The respiratory system brings air into the lungs supplying the blood with oxygen. Oxygen is carried throughout the body to the organs and keeps the heart pumping and when the blood circulates back to the lungs the process starts over again.
D. He controlled variables by studying one or two traits at a time
Answer:
Phenotypes
Explanation:
'Phenotype' is just a fancy way of saying 'what an organism looks like'. Those words describe an organism's physical appearance and not its genetic makeup--that's its genotype.
You can remember it this way:
Phenotype
Physical characteristics/appearance
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Genotype
Genetic makeup