51. Among the following, which is sex linked disorder
(1) Night blindness    (2) color blindness
(3) Cretinism             (4) Myxedema

Introduction:
Sex-linked disorders are genetic conditions associated with genes located on the sex chromosomes, primarily the X chromosome. Among various genetic disorders, color blindness is a well-known example of a sex-linked disorder, especially affecting males more commonly. This article explains the options given in the question, highlights which disorder is sex-linked, and provides an SEO-friendly structure for better understanding.

Explanation of Options:

1. Night Blindness:
   Night blindness is a condition where an individual has difficulty seeing in low light or darkness. It is primarily caused by vitamin A deficiency or retinitis pigmentosa, a retinal disease. Night blindness is not linked to sex chromosomes and hence is not a sex-linked disorder.

2. Color Blindness:
   Color blindness occurs due to mutations in genes located on the X chromosome, particularly affecting the red and green color receptors in the eye. This makes it an X-linked recessive disorder. Since males have only one X chromosome, if they inherit the defective gene, they will express the condition, whereas females need two copies of the defective gene. Thus, color blindness is a classic example of a sex-linked disorder.

3. Cretinism:
   Cretinism results from severe iodine deficiency leading to hypothyroidism and impaired physical and mental development. It is not associated with sex chromosomes, so it is not a sex-linked disorder.

4. Myxedema:
   Myxedema is a condition caused by hypothyroidism (underactive thyroid gland) in adults, characterized by swelling and metabolic slowing. Like cretinism, it is unrelated to sex chromosomes and is not sex-linked.

In summary, among the given options, color blindness is the sex-linked disorder caused by a gene on the X chromosome, making it predominantly affect males. The other conditions listed are not linked to sex chromosomes.

References:
Color blindness is an X-linked recessive disorder caused by mutations in genes on the X chromosome, affecting red-green color perception and predominantly found in males. Night blindness, cretinism, and myxedema are not linked to sex chromosomes but have other causes.​

This explanation and SEO-friendly article format correctly addresses the question and supports effective learning and online visibility.