A recessive gene expresses only in homozygous condition. Some times, a single recessive gene can express itself when it is

Options

A. autosomal gene
B. located on Xchromosome of female
C. located on X-chromosome of male
D. all of the above

Answer

### Step-by-Step Solution:

1. **Understanding Recessive Genes**: A recessive gene typically expresses its traits only when present in a homozygous condition, meaning two copies of the recessive allele are required for the trait to be visible.

2. **Identifying Exceptions**: There are exceptions where a single recessive gene can express itself. This often occurs in sex-linked traits, particularly those associated with the X chromosome.

3. **Example of Color Blindness**: Color blindness is an example of a trait that is primarily expressed in males. This is due to the fact that the gene responsible for color blindness is located on the X chromosome.

4. **Understanding Hemophilia**: Hemophilia is another sex-linked recessive disorder that illustrates this concept. The gene for hemophilia is also located on the X chromosome.

5. **Male vs. Female Expression**: In males (XY), if they inherit one X chromosome with the hemophilia gene, they will express the condition because they do not have a second X chromosome to mask the recessive gene. In contrast, females (XX) would need both X chromosomes to carry the hemophilia gene to express the condition. If they have one normal X chromosome, they will be carriers but not express the disease.

6. **Conclusion**: Therefore, a single recessive gene can express itself when it is located on the X chromosome in males, as they have only one X chromosome.