Explain why the genome of the eastern grey kangaroo (Macropus giganteus) must contain 12 per cent cytosine if 38 per cent of the genome contains adenine.

The explanation is based on Chargaff's rules, which state that in DNA, the amount of adenine (A) is equal to the amount of thymine (T), and the amount of cytosine (C) is equal to the amount of guanine (G). This is due to the specific base pairing in DNA where A pairs with T and C pairs with G.

If 38% of the genome contains adenine, then 38% must also contain thymine (because A=T). This accounts for 76% of the genome (38% A + 38% T).

The remaining 24% of the genome must therefore be made up of cytosine and guanine (because the total must add up to 100%). Since cytosine pairs with guanine, they must be present in equal amounts. Therefore, 12% of the genome contains cytosine and 12% contains guanine (because C=G).

So, if 38% of the genome contains adenine, then 12% of the genome must contain cytosine.