The pacemaker of the human heart is the: MCQ with Answer and Explanation

Explanation:
The cohesion-tension theory (transpiration pull) is the primary mechanism for water ascent in tall trees. Transpiration creates negative pressure (tension) in leaves, water molecules cohere, and the column is pulled up. Root pressure and capillary action play minor roles. The cohesion of water is due to hydrogen bonds.

Explanation:
The iris contains smooth muscle fibers (sphincter and dilator) that regulate the diameter of the pupil. The cornea and lens focus light, retina senses it.

Explanation:
Pleiotropy occurs when a single gene affects two or more distinct phenotypic traits. This happens because the gene product (protein) may be used in multiple biological pathways or expressed in different tissues. Examples include phenylketonuria (PKU), sickle cell anemia, and Marfan syndrome. Epistasis involves the interaction of different genes, polygenic inheritance involves multiple genes affecting one trait, and incomplete dominance is a single gene interaction.