The correct answer is B. Generally, people have 23 pairs of chromosomes. These chromosomes contain about 20,000 genes in total.

The correct answer is C. Scientists use DNA sequencing to determine the order of bases in a DNA segment. Sequencing techniques require many copies of the same DNA segment. Researchers often use PCR to create these copies.

The correct answer is D. In the first step of gene expression, called transcription, the cell creates messenger RNA (mRNA). In the second step, known as translation, the cell uses mRNA as instructions to build a protein.

The correct answer is A. Most DNA is the same in all people, but we each have variants that make us unique. Variants can be the basis of physical traits, affect how people respond to medicines, and cause diseases.

The correct answer is A. Mitochondria have a small amount of DNA that includes 37 genes. Mitochondrial DNA is almost always inherited through the egg cell, meaning it’s usually inherited from only a person’s biological mother.

The correct answer is C. CRISPR is a commonly used gene-editing technique. Scientists discovered it through research on how bacteria defend themselves against viruses.