The key to understanding Mendelian genetics is to understand what ratios are and how to determine them. Let's do a few exercises to insure that you are familiar with ratios.
In ratios, order means everything. In the example of Mendel's classic experiment following the inheritance of flower color in garden peas, the F2 generation had a 3:1 ratio of individuals with purple to white flowers. The same results could be written as a 1:3 ratio of white to purple flowers. Therefore, the order of the traits listed within the ratio are important because the first number in the ratio is related to the first trait listed and the second number in the ratio corresponds to the second trait.
Some of Mendel's actual data for individual pea plants are listed below. Note that the ratios are obtained by dividing the larger of the two counts by the smaller.
| Data for Individual Pea Plants | |
|---|---|
| Flower Color | Pod Shape |
| Purple = 705 individuals | Inflated = 882 individuals |
| White = 224 individuals | Constricted = 299 individuals |
| 705/224 = 3.15 | 882/299 = 2.95 |
| Ratio of purple:white | Ratio of inflated:constricted |
| Flowers is 3.15:1 | Pods is 2.95:1 |
An examination of the ratios listed above shows that they are all approximately a 3:1 ratio, one slightly higher and one slightly lower. The inheritance of traits is governed in some respects by probability (as you will learn in upcoming exercises), therefore, ratios are often rounded to the nearest whole number.
The relationship between ratios and fractions can be confusing. A ratio of 1:3 means that there are four "units" being considered. In other words, 1:3 does not mean 33% have one trait, while 66% have the other. Rather, 1:3 means that 25% (1 unit) have one trait and 75% (3 units) have the other. Observe the punnett square in this image illustrating the self-pollination of an F1 pea plant for the character of flower color. We’ll learn more about punnett squares in the coming tutorials, just focus on the ratio versus the units.
Just as you can calculate ratios from counts of offspring, you can also work backwards and estimate offspring counts from ratios. Let's do an example to demonstrate.
You cross two true-breeding parents, one with purple flowers and one with white flowers, plant the seeds produced, grow the F1 generation an allow the plants to self-pollinate, and then count the number of offspring (the F2 generation) with purple and white flowers. If you counted 1,000 offspring and there was a 3:1 ratio of purple:white flowers, how many offspring would have purple flowers? How many would have white flowers?
Pull out a piece of paper and pencil (and/or your calculator) to see if you can get the right answer before viewing the solution in the next section.
n
= total number of offspring = 1,000
3:1 ratio = total of 4
"units" (3 + 1 = 4)
3
/4
of units have purple
flowers, 1
/4
have white flowers (Recall, there
was a 3:1 ratio of purple:white flowers.)
3
/4
=
75%, 1
/4
= 25%
75% purple flowers, 25%
white flowers
Convert to decimal equivalent: 0.75 purple flowers, 0.25
white flowers
Numbers = n
X 0.75 for purple, n
X
0.25 for white
purple = 1,000 X 0.75 = 750, white = 1,000 X 0.25 = 250
Estimate = 750 offspring with purple flowers, 250 with white flowers
Work through these slides to reinforce the concept of a Mendelian cross: