Answer:
The blood brain barrier blocks chemicals from entering brain tissue.
Explanation:
The structural unit of compact bone is called an osteon, or sometimes referred to as a Haversian system. Compact bone is made up of many osteons
Answer: A recessive allele is only expressed if the individual has two copies and does not have the dominant allele of that gene.
Explanation: Recessive alleles are represented by a lower case letter, for example, a. The allele for blue eyes is recessive.