Mendel’s Dihybrid Cross Experiments

Proper credit must be given to the idea of independent assortment. Gregor Mendel was the first to put this idea down on paper based on what he observed with his pea experiments. Furthermore, Mendel performed additional experiments to back up his ideas. Let’s examine his experiments with peas from the late 1800’s.

The outline below describes Mendel’s dihybrid cross experiments. The pattern observed in the results should look familiar!

Parents: round seeds, yellow seeds X wrinkled seeds, green seeds; F1:  All round and yellow seeds; F2 table row 1 round, yellow  number 315,  fraction 9/16; Row 2 round, green  Number 108  Fraction 3/16; Row 3 Wrinkled, yellow  Number 101  Fraction 3/16;

Mendel explained his results as follows:

The F1 plants have the genotype RrYy and can make four kinds of gametes RY, Ry, rY and ry

These gametes are made in equal frequencies in the male and female parts of the plant:

Possible Gametes, Subtitle F1 Genotype = big r little r big y little y, Row 1 Male: 1/4 big R big Y  + 1/4  big R little y + 1/4 little r big Y + 1/4 little r little y, Row 2 Female: 1/4 big R big Y  + 1/4  big R little y + 1/4 little r big Y + 1/4 little

Punnett square graphically demonstrates the possible genotypes produced when the RrYy dihybrid is selfed.

Note that with both the Mendel algebra and Punnett square, the RRYY genotype occurs one time and the RrYy genotype occurs four times. Mendel’s algebra and Punnett’s squares can be summarized to give the same results.