Understanding Recessive Sex-Linked Traits

This question asks us to identify which of the listed conditions are transmitted as recessive, sex-linked traits. A recessive, sex-linked trait is a characteristic caused by a gene located on the X chromosome, which requires two copies of the mutated allele (one on each X chromosome) to be expressed in females, and only one copy in males (since they possess only one X chromosome).

Analysis of Conditions

Let's examine each condition mentioned:

• I. Retinitis pigmentosa: While Retinitis pigmentosa encompasses various genetic forms, several types are known to be inherited in an X-linked recessive pattern. This means they fit the criteria of being recessive and sex-linked.
• II. Colour blindness: The most common forms of colour blindness, particularly red-green colour blindness, are classic examples of X-linked recessive traits.
• III. Cystic fibrosis: Cystic fibrosis is a well-known genetic disorder, but it is inherited in an autosomal recessive manner. This means the gene responsible is located on an autosome (a non-sex chromosome), not the X chromosome. Therefore, it is not a sex-linked trait.
• IV. Duchenne muscular dystrophy: Duchenne muscular dystrophy (DMD) is another prominent example of an X-linked recessive disorder. It primarily affects males.

Identifying Correct Combinations

Based on the analysis:

• Retinitis pigmentosa (some forms) is X-linked recessive.
• Colour blindness (common forms) is X-linked recessive.
• Cystic fibrosis is autosomal recessive.
• Duchenne muscular dystrophy is X-linked recessive.

Therefore, the conditions that are transmitted as recessive, sex-linked traits from the given list are Retinitis pigmentosa (I), Colour blindness (II), and Duchenne muscular dystrophy (IV).

Conclusion

The correct combination representing recessive, sex-linked traits is I, II, and IV.