Basic Principles of Genetics

Traits, Heredity, and Genetics

People tend to look like other members of their family because they share similar traits. A person’s-or any other organism’s-traits are their heritable characteristics. The color of your eyes, the shape of your nose, and the texture of your hair are just some of your many traits. Some traits are not visible. For example, your blood type is a trait, but not one you can see just by looking. Traits are different from learned characteristics. Learned characteristics, such as the ability to play an instrument or drive a car, are things you learn and are not heritable.

Traits are passed from parents to offspring. This passing of traits from one generation to the next is known as heredity. The field of biology devoted to studying heredity is called genetics. An understanding of genetics on a molecular level is relatively new because of the complex technology scientists needed to study structures as small as the DNA within cells. That does not mean the study of genetics is new, however. For thousands of years, people have been using their observations to breed plants and animals with specific traits.

Mendel and the History of Genetics

In the mid-nineteenth century, an Austrian monk named Gregor Mendel became one of the first people to diligently study genetics. For his studies, Mendel observed pea plants from his garden. Mendel could study how specific traits are inherited from one generation to the next generation. For example, he could observe the flower color in the plants produced when a plant with white flowers was bred with a plant with purple flowers. This type of controlled breeding is called cross-pollination.

Dominant and Recessive Traits

In one of his earliest experiments, Mendel crossed a short pea plant with a tall pea plant. The parent plants that are first crossed are known as the P1 generation. Mendel planted the seeds that resulted from the cross. The offspring plants are called the F1 generation. Mendel discovered that all the offspring in the F1 generation grew to be tall. It was as if the shortness trait had disappeared.

Mendel then allowed the plants of the F1 generation to self-pollinate. Again, he planted the seeds that were produced. In this generation, known as the F2 generation, some plants were tall, and some were short. The shortness trait had reappeared. Mendel conducted similar crosses to test for other traits. He tested for seed shape, seed color, flower color, and flower position. In every case, Mendel discovered that one trait seemed to disappear in the F1 generation and reappear in the F 2 generation. Mendel concluded that each trait is controlled by two factors. He called the form of a trait that appeared in the F1 generation the dominant trait. He named the form of the trait that disappeared until the F2 generation the recessive trait.

Medalls Peas

Think about Science

Directions: Fill in the blank.

  1. In the pea-plant example, the tall form of the trait is the [ fill-in-the-blank ] trait.
  2. The short form of the trait is the [ fill-in-the-blank ] trait.

Chromosomes, Genes, and Alleles

The factors Mendel described are now called genes. And it is now understood that a gene is a segment of DNA that contributes to the traits of an organism. Recall that DNA is an organic molecule that carries the information of heredity. One way of thinking about genes is that a gene determines a particular trait. The reality is much more complex, however. Genetics is a very active area of research, and scientists continue to find new evidence that helps explain how traits are inherited. A single organism can have tens of thousands of different genes. Genes are found within chromosomes, and a chromosome can contain thousands of genes.

Different forms of a gene are called alleles. Pea plants, for example, have two different alleles for the gene for height: one allele for tall height and one allele for short height. Mendel found that the tall trait in peas is dominant and the short trait is recessive. Each plant receives two alleles for a trait-one from each parent. The dominant trait will appear if the plant has at least one allele for the dominant trait. In other words, the plant could have two alleles for the dominant trait, or one allele for the dominant trait and one allele for the recessive trait. In both cases, the plant would exhibit the dominant trait. The recessive trait will appear only if the plant has two recessive alleles. As shown in the table, Mendel studied several different traits in pea plants because they are easily grown and the traits easily observed.

In pea plants, tall height is dominant over short height. It is important to note that a dominant trait is not necessarily a better trait or even a more common trait. A dominant trait is simply the trait that is expressed if a plant has at least one dominant allele. Some traits, including the height of pea plants and the other traits Mendel studied, are inherited based on a single gene that has two alleles.

Mendal's Data

Mendel studied seven traits of pea plants. Each trait had a dominant allele and a recessive allele.

