A chromosome with an extremely short and a very long arm is called ______.

Based on the position of the centromere, chromosomes are classified into four types:

Metacentric: The centromere is in the middle, forming two equal arms.

Submetacentric: The centromere is slightly away from the middle, resulting in one shorter arm and one longer arm.

Acrocentric: The centromere is situated close to its end, forming one extremely short and one very long arm.

Telocentric: The centromere is at the terminal end.

Therefore, a chromosome with an extremely short and a very long arm is called acrocentric.

Answer: Acrocentric