Cracking the AP Biology Exam

8

Heredity

GREGOR MENDEL: THE FATHER OF GENETICS

What is genetics? In its simplest form, genetics is the study of heredity. It explains how certain characteristics are passed on from parents to children. Much of what we know about genetics was discovered by the monk Gregor Mendel in the 19th century. Since then, the field of genetics has vastly expanded. As scientists study the mechanisms of genetics, they’ve developed new ways of manipulating genes. For example, scientists have isolated the gene that makes insulin, a human hormone, and now use bacteria to make large quantities of it. But before we get ahead of ourselves, let’s study the basic rules of genetics.

Let’s begin then with some of the fundamental points of genetics:

• Every trait—or expressed characteristic—is produced by hereditary factors known as genes. A gene is a segment of a chromosome. Within a chromosome, there are many genes, each controlling the inheritance of a particular trait. For example, in pea plants, there’s a gene on the chromosome that codes for seed coat. The position of a gene on a chromosome is called a locus.
• Diploid organisms (organisms that have two sets of chromosomes) usually have two copies of a gene, one on each homologous chromosome. These copies may be different from one another—that is, they may be alleles, or alternate forms of the same gene. For example, if we’re talking about the height of a pea plant, there’s an allele for tall and an allele for short. In other words, both alleles are alternate forms of the gene for height.
• An allele can be dominant or recessive. In simple cases, an organism can express contrasting conditions. For example, a plant can be tall or short. The convention is to assign one of two letters for the two different alleles. The dominant allele receives a capital letter and the recessive allele receives a lowercase of the same letter. For instance, we might give the dominant allele for height in pea plants a “T” for tall. This means that the recessive allele would be “t.”
• When an organism has two identical alleles for a given trait, the organism is homozygous. For instance, TT and tt both represent homozygous organisms. TT is homozygous dominant and tt is homozygous recessive. If an organism has two different alleles for a given trait, Tt, the organism is heterozygous.
• When discussing the physical appearance of an organism, we refer to its phenotype. The phenotype tells us what the organism looks like. When talking about the genetic makeup of an organism, we refer to its genotype. The genotype tells us which alleles the organism possesses.

One of the major ways ETS likes to test genetic information is by having you do crosses. Crosses involve the mating of hypothetical organisms with specific phenotypes and genotypes. We’ll look at some examples in a moment, but for now, keep these test-taking tips in mind:

• Label each generation in the cross. The first generation in an experiment is always called the parent, or P1 generation. The offspring of the P1 generation are called the filial, or F1 generation. The next generation, the grandchildren, is called the F2 generation.
• Always write down the symbols you’re using for the alleles, along with a key to remind yourself what the symbols refer to. Use uppercase for dominant alleles and lowercase for recessive alleles.

Now let’s look at some basic genetic principles.