Answer:
Alleles.
Explanation:
Gene may be defined as the functional segment of DNA that codes for the particular protein. Genes are responsible for the phenotype and genotype of the organism. Each gene may exist in multiple forms. 
The variants forms of the genes are known as alleles. Alleles are the alternative form of the same gene. A single gene may have atleast two alternative form. These alternative forms together contribute the functional aspect of the organism.
Thus, the correct answer is alleles.
 
        
             
        
        
        
Ice age. It caused saber tooth and mammoth to die
        
             
        
        
        
Answer:
Explanation:
B.  có nhiều hệ sinh thái khác nhau trên trái đất. mỗi hệ sinh thái có một đặc tính riêng và chúng có thể kết nối, tác động qua lại với các hệ sinh thái khác
 
        
             
        
        
        
Chemicals produced by bacteria that limit the growth of other bacteria are called antibiotics.