Mendel’s first cross

Mendel pollinated a pure tall pea plant TT with a pure short pea plant tt. Another word for “pure” in the world of genetics is homozygous. Homozygous traits are traits that have two of the same alleles like two tall alleles, TT, or two short alleles, tt.

First we draw a box like below and put the parent’s alleles along the side:

A square divided into four sections. Along the top of the square is two capital T that are labeled tall plant. Along the left side of the square are two lower case t that are labeled short plant. Each section of the main square is empty.

Next we combine the alleles inside the box. These will be the possible outcomes for the offspring. You see each parent can donate one allele to the offspring. The Punnett square helps us sort out all the possibilities.

A square divided into four sections. Along the top of the square is two capital T. Along the left side of the square are two lower case t. Each of the four sections of the main square contain Tt.
Well, look at that. All the letters are the same inside the Punnett square. Tt There is one dominant allele and one recessive allele. We call this a heterozygous. The important thing to note is that because there is one dominant gene all the plants will be tall. We call the description of the trait the phenotype. So the phenotype for height in this Punnett square is 100% TALL.

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