An organism's phenotype is the form of a gene which is expressed. Phenotypes disregard the allele pairing for a gene and focuses only on the trait. We can figure out an organism's phenotype from physical tests, but its genotype can only be determined with genetic analysis.
In Genetics there's two tems that are often used and confused by students. The first is genotype and the other one is phenotype. In this case I'm going to focus in on phenotype.
The phenotype is an organism's actual physical trait. So what you can see, it has the same root word phen that is seen in words like phenomena or phenomenal, that stuff that you see or observe. So this is caused by genotype. So genotype is the types of gene you have that causes your phenotype.
The other thing that can influence phenotype however besides just your DNA is your environment. Now the way genotype influences it is that if you get a dominant allele then you will show the dominant phenotype. So if somebody is big r big r, for example with r representing say the allele to roll your tongue, then you will have the dominant tongue rolling ability or phenotype. The only way you can have the non-rolling phenotype, the inability to roll your tongue like this for example. You would need to have two combinations sorry, two of the recessive allele. So you need to be homozygous recessive. So again homozygous dominant and heterozygotes they show the dominant phenotype while homozygous recessive show the recessive phenotype.
Now, I mentioned that your environment can influence your phenotype. Some simple examples might be my hair is brown. That's my phenotype. If I dye it red, then it's red. Was that my genes? No. That was my environment. But generally in Genetics we're not going to be talking about it. So, you got a guinea pig and you dye it red, what causes because we know that you dyed it. What is more common to be seen and studied in genetics is the influence of environmental factors such as the temperature, the amount of food, etcetera. Things that will alter the way your body behaves, it may turn on genes, it may turn off some genes. For example, in my genetic background, there is propensity to have diabetes as you age. Type two diabetes. I know that I can keep my phenotype, my current phenotype is non-diabetic. I can keep it that way if I have a good environment by maintaining the proper combination of resources. Eat right, excercise, everything else.
So there you go. That's phenotype.