Question

Which of the following could be evidence that an organism's growth is influenced by the organism's genes?

(Hint: Identical twins have identical genes. Non-identical twins have about half the same genes.)

Children are likely to have friends who are about the same height.
Identical twins are more likely to be about the same height than non-identical twins.
Identical twins often have slightly different weights at birth.
A dog breed can be made smaller by breeding together only the smallest dogs.

Answer 1

The 2nd and 4th statements could be evidence that genes influence growth:

- Identical twins are more likely to be about the same height than non-identical twins. (Because identical twins share all their genes, greater similarity than fraternal twins suggests a genetic effect.)
- A dog breed can be made smaller by breeding together only the smallest dogs. (Selective breeding changing a trait shows that trait is heritable — controlled at least partly by genes.)

Why the others are not good evidence:
- Children having friends about the same height can be due to social/age grouping or environment, not genetics.
- Identical twins often having slightly different birth weights points to environmental or developmental variation, not genetic influence.