Not all patterns of inheritance are so simple. For example, the expression of some traits is related to gender--being male or female. The genes for these traits are located on the sex chromosomes.
Human beings have 46 chromosomes, which are grouped into 23 pairs. Pairs 1 to 22 are alike in both men and women. But there are two forms of the 23rd pair, the sex chromosomes: the "X" chromosome and the "Y" chromosome. Females have two copies of the X chromosome--an XX genotype; males have one X and one Y--an XY genotype. The X and Y chromosomes are pictured below. As you can see, the X chromosome is much larger than the Y.
Certain genes are found only on the X chromosome. These are referred to as "sex-linked genes." Because a woman has two copies of the X chromosome, she will get two copies of these genes, but men, who have only one X, will get only one copy.
One example is the gene is for red-green colorblindness. The colorblindness trait is recessive, which means that a female must get two damaged copies of the gene in order to have colorblindness. A male, however, needs to get only one damaged copy in order to have the condition.
A healthy copy of the gene is represented by XB, and a damaged copy is represented by Xb. Therefore:
The genotype of a male with colorblindness is XbY.
The genotype of a male with normal vision is XBY.
The genotype of a female with colorblindness is XbXb.
The genotype of a female who has normal vision but is a carrier of the disease gene is XBXb.
The genotype of a female who has normal vision and is not a carrier is XBXB.
Let's examine a Punnett square depicting the possible genotypes of the offspring of a female carrier and a male with normal vision.
As you can see from the Punnett square, these parents have a 25% chance of having a child who is colorblind; that child would have to be male (genotype XbY). The female children would be either XBXB or XBXb, and none would be colorblind. One-half of male children would have the XBY genotype and would have normal vision.
Would you like to give it a try?