Hemophilia is crossed in a 2 x 2 Punnet square using the technique for single hybrid, sex-linked crosses. The trait in question, hemophilia, should be denoted with a superscript on the X chromosome. Typically a capital H is used for the normal gene; the lower case h is used to denote the gene for hemophilia. Filling out the Punnet square gives the predicted percentages of normal, carrier or hemophiliac children.
In order to set up the Punnet square properly, there is some information necessary about the gene in question. For hemophilia, it is important to know that it is sex-linked, carried on the X chromosome and recessive. This means that women will only have hemophilia if both X chromosomes contain the hemophilia gene because a normal gene on one of the X chromosomes is dominant over the hemophilia gene. Women with only one hemophilia gene are carriers. Men with the hemophilia gene are afflicted with hemophilia because they only have one X chromosome.
To use a Punnet square on a cross between two people, draw a 2 x 2 square. Write the woman's chromosomes down the left side, noting if each chromosome is XH or Xh. Next, write the man's chromosomes across the top, noting if his X chromosome is XH or Xh. Then, carry out the cross as you would in any other Punnet square. As an example, when crossing a man with hemophilia and a woman who is a carrier, the children will be as follows: 25 percent female carrier, 25 percent female with hemophilia, 25 percent normal male, and 25 percent male with hemophilia.