Cracking the AP Biology Exam
One of Mendel’s hobbies was to study the effects of cross-breeding on different strains of pea plants. Mendel worked exclusively with true-breeder pea plants. This means the plants he used were genetically pure and consistently produced the same traits. For example, tall plants always produced tall plants; short plants always produced short plants. Through his work he came up with three principles of genetics: the law of dominance, the law of segregation, and the law of independent assortment.
THE LAW OF DOMINANCE
Mendel crossed two true-breeding plants with contrasting traits: tall pea plants and short pea plants. This type of cross is called a monohybrid cross, which means that only one trait is being studied. In this case, the trait was height.
To his surprise, when Mendel mated these plants, the characteristics didn’t blend to produce plants of average height. Instead, all the offspring were tall.
Mendel recognized that one trait must be masking the effect of the other trait. This is called the law of dominance. The dominant, tall allele, T, somehow masked the presence of the recessive, short allele, t. Consequently, all a plant needs is one tall allele to make it tall.
A simple way to represent a cross is to set up a Punnett square. Punnett squares are used to predict the results of a cross. Let’s construct a Punnett square for the cross between Mendel’s tall and short pea plants. Let’s first designate the alleles for each plant. As we saw earlier, we can use the letter “T” for the tall, dominant allele and “t” for the short, recessive allele.
Since one parent was a pure, tall pea plant, we’ll give it two dominant alleles (TT homozygous dominant). The other parent was a pure, short pea plant, so we’ll give it two recessive alleles (tt homozygous recessive). Let’s put the alleles for one of the parents across the top of the box, and the alleles for the other parent along the side of the box.
Now we can fill in the four boxes by matching the letters. What are the results for the F1 generation?
Each offspring received one allele from each parent. They all received one T and one t. They’re all Tt! Our parents had duplicate copies of single alleles, TT and tt, respectively. We could therefore refer to them as homozygous. The offspring, on the other hand, are heterozygous: They possess one copy of each allele.
Let’s compare the results of this cross with what we already know about meiosis. From meiosis, we know that when gametes are formed the chromosomes separate so that each cell gets one copy of each chromosome. We now know that chromosomes are made up of genes, and genes consist of alleles. We’ve just seen that alleles also separate and recombine. We can say, therefore, that each allele in a Punnett square also “represents” a gamete:
When fertilizaton occurs, chromosomes—along with the alleles they carry—get paired up in a new combination.
THE LAW OF SEGREGATION
Next, Mendel took the offspring and self-pollinated them. Let’s use a Punnett square to spell out the results. This time we’re starting with the offspring of the first generation—F1. Take a look at the results:
One of the offspring could be a short pea plant! The short-stemmed trait reappeared in the F2 generation. How could that happen? Once again, the alleles separated and recombined, producing a new combination for this offspring. The cross resulted in one offspring with a pair of recessive alleles, tt. Because there is no T (dominant) allele around to mask the expression of the short, recessive allele, our new plant could wind up short.
Although all of the F1 plants appear to be tall, the alleles separate and recombine during the cross. This is an example of the law of segregation.
What about the genotype and phenotype for this cross? Remember, genotype refers to the genetic makeup of an organism, whereas phenotype refers to the appearance of the organism. Using the results of our Punnett square, what is the ratio of phenotypes and genotypes in the offspring?
Let’s sum up the results. We have four offspring with two different phenotypes: three of the offspring are tall, whereas one of them is short. On the other hand, we have three genotypes: 1 TT, 2 Tt, and 1 tt.
Here’s a summary of the results:
- The ratio of phenotypes is 3 : 1 (three tall: one short).
- The ratio of genotypes is 1 : 2 : 1 (one TT: two Tt: one tt).
THE LAW OF INDEPENDENT ASSORTMENT
So far, we have looked at only one trait: tall versus short. What happens when we study two traits at the same time? The two traits also segregate randomly. This is an example of independent assortment. For example, let’s look at two traits in pea plants: height and color. When it comes to height, a pea plant can be either tall or short. As for color, the plant can be either green or yellow, with green being dominant. This gives us four alleles. By the law of independent assortment, these four alleles can combine to give us four different gametes:
TG Tg tG tg
Keep in mind that the uppercase letter refers to the dominant allele. Therefore, “T” refers to tall and “G” refers to green, whereas “t” refers to short and “g” refers to yellow. Now let’s set up a cross between plants differing in two characteristics—called a dihybrid cross—using these four gametes and see what happens.
Each trait will act independently, meaning that a plant that is tall can be either green or yellow. Similarly, a green plant can be either tall or short.
Here is the Punnett Square for a cross between two double heterozygotes (Tt Gg):
This is an example of the law of independent assortment. Each of the traits segregated independently. Don’t worry about the different combinations in the cross—you’ll make yourself dizzy with all those letters. Simply memorize the phenotype ratio of the pea plants. For the 16 offspring there are:
- 9 tall and green
- 3 tall and yellow
- 3 short and green
- 1 short and yellow
That’s 9 : 3 : 3 : 1. Since Mendel’s laws hold true for most of the traits they’ll ask you about on the AP test, simply learning the ratios of offspring for this type of cross will help you nail any questions that come up.
The Punnett square method works well for monohybrid crosses and helps us visualize the possible combinations. However, a better method for predicting the likelihood of certain results from a dihybrid cross is to apply the law of probability. For dihybrid ratios, the law states that the probability that two or more independent events will occur simultaneously is equal to the product of the probability that each will occur independently. To illustrate the product rule, let’s consider again the cross between two identical dihybrid tall, green plants with the genotype TtGg. To find the probability of having a tall, yellow plant, simply multiply the probabilities of each event. If the probability of being tall is and the probability of being yellow is , then the probability of being tall and yellow is × = .
One more thing: Probability can be expressed as a fraction, percentage, or a decimal. Remember that this rule works only if the results of one cross are not affected by the results of another cross.
Let’s summarize Mendel’s three laws.
Suppose we want to know if a tall plant is homozygous (TT) or heterozygous (Tt). Its physical appearance doesn’t necessarily tell us about its genetic makeup. The only way to determine its genotype is to cross the plant with a recessive, short plant, tt. This is known as a test cross. Using the recessive plant, there are only two possibilities: (1) TT × tt or (2) Tt × tt. Let’s take a look.
If none of the offspring is short, our original plant must have been homozygous, TT. If, however, even one short plant appears in the bunch, we know that our original pea plant was heterozygous, Tt. In other words, it wasn’t a pure-breeding plant. A test cross uses a recessive organism to determine the genotype of an organism of unknown genotype.