Genetics is the study of genes and heredity and the differences in organisms that arise from their genes. The genetic traits passed on to us from our parents determine how we look, influence what diseases we have, and may also affect our behavior. By studying genes and how they are inherited, researchers can understand many diseases and learn how to treat and possibly prevent them. In this activity, you will discover the history of the science of genetics and learn simple tools that will help you understand how genes are passed on from one generation to the next.

Gregor Mendel

Long before DNA was discovered and recognized as the genetic material of living things, researchers understood the basics of how traits are passed from parent to child and from generation to generation. This was largely due to the contributions of an Austrian monk, Gregor Mendel (1822-1884). Mendel is considered the father of genetics because he was the first to understand how genetic traits are passed from one generation to the next.

In 1856, Mendel, a monk at the Abbey of St. Thomas in Brno and later its abbot, began breeding and cultivating varieties of the garden pea plant, Pisum sativum, in an experimental garden on the abbey grounds. Mendel bred 34 varieties of the plants and kept detailed records of the variations that appeared when plants with different physical features were crossed. He eventually identified seven characteristics in the pea plants that were inherited by successive generations in predictable ways. He developed seven pure lines of garden peas with these traits, which included features like seed color, plant height, and pea pod shape. These traits were passed on from plant parents through what Mendel called "factors." We now know that these factors are actually genes.

More on Mendel's Discovery

In one of his many experiments, Mendel decided to cross a tall pea plant with a short one. He expected that the height of the offspring would be an average of the height of the parent plants. Instead, all of the offspring were tall.

Mendel got a bigger surprise when he crossed two of those offspring. He expected that breeding two tall plants would lead to only tall plants, but he instead got tall and short plants in a 3 to 1 ratio--three tall plants for every short one.

This result and the outcome of many other crosses led Mendel to a stunning realization: there were two copies of a gene for each of the seven traits. The genes come in two forms: dominant and recessive. The dominant gene (for example, the gene for tallness in garden peas) determines the trait that you will see in a plant, and the recessive gene (for shortness) will only produce an observable trait if the plant has two copies of it, with no dominant gene present.

Mendel also came up with two concepts explaining the patterns he saw. These are now known as Mendel's Laws of Heredity.

The Law of Segregation states that two genes in a pair separate from each other in sex cells, or gametes--the eggs and sperm--so that each sex cell will have only one gene for each trait. Therefore an egg will have either a gene for tallness or a gene for shortness, but not both, and a sperm will also carry only one or the other. One half of the sex cells will have one type of gene; the other half, the other version.

The Law of Independent Assortment states that the separation of the genes for one trait (for example, height) into two sex cells will occur independently of the separation of the gene pairs for other traits (for example, flower color). Therefore, the inheritance of one trait does not affect the inheritance of any other trait.

Punnett Square

An English geneticist, Reginald Punnett, devised an easy way to calculate the probability that a certain trait will be inherited. It involves what is now called a Punnett square. In order to use a Punnett square, it helps to know some common terminology:

Allele: The alternative forms of a gene, like the "tall" and "short" versions of the gene for height in garden peas.

Dominant: An allele that produces the visible or measurable trait in an organism and is expressed over recessive genes. Dominant alleles are represented by a capital letter ("T").

Genotype: The specific combination of alleles possessed by an individual. Example: "homozygous dominant," which means possessing two copies of the dominant allele.

Homozygous: Possessing two copies of the same allele, both dominant or both recessive. Example: "TT" or "tt."

Heterozygous: Possessing two different alleles. Example: "Tt."

Phenotype: The detectable or measurable characteristic of an organism. Example: tall. The phenotype can, but doesn't always, indicate the genotype.

Recessive: An allele that is expressed only when the dominant allele is not present. Recessive alleles are represented by a lowercase letter ("t").

Trait: A feature or characteristic of an organism that can be tested for or observed.

Sex Linkage


Complete a Punnett square showing the possible genotypes of children born to a colorblind mother and a father with normal vision.

Complex Inheritance Patterns

In the 150 years since Mendel's first experiments, geneticists have discovered that genetic traits usually don't follow simple inheritance patterns. Some traits, like human eye color and stature, are produced by the interplay of many sets of genes; these are known as "polygenic traits." Other traits show a phenomenon called intermediate expression, in which a heterozygous genotype will produce a phenotype that's different from homozygous dominant genotypes. In snapdragons, for example, a homozygous dominant genotype makes red flowers, homozygous recessive makes white flowers, and heterozygous individuals are pink.

In addition, some genes have more than two different versions. This type of inheritance is called multiple allelic inheritance. Our blood types are transmitted in this way. In the ABO blood group there are three alleles; two of the alleles represent varieties of antigens -- substances that trigger an immune response. One allele, IA, represents the A antigen. Another allele, IB, codes for the B antigen. The third allele, i, indicates the absence of either antigen. The body makes antibodies against any antigen that it doesn't have. Combinations of these three antigens produce the four different blood types -- A, B, AB, and O.

Now that you know a little about blood types, let's learn about the genes that code for them.

Multiple Alleles

As you can see, there are two possible genotypes for the A and B blood types. One is homozygous (with two of the same alleles) and one is heterozygous (with two different alleles).
Multiple Alleles: Sample Problem
Using a Punnett square, we can figure out the possible blood types of the offspring of parents even when we don't know the exact genotypes of the parents. For example, let's say the parents have type O and type A blood. We know the genotype of the type O parent--ii--but the parent with type A blood could have one of two genotypes, IAi or IAIA. That means we have to do two Punnett squares.

Now, let's say that these parents have a child with type O blood. What does that tell us about the type A parent?

His or her genotype is IAi.

Now that you've learned a little about genetics, let's take a look at the building blocks of genes: DNA.


Anonymous said...

Keep going...