A student conducted a series of experiments with pure breeding red tomatoes and pure breeding yellow tomatoes.  The pollen from the yellow tomatoes was allowed to pollinate the stigmas of the red tomatoes. The seeds were collected and grown to determine the colour of the fruits of the first generation. All the first generation produced red fruit. The first generation plants were allowed to interbreed and the colour of the second generation fruit was determined. The results were:

red fruit = 360
yellow fruit = 119

(a) Explain why all the first generation produced red fruit.​