Chromosomes and Mutations

DNA is the “blueprint for life” because it contains the instructions for building all living things and ensuring that organisms function properly. Because understanding traits and inheritance begins first with an understanding of DNA and genes, students organize human karyogram models to observe how structural changes to the DNA code affect certain traits.

• Traits are the physical and behavioral characteristics of an organism, and they can be passed down from parents to offspring.
• DNA is a large molecule made up of smaller molecules.
• An organism’s DNA determines what an organism looks like, how its body functions, and often how it behaves.
• Sometimes the specific structure of a gene can change. This is called a mutation, and it sometimes affects the production of proteins.
• Scientists use karyotypes and karyograms to analyze chromosomes.

• Why do parents and their offspring often resemble one another?
• How are inherited traits different from learned traits?
• Why do scientists need to understand DNA if they want to understand why an organism has the traits it has?
• How are the two strands of DNA connected together?
• How does an organism’s body know how to make the different proteins?
• What is the relationship between a chromosome, a gene, and a trait?
• Why are chromosomes generally found in pairs in humans?
• Why are there small differences between the genes on each of the individual chromosomes?
• Why are karyotypes important?
• How might a scientist be able to tell if there is a mutation in a karyotype?
• Why aren’t mutations always a cause for concern?

Misconception: Organisms grow because cells get bigger.

Fact: Organisms grow because cells divide. 

Misconception: Cell division results in a decrease in the number of chromosomes.

Fact: In mitosis, cells divide to create exact replicas of themselves, with the same number and kind of chromosome. In meiosis, the daughter cells have half the number of chromosomes as the parent cell.

Misconception: Mutations are always harmful. 

Fact: Mutations are not uncommon, and they can sometimes lead to beneficial traits. Other mutations don’t change how the trait is expressed. These mutations are called silent or neutral mutations.

Adaptation : a trait that helps an organism survive in its environment

Chromosome : a threadlike structure of DNA and protein found in the nucleus of eukaryotic cells; a discrete package of genetic material

Chromosome pair : a pair of chromosomes that possess the same genes but may have different alleles (alternative forms of a gene) because each chromosome comes from a different parent

DNA : deoxyribonucleic acid; a molecule that contains the genes for an organism’s traits and is passed on from parents to children

Gene : a specific pattern of nucleotides that produces a specific protein, which in turn codes for a trait

Heredity : the passing on of traits from one generation to the next in a family

Karyogram : a visual representation of an organism’s complete set of chromosomes, arranged in pairs and in a numbered sequence according to the size and the position of the centromeres

Mutation : any permanent change in the DNA

Polymer : a large molecule made up of many smaller molecules bonded together in a repeating chainlike pattern

Trait : a physical or behavioral characteristic of an organism

Catching a Blue Lobster

Wayne Nickerson was fishing off of the coast of Massachusetts when he caught a lobster. This wasn’t a typical lobster, however. Most lobsters are usually a greenish-brown color until they’re cooked, when they turn bright orange/red.

The lobster caught by Wayne Nickerson was bright blue. Bright blue lobsters are rare. Only 1 out of every 2 million lobsters is blue. The lobster Wayne caught was so unusual that Wayne’s wife, Jan, named it “Bleu,” which is French for blue. The couple wants to donate the lobster to the New England Aquarium.

The unusual color of the lobster is a physical trait. Traits are the physical and behavioral characteristics of an organism. A lobster’s crawling to move around is an example of a behavioral trait. Traits can be passed down from parents to offspring, which is called heredity. Traits that are passed down are called inherited traits.

Environmental factors can shape inherited traits. For example, height is an inherited trait that is determined by our genes. However, poor nutrition—an environmental factor—can stunt growth. Learned traits are those traits that are developed during your lifetime, so they are not inherited. Language is an example of a learned trait in humans, as is the ability to ride a bike. Just because your parents can ride a bike does not mean that you are born knowing how to ride a bike. You must first learn how to do it.

Mutations

The path from a gene to a protein is very complex, and there are many steps along the way. Sometimes, the instructions in the DNA can become changed in the process. Any permanent change in the DNA is called a mutation.

Genetic mutations that are inherited from a parent are called hereditary mutations. Other mutations are not hereditary. These are called acquired mutations and they occur in the DNA of individual cells at some time during a person’s life. They can be caused by environmental factors or a mistake made as DNA copies itself (we’ll explore this idea more in the next section). While some genetic mutations are very rare, others are common in the population. Many of the common mutations cause the normal variations among people, including hair and eye color.

Other mutations don’t have any effect on you. These are called neutral, or “silent,” mutations because they don’t change the expression of any gene. The blue lobster’s color is an example of a mutation that is harmful to the organism. In this particular mutation, the genes that code for color cause the lobster’s cells to make too much of one particular protein, which makes the lobsters that unique blue color. Very few blue lobsters survive in the wild because their bright blue color makes them stand out against the ocean floor. This makes it easy for predators to spot them and eat them.

In contrast, the brownish color of most lobsters is a useful adaptation that allows them to blend into their surroundings and hide better from predators. An adaptation is a trait that helps an organism survive in its environment.

Scientists can study an organism’s karyotype, which is the number, form, and size of all of an organism’s chromosomes. Scientists use a karyotype to identify and evaluate the size, shape, and number of chromosomes in a sample of body cells.

Scientists can organize an organism’s chromosomes in a karyogram, a visual representation of an organism’s complete set of chromosomes, arranged in pairs and in a numbered sequence according to the size and the position of the centromeres. The centromere is the part of the chromosome that links the individual chromosomes within a pair.

In humans, the first 22 chromosome pairs are the autosomes, which are chromosomes that everyone has. They are numbered according to size. The gender-specific sex chromosomes are the final chromosome pair. Biological females have an XX combination, and biological males have an XY combination.

Studying karyotypes and karyograms is important for scientists interested in the structure of cells, as well as the study of heredity and genetics. For example, scientists can sometimes see mutations that are known to sometimes cause various illnesses.

As the main activity for this lesson, students create human karyogram models to analyze how structural changes to chromosomes can affect the physical traits of organisms. During this activity, students analyze the karyogram models and figure out the relationship between DNA, chromosomes, mutations, and traits.