Why has two different alleles for the same trait?

Select the correct answer from each drop-down menu the images show fossils of a modern bird and two extinct organisms (a tyrannosaurus rex and an archaeopteryx). based on the structure of the three organisms, it can be concluded that the archaeopteryx and bird have wings, while dinosaurs have limhe se it can be concluded that

Which of the characteristics of life does a virus display

The backbone of the double helix has an alternating molecules. what do you call these alternating molecule?