Always start with the genotypic ratios, because they'll be the same. Since hidden variation is popping out, you can assume that both of the parents are heterozygous. The problems you get will be the F2 ratio of the cross between two homozygous individuals.
1:2:1 is the genotypic ratio for one locus. AA:Aa:aa.
1:2:1 is the phenotypic ratio for incomplete dominance. Incomplete dominance means that the heterozygote expresses differently from either homozygote.
3:1 is the phenotypic ratio for complete dominance.
2:1 is the phenotypic ratio for lethal recessive.
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The answer is D. The ribs PROTECT the lungs and the heart. They allow you to move and they store minerals such as calcium. However it is the brain and the blood vessels that regulate the body temperature not the skeletal system.
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