Genotype refers to the alleles that a particular organism has — the actual sequences of DNA in its genome, such as a gene for growth hormone. Phenotype refers to the physical characteristics of the organism, such as the organism's height.
Genotype and phenotype are often connected, but the important thing to remember is that the connection is not always absolute. Organisms with the same phenotypes may have different genotypes; similarly, organizations with the same genotypes may have different phenotypes. What this means is that you can't always determine what DNA sequences are at play simply by identifying outward characteristics; neither can you always know whether an organism's characteristics are the result of genetics or of something else.
Different phenotypes, same genotype
An organism's phenotype — its physical characteristics — is not always determined by its genes alone. Grasping this concept is especially important when you think about evolution by natural selection. Environmental factors — like how much food we get or how big our pond is — interact with genotype to produce an organism's phenotype.
Imagine a pack of cheetahs chasing a gazelle across the African plains. One by one, the cheetahs get tired and give up. But one cheetah keeps at it and eventually catches the gazelle. It's a tough year out there in the Serengeti, and the mere difference of a single gazelle can determine whether a cheetah is able to reproduce.
Why this cheetah captured the gazelle while the others fell away, I have absolutely no idea. Are her genes especially good? Does she have different genes that make her go extra fast? Perhaps . . . but perhaps not. It could be that all the cheetahs have the same genes, but this particular cheetah was lucky enough to have been very well fed when she was a cub and, as a result, grew up to be faster and stronger. If that's the case, all these cheetahs have the same genotype, but the phenotype — in the successful cheetah's case, the strength and stamina to continue the chase — differs.
Same phenotype, different genotypes
Occasionally, organisms have the same phenotype (characteristics) but different genotypes (gene sequences). Consider the human blood-type alleles A, B, and O. Each of us has two of these alleles, receiving one from each parent. Six pairs of alleles are possible: AA, AO, BB, BO, AB, and OO. Yet only four blood types exist: A, B, AB, and O. How does that work?
Well, in one way, it's similar to the snapdragon with a red, white, and pink flower. Whether you have A, B, or AB blood depends on the genotype:
i The AA genotype (you got an A allele from each parent) gives you type A blood.
i The BB genotype (you got a B allele from each parent) gives you type B blood.
i The AB genotype (you got an A allele from one parent and a B from the other) gives you type AB blood.
To understand the others, you need to know that A and B are dominant over O, but neither is dominant over the other. Therefore:
i The AO genotype gives you type A blood. i The BO genotype gives you type B blood. i The OO genotype gives you type O blood.
Knowing the phenotype sometimes gives you complete information about the genotype, such as the phenotypes for type O blood and type AB blood. But in the cases of blood type A and blood type B, two possible genotypes could bring about each phenotype.
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