Introduction:
Mendelian disorders, also known as monogenic disorders, are genetic conditions caused by alterations or mutations in a single gene. These disorders are inherited in a predictable manner following Mendel's laws of inheritance. In this context, we will discuss the options given and determine which one is not a Mendelian disorder.

Explanation:
A. Colour blindness:
- Colour blindness is a genetic disorder that affects an individual's ability to perceive certain colors.
- It is caused by mutations in the genes responsible for the production of photopigments in the cones of the retina.
- The inheritance pattern of color blindness follows Mendel's laws and is usually X-linked recessive, meaning it is more common in males.

B. Down syndrome:
- Down syndrome, also known as trisomy 21, is a chromosomal disorder where individuals have an extra copy of chromosome 21.
- It is not a Mendelian disorder because it is caused by a chromosomal abnormality rather than a mutation in a single gene.
- The extra chromosome 21 can occur due to non-disjunction during meiosis, resulting in an extra copy in the developing fetus.

C. Haemophilia:
- Haemophilia is a genetic disorder characterized by impaired blood clotting, leading to prolonged bleeding and easy bruising.
- It is caused by mutations in the genes responsible for producing clotting factors, specifically factor VIII (in hemophilia A) or factor IX (in hemophilia B).
- The inheritance pattern of haemophilia is X-linked recessive, following Mendel's laws.

D. Thalassemia:
- Thalassemia refers to a group of genetic blood disorders that result in reduced or absent production of hemoglobin, the protein responsible for carrying oxygen in red blood cells.
- It is caused by mutations in the genes involved in the production of globin chains, which make up the hemoglobin molecule.
- The inheritance pattern of thalassemia can vary, but it is primarily an autosomal recessive disorder, following Mendel's laws.

Conclusion:
Among the given options, B. Down syndrome is not a Mendelian disorder. It is caused by a chromosomal abnormality (trisomy 21) rather than a mutation in a single gene. The other options, colour blindness, haemophilia, and thalassemia, are Mendelian disorders as they are caused by mutations in specific genes and follow Mendel's laws of inheritance.

(B) Down syndrome