent across the top.

Write the possible gametes of the other parent down the side.

Since the genes are the same, only one letter is used in this problem.
 

 

 

3. Draw the lines of your Punnett Square. This is the first time you know how many squares are inside the Punnett Square. This problem has only one!

Combine the gametes from each parent in the squares. Always write capital letters first.
 

 

 

4. Count the number of each possible combination present in your Punnett Square. These are the offspring genotypes.
1   R r

Since the genes are not the same,
this trait is heterozygous.

 

5. The phenotypes are the way the offspring appear.
1   red
 
 
6. Divide the number of each combination type by the total combinations.

You now have the probability ratio for the offspring of the parents.

1 / 1

One out of one, or all, of the offspring will be heterozygous red, R r
 

 

 
Sample Problem 2:

  • Draw a Punnett square showing a cross between two of the offspring in problem 1. check
     
  • What genotypes are predicted by the square? check
     
  • What phenotypes are predicted by the square? check
 

 

 
Sample Problem 3:

In carnations, dominance is incomplete. What are the possible results when two pink carnations are crossed?

  • Incomplete dominance means neither trait (color in this example) is dominant. Therefore BOTH colors are expressed equally.
     
  • Since neither trait is dominant, no capital letters are used in the Punnett square. Use the first letter of each trait to indicate that gene in the square.
     
  • Show the genotypes for both parents. check
     
  • Draw the Punnett square. check
     
  • What are the phenotypes of the offspring? check