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Why doesn’t the whole world have brown eyes since it is a dominant trait? That question brought about Punnett squares. Punnett squares are a picture representation for predicting what offspring will look like.
Mendel used peas because they grow fast, little time and investment was involved and if you totally messed up, you can just throw the peas in a pot and eat them for dinner. Waste not, want not.
To predict offspring, we use letters to represent the alleles or genes. When Mendel crossed his pure tall pea plants with his pure short pea plants, only tall pea plants were produced. We call the tall trait dominant because that trait is always seen. We use a capital letter to represent dominant alleles. The traits that are not always seen are called recessive and we use a lower case letter to represent the recessive alleles. In Mendel’s case, his short pea plants were recessive.
For pea plant height we will use the letter T (for tall because it is dominant). Let the tall allele be represented with a capital “T,” and the short allele will be a lower case “t.” (Please note that we do NOT use an “s” for short. The letters must be the same)
In sexual reproduction an offspring has two parents. They get one allele from each parent. In Mendel’s pure pea patches the tall pea alleles are TT (one T from each parent) and the short peas alleles are represented by tt (they got one t from each parent). Looking at the alleles in this way is called the organism’s genotype. The genotype is what the gene says the trait will be.