Most traits, however, have many alleles and depend on much more complex patterns of inheritance as well as on environmental factors. This is why most traits, including human height, have a continuous range of possibilities and not just two alternatives, such as tall or short.

Representing Alleles with Text

To describe alleles, scientists use an uppercase letter for the dominant allele. The lowercase version of the same letter is used for the recessive allele. The allele for tall height, therefore, is written as T, and the allele for short height is written as t. If an individual has a dominant allele and a recessive allele, the dominant allele is usually written before the recessive allele (Tt).

Think again about Mendel’s crosses. He selected true-breeding plants, which means that the plants had two copies of the same allele for a trait. The tall plants had two alleles for tall height (TT), and the short plants had two alleles for short height (tt). The plants in the F1 generation received one allele from each parent. Therefore they received an allele for tall height from the tall parent and an allele for short height from the short parent. As a result, every plant in the F 1 generation had both types of alleles (Tt). Because tall height is dominant over short height, all the plants were tall.

Mendal's Plants

Representing Alleles on Chromosome Diagrams

Bacteria and other prokaryotes have a circular chromosome. Eukaryotes have linear chromosomes that occur in pairs called homologous chromosomes. For sexually reproducing organisms, one chromosome in each pair came from the male parent and the other came from the female parent. The number of chromosomes is different from one organism to another. Humans, for example, have 23 pairs of chromosomes. Dogs have 39 pairs of chromosomes. Both chromosomes have genes for the same trait arranged in the same order. However, there may be different alleles for a gene on each chromosome. Therefore homologous chromosomes are not genetically identical.

Chromosome 1

The illustration above shows a pair of homologous chromosomes for a pea plant that carries genes for flower position, pea-pod shape, and height. In reality, there would be many more genes along these chromosomes, but this diagram is helpful for learning about the basics of genetics. The shaded areas represent genes. The gene for height, for example, is the shaded area at the bottom end of the chromosomes. Notice that each chromosome has a different allele for height. One chromosome has an allele for tall height (T), and the other has an allele for short height (t). In other words, the diagram shows visually what is meant by Tt.

Homologous Chromosomes Versus Sister Chromatids

Recall that chromosomes replicate before cell division. The replicated chromosome is made up of two sister chromatids, which are exact copies of one another. To understand genetics, it is important to distinguish between homologous chromosomes and sister chromatids. Look at the illustration. On the left you see a pair of homologous chromosomes. They are un-replicated. Homologous chromosomes have the same genes but are not identical, because they may carry different alleles for each gene. Both chromosomes in the homologous pair make exact copies of themselves during replication. After replication, each chromosome is made of two sister chromatids. The sister chromatids of a replicated chromosome are identical copies.

Chromosome 2

Inheritance and Meiosis

Mendel’s observations can be explained by the cellular process of meiosis. Recall that in sexually reproducing organisms, the process of meiosis produces gametes: sperm and eggs. Each gamete contains a unique genetic makeup. Fertilization occurs when sperm and egg combine, resulting m a zygote that divides through mitosis to eventually become a multi-cellular organism. The inheritance of traits occurs through the physical processes of meiosis and fertilization.


The diagram shows two peas and their color alleles. Each parent plant has two alleles, one on each chromosome within a homologous pair. Each chromosome replicates before meiosis. Meiosis separates each homologous pair and separates the sister chromatids. Only one of the original pair of homologous chromosomes ends up in a single gamete. The parent plant with yellow peas (YY) has Y alleles on both chromosomes; therefore, it only forms gametes with the Y allele. The parent plant with green peas (yy) has y alleles on both chromosomes; therefore, it only forms gametes with the y allele. The Y and y gametes join through fertilization, forming offspring with Yy alleles. All offspring plants have the dominant Y allele, so they will produce yellow peas. In this way, traits are passed from parent to offspring through chromosome movement.

Think about Science

Directions: Answer the following questions.

  1. Which alleles does a plant that produces green peas have? A. YY B. Yy C. yY D. yy
  2. Which alleles does a plant that produces yellow peas have? A. YYor Yy B. Yy or yy C. yY or yy D. yy or Yy