In this explainer, we will learn how to describe the inheritance of genes from parent to offspring and explain the difference between dominant and recessive genes.
DNA (deoxyribonucleic acid) is a molecule contained within each cell that contains all the information necessary to build an individual. DNA is a very long helical molecule that contains genes. Genes are DNA sequences that contain instructions to create molecules and proteins. Imagine your DNA as a unique cookbook used to make you who you are. Genes would be like the recipes contained within that DNA cookbook. Humans have over 23 000 genes carrying all the instructions needed for building every cell in their bodies. This makes for a giant cookbook, right? Generally, we receive our genes from each of our biological parents. This means that we have two versions of each recipe. We will see how these versions are used to build the unique person we are.
Definition: Gene
A gene is a sequence of DNA that contains the information needed to produce a certain characteristic (e.g., eye color).
Definition: Characteristic
A characteristic is an observable feature, like eye or hair color, determined by our DNA.
Genes allow for the inheritance of certain traits from one generation to the next. When an organism passes down genes to the children (which are called offspring), the offspring are said to inherit their parents’ genes. The proteins that genes encode create observable characteristics, traits, and physical features that can be transmitted from the parents to their children.
Definition: Inheritance
Inheritance is the passing down of traits from parents to their offspring.
DNA molecules are incredibly long. The human DNA molecule from just one cell is between two and three metres long. As human cells are microscopic, cells package this long DNA into incredibly compact, threadlike structures called chromosomes, as shown in Figure 1. Chromosomes package DNA so tightly that all the DNA of a cell can fit inside the nucleus, where human chromosomes are found.
Definition: Chromosomes
Chromosomes are long molecules of DNA that are coiled around proteins.
Example 1: Identifying Coiled DNA Found in Cell Nuclei
Inside the nucleus of your cells, DNA is wound and coiled into long strands. What are these long strands of DNA called?
- Helices
- Genes
- Nucleosomes
- Chromosomes
- Ribosomes
Answer
DNA is a long, double-stranded, helical molecule that contains genes. Within the DNA, DNA sequences code for protein products that result in an organism’s characteristics. DNA is a very long molecule, measuring 3 metres in length. To fit this DNA into a microscopic cell, the cell packages the DNA into tightly wound coiled structures called chromosomes. Chromosomes look like threadlike structures when viewed under the microscope and are always found in the cell’s nucleus.
Therefore, the correct answer is chromosomes.
The offspring inherit their genes, and hence the characteristics, from their biological mother and father. The genes of the offspring are unique combinations of their parents’ genomes, and children display a mix of their parents’ characteristics.
Chromosomes play a central role in gene inheritance. Normal human body cells have 46 chromosomes consisting of 23 chromosome pairs. These 23 chromosome pairs are shown in Figure 1. However, special reproductive cells (sperm and egg cells) have only 23 chromosomes in total.
When two organisms reproduce to produce offspring, their reproductive cells combine in a process called fertilization. During this process, a sperm cell from the father fuses with an egg cell from the mother to produce a new cell. This process is depicted in Figure 2.
This new cell, called a zygote, therefore has a full set of 46 chromosomes. The zygote then divides into many cells that later develop into an adult organism. When the zygote divides into more cells, each daughter cell also carries 46 chromosomes. This is why all somatic (body) cells each contain 46 chromosomes in their nuclei.
Example 2: Determining the Number of Chromosomes in a Cell Nucleus after Fertilization
The diagram shows a basic outline of the process of fertilization.
After fertilization, a zygote forms. The zygote shown in the diagram has 4 cells. How many chromosomes will be in the nucleus of each of these cells?
Answer
During fertilization, a sperm cell from the biological father fertilizes an egg cell from the biological mother. Both cells combine the parents’ genetic material to form a new cell called the zygote. Half of the zygote’s genome comes from the father, and the other half comes from the mother.
Both the sperm cell and the egg cell each carry 23 chromosomes. Chromosomes are threadlike structures consisting of tightly wound DNA molecules. The sperm and egg cells combine to form a zygote during fertilization, which carries all 46 chromosomes.
The zygote then starts cell division. It divides into two cells, then into four cells, and then into eight cells, and