the boys in this picture look alike because their bodies contain identical

DNA?

I'm guessing the boys who look the same are twins, since there's no photo of them..

Hope this helps

Nucleic acids.

Explanation:

dna is found in nucleic acids

genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism's offspring).

when you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. this increases the space in your chest cavity, into which your lungs expand. the intercostal muscles between your ribs also enlarge the chest cavity. they contract to pull your rib cage both upward and outward when you inhale.