Which of the following describes the allele that causes colour blindness?

Options

A. Recessive
B. Carried on the Y chromosome
C. Carried on the X chromosome
D. Present only in males

Answer

To determine which of the following describes the allele that causes color blindness, we need to analyze the characteristics of the allele responsible for this condition.

### Step-by-Step Solution:

1. **Understanding Color Blindness**:
- Color blindness is a genetic condition that affects the ability to perceive colors correctly. It is primarily associated with the X chromosome.

2. **Identifying the Type of Inheritance**:
- Color blindness is classified as a **sex-linked** disorder, meaning the allele responsible for it is located on a sex chromosome (the X chromosome).

3. **Determining the Nature of the Allele**:
- The allele that causes color blindness is **recessive**. This means that an individual must inherit two copies of the allele (one from each parent) to express the condition, or in the case of males, just one copy since they have only one X chromosome.

4. **Genotype of Affected Individuals**:
- In females, who have two X chromosomes, the genotype for color blindness would be represented as **XcXc** (homozygous condition) or **XcX** (heterozygous condition, where Xc is the allele for color blindness and X is the normal allele).
- In males, who have one X and one Y chromosome, the genotype would be **XcY** (where Xc is the allele for color blindness).

5. **Carriers of the Allele**:
- Females can be carriers of the color blindness allele (XcX) without being color blind themselves, as they have one normal allele (X). Males cannot be carriers; they either have color blindness (XcY) or do not (XY).

6. **Conclusion**:
- The allele that causes color blindness is **recessive** and is **carried on the X chromosome**. It is not exclusive to males; females can also be affected or be carriers.

### Final Answer:
The allele that causes color blindness is **recessive** and **carried on the X chromosome**.

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