Genome Mapping

The human genome contains approximately 3.2 billion base pairs and protein-coding genes make up about 2% of the total DNA.

We need to find 2% of 3.2 billion: $2\%\times 3.2\times 10^9=0.02\times 3.2\times 10^9$

$0.02\times 3.2\times 10^9=0.064\times 10^9=6.4\times 10^7=64,000,000$ base pairs, which is 64 million base pairs.

The answer is 64 million base pairs. This highlights that the vast majority of human DNA is non-coding but still plays important regulatory roles.

We need to calculate 2% of 3.2 billion base pairs to find the number of base pairs in protein-coding genes.

3.2 million would be only 0.1% of 3.2 billion — far too small to represent 2% of the genome. This can be eliminated.

640 million would represent 20% of 3.2 billion (since $640÷3200=0.2$), not 2%. This is 10 times too large.

320 million would represent 10% of 3.2 billion ($320÷3200=0.1$), not 2%. This is also too large.

64 million base pairs is correct: $0.02\times 3,200,000,000=64,000,000$, which equals 64 million — representing 2% of the genome.

This question tests genetic (linkage) mapping, which determines the relative positions of genes based on how often they are inherited together.

Genes that are physically close together on the same chromosome are less likely to be separated during recombination (crossing over), so they tend to be inherited together more often.

If two genes are almost always inherited together, this is strong evidence that they are closely linked — meaning they are located near each other on the chromosome.

The correct answer is they are located close together on the chromosome. This is the fundamental principle underlying genetic linkage mapping.

We need to explain why two genes on the same chromosome are almost always inherited together — a question about genetic linkage.

Different genes code for different proteins by definition. Two separate genes coding for the same protein would be unusual and doesn't explain co-inheritance. Eliminate this option.

The question states they are on the same chromosome. Genes on different chromosomes assort independently and would not reliably be inherited together.

Two different genes having identical sequences is not a meaningful genetic concept and would not explain co-inheritance patterns. Eliminate this option.

They are located close together on the chromosome is correct. Physical proximity reduces the chance of being separated by recombination, so closely linked genes are inherited together.