Answer the following question given this paragraph: 

Humans have an estimated 20,000 to 22,000 genes. This may sound like a lot, but it really isn’t. Far simpler species have almost as many genes as humans. However, human cells use splicing and other processes to make multiple proteins from the instructions encoded in a single gene. Of the 3 billion base pairs in the human genome, only about 25 percent make up genes and their regulatory elements. The functions of many of the other base pairs are still unclear. To learn more about the coding and noncoding sequences of human DNA, watch the animation at this link: http://www. hhmi. org/biointeractive/dna/DNAi_coding_sequences. html .


Q: Humans have an estimated 20,000 to 22,000 of what units of heredity?

 Choices:

- eggs

- cells

- chromosomes

- genes

A:
genes
