Heredity: Genetic Variations and Expression

Genetic Variation

Think for a moment about the variation of human and animal species on Earth. Millions of different species as well as individuals within one species can show a variety of traits. Humans have different eye colors hair colors heights, and shapes. Animals also have a large variety of traits’. Dogs, for ‘ example, can be very small like a Chihuahua or very large like a Great Dane. They have different fur colors, patterns, and shapes. This tremendous variation can be traced back to genes, the expression of genes, and the interaction of genes and the environment.

Mitosis and Meiosis

You learned about mitosis and meiosis in Chapter 3. Recall that mitosis is the process in which one cell divides into two cells with the same genetic information. These two cells have the same number of chromosomes as the original cell. Recall that meiosis is the process in which one cell is divided into four cells. These cells are gametes, sperm or egg cells. Gametes have half as many chromosomes as the original cell and have genetic information that is different from the original cell.

Crossing Over

Genetic recombination, or the redistribution of genes in cells, is what causes gametes to have different genetic information. An additional source of genetic variation occurs during Prophase I of meiosis, when DNA from the male parent and DNA from the female parent exchange parts of chromosomes in a process called crossing over. This process is shown in the illustration. Crossing over results in each gamete-egg cell or sperm cell-receiving new combinations of genes. Genetic recombination leads to an increase in the variety of traits within a population.

Crossing Over

In this diagram, genetic material from the male parent is blue, and genetic material from the female parent is pink When the two chromosomes cross over, genetic material from each parent detaches and then attaches to the opposite parent’s chromosome. This gives each set of chromosomes a different combination of genes.


Before mitosis or meiosis occurs, an extra copy of DNA is made in a process called DNA replication. During DNA replication, the two strands of a DNA molecule separate from one another in a process that is similar to the unzipping of a zipper. The “teeth” of the zipper are nucleotides. DNA is made up of four different nucleotides defined by the following bases:

  • adenine (A)
  • guanine (G)
  • cytosine (C)
  • thymine (T)

These pair up with one another according to base pairing rules: A pairs with T, and C pairs with G. During replication, new bases attach to each separated strand of DNA, according to the base pairing rules. For example, if the base on the old strand was adenine (A), the rule requires the base thymine (T) to attach to it. The process results in two exact copies of the original DNA molecule. The key enzyme responsible for DNA replication is called DNA polymerase.

DNA Replication


A mutation is a random change that occurs in the genetic material of a cell. Mutations can be caused by errors that occur during replication, or other normal processes related to genetic material. Environmental sources, including electromagnetic radiation with high energy, such as ultraviolet light or x-rays; radiation of particles from radioactive materials or nuclear reactions; and chemicals that react readily also cause mutations.

White Tiger

A white tiger is a Bengal tiger that has a recessive mutation in the gene for color. The several hundred white tigers that exist today can be traced back to the same tiger caught years ago in India.

Mutations are copied during the process of replication and are passed down to daughter cells during mitosis. This means the number of cells that have the mutation will multiply. Cancer cells pass along mutated DNA in this way. Mutations occurring from mitotic division affect body cells, not gametes. Therefore they do not get passed to offspring.

Mutations can also occur in gametes-egg or sperm cells. If one of these cells is combined in a fertilized egg, all the cells of the new individual will have the mutated DNA Mutations in the gametes often result in serious diseases, such as cystic fibrosis.

Mutations may occur in a single allele-one form of a gene. Recall that an allele is an alternative form of a gene that may occur at a given locus, or location. The mutation of a single allele may not seem serious, but it may change the code by which a protein is made. One change in a protein can stop its ability to function in the way it was intended. Suppose an enzyme, which is a protein, that is essential to cellular respiration is rendered inactive by a single gene mutation. Cellular respiration could no longer occur in the affected cells, which would inhibit the cells from getting the energy they need to function. This eventually results in the death of the cells.

Mutations can also occur at the chromosome level. These types of mutations can change a large number of genes at once. Large amounts of genetic information can be lost, switched around, or duplicated, causing abnormalities. Sometimes entire chromosomes are lost or extra chromosomes are added. Down syndrome, for example, is the result of an extra copy of chromosome 21.

Think about Science

Directions: Answer the following questions.

  1. How does crossing over create genetic variation?
  2. What are some things that cause mutations?

Genetic Expression and the Environment

Having a particular gene does not always mean that gene will be expressed in that individual. Many cellular processes regulate gene expression. Some genes become active only under certain environmental conditions. For example, the color of the light a certain butterfly is exposed to as a caterpillar determines the intensity of the color of its wings as an adult. Additionally characteristics of individuals are not simply the result of inheritance but may be strongly influenced by their environment. A person’s body weight may be affected by his or her genetics. However, a diet and exercise can also influence a person’s body weight. Although hair color may be inherited, it can also be changed through chemical processes and sunlight.

Chemical Environment

The chemical environment of the cell can have an effect on the cellular reactions in which DNA is involved, and, particularly, in the expression of genes. Epigenetics is the study of how the chemicals that are not part of the DNA sequence affect the expression of genes (phenotype). For example, the foods we eat, the things we do, and how we feel have an effect on the chemicals in our body. These chemicals can cause genes to be expressed and repress expression of genes. So, in a very direct way, our environment can affect our physical characteristics.

Genetic Testing

Genetic testing is a group of procedures performed on blood or other biological tissue to find genetic variations related to disease. Genetic testing may reveal the risk a person has of developing a particular disease, but not whether a person will actually develop the disease. Developing the actual disease often depends on environmental factors. One type of genetic test is used to determine if certain genes have mutations that increase the risk of breast cancer. Whether cancer actually develops depends on the interaction of genes with the environment.

As genetic testing becomes more common, people are faced with some difficult choices. Some couples may be faced with the decision of whether they should have children if there is a chance they could pass on a specific disease to them. Genetic counselors are medical professionals who help people understand their risks of heritable disease and help them consider their options when faced with these decisions.

Think about Science

Directions: Answer the following questions.

  1. How can the food we eat affect genetic expression?
    A It causes gene mutations. B. It causes replication errors. C. It changes when crossing over occurs. D. It changes the chemicals in our cells.
  2. What is one benefit of genetic testing?
    A You can find out if you will get a particular disease. B. You can find out how to prevent a particular disease. C. You can find out all the diseases you will get. D. You can find out if you are at risk for a particular disease.