What is a gene? A-the different forms of a trait

B-a segment of DNA that is the basic unit of heredity

C-the appearance of characteristics that are seen in an organism

D- an organism allele combination

B: a segment of DNA that is the basic unit of heredity

Explanation:

answer; a male inherits only one allele of the x-linked gene controlling hair color;

blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium of the heart. as the ventricle contracts, blood leaves the heart through the pulmonic valve, into the pulmonary artery and to the lungs where it is oxygenated.