Most color pigmentation goes towards hair and not much is left for skin.
There is 50% probability that their daughter's son will have the disease, because daughter chromosomes are XX , and the X from father will have x- linked disease, now in next generation the son will have XY chromosomes, Y is from father and X will from mother, now mother have two "XX", out of these two one carry x- linked disease, that means 50 percent chances of having disease in son.
Her conclusion is invalid because using sand instead of soil slows down plant growth. Fertilizer, on its own, does not 'greatly' increase the size of the plant.
The correct way to do this experiment would be to keep all other parameters same, except the fertilizer. This will give her accurate results.
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HIV is the only disease listed here to be sexually transmitted.
-Tiara