A common cold i believe .
        
             
        
        
        
The answer would be algae or phytoplankton
        
                    
             
        
        
        
Answer:
Because natural selection selects for it. Thus it persists.
Explanation:
You have to understand heterozygote advantage. Basically, it's where heterozygotes have an advantage over homozygotes. In the case of sickle cell disease, heterozygotes have an advantage, and natural selection favors whatever is advantageous. Thus, because heterozygotes each have one recessive sickle-cell allele, as natural selection favors the heterozygotes, the recessive sickle-cell allele persists and remains in the gene pool.
 
        
             
        
        
        
Answer:
DNA can be described as the molecules which carries the genetic code for every individual. The DNA is made up of nucleotide bases which join the two strands of DNA. The sugar and phosphate form the backbone of the DNA. 
Genes are the hereditary molecules which are made up of DNA. The alleles of a gene represent which traits will be carried by an organism. 
Chromosomes are made up of genes. During reproduction, crossing over of DNA occurs and alleles of the genes assort independently due to which variations occur. 
 
        
             
        
        
        
Answer:
changes in temperature that happen with increasing altitude.