Authors
Dr. DeWayne Derryberry
Idaho State University
Objective
Use the data sets provided to explore Mendel’s Laws of Inheritance for dominant and recessive traits.
Background
Gregor Mendel began breeding garden peas in about 1857. His scientific training prepared him to focus on experimentation and observation. He began to observe key laws of inheritance that could only be fully explained much later, when it was possible to link reproduction and chromosomes. (At around 1902 the chromosome theory of inheritance began to take form. Historical and biological information was taken from Biology, Fourth Edition, 1996, Neil Campbell.)
Mendel noticed that traits (observable characteristics) of peas, such as color and shape, seem to follow a regular pattern in reproduction. Mendel must have noticed the existence of dominant and recessive traits. A plant carries a pair of traits, and the outward appearance is based on the pair combination.
For example, for some of the peas studied by Mendel the colors were green or yellow. Yellow (Y) is dominant and green (y) is recessive. A plant can have any combination (YY, Yy, yY and yy). But YY, Yy and yY will all appear yellow, while only yy will appear green. In this sense yellow is dominant and green is recessive. The shapes were round (R) and wrinkled (r), with round the dominant trait, given our notation.
Suppose a plant carries the combination (Yy, Rr) and self-pollinates. In this case the seeds get random combinations of the pairs, which occur in approximately equal proportions (there are a lot of seeds). There are 16 possible combinations: YYRR, YYRr, YYrR, YYrr, YyRR, YyRr, YyrR, Yyrr, yYRR, yYRr, yYrR, yYrr, yyRR, yyRr, yyrR, yyrr.
The outward appearance follows the dominance-recessive pattern:
| Underlying pair combination (genotype) | Outward appearance (phenotype) |
| YYRR, YYRr, YYrR, YyRR, YyRr, YyrR, yYRR, yYRr, yYrR | YR (Yellow, Round) |
| YYrr,Yyrr, yYrr | Yr (Yellow, Wrinkled) |
| yyRR, yyRr, yyrR | yR (Green, Round) |
| yyrr | yr (Green, Wrinkled) |
If every combination is equally likely, we would expect the observed appearances YR, Yr, yR, and yr to appear in a ratio of 9:3:3:1.
This is actually based on a more basic idea. We expect any dominant-recessive pattern to appear as 3:1 or (3/4, 1/4). The reproduction process produces the overall pattern (9/16, 3/16, 3/16, 1/16), which is the multiplication of the probabilities, suggesting some relationship to independence. In other words, outward appearances follow probability rules for independent events: the Pr(Y) = 3/4 and Pr(R) = 3/4, so Pr(Y and R) = 3/4 x 3/4 = 9/16. In biology, this is called independent assortment.
The Task
We will assess whether Mendel’s data support the 9:3:3:1 ratio and a second set of data to assess whether other traits display a 3:1 ratio between the dominant and recessive traits.