Assertion: $H{b}^{S}H{b}^S$ is homozygous condition of sickle cell anaemia.
Reason: It occurs due to substitution of glutamic acid by valine at sixth position in $\beta$-chain of haemoglobin.

Sure, I can provide step by step solutions. Here they are: 1. The given term is: $H{b}^{S}H{b}^S$. 2. This term represents the homozygous condition of sickle cell anaemia. 3. Sickle cell anaemia is caused by a substitution of glutamic acid by valine at the sixth position in the beta-chain of haemoglobin. 4. Therefore, the term $H{b}^{S}H{b}^S$ represents the presence of two copies of the sickle cell haemoglobin gene. 5. Both copies of the gene have the substitution of glutamic acid by valine at the sixth position in the beta-chain of haemoglobin. 6. This homozygous condition results in the production of abnormal haemoglobin molecules, which can cause red blood cells to become sickle-shaped. 7. Sickle-shaped red blood cells can cause blockages in blood vessels, leading to tissue damage and other complications. 8. Therefore, individuals with the homozygous condition of sickle cell anaemia may experience a range of symptoms, including fatigue, pain, and organ damage. 9. In summary, the term $H{b}^{S}H{b}^S$ represents the homozygous condition of sickle cell anaemia, which is caused by a substitution of glutamic acid by valine at the sixth position in the beta-chain of haemoglobin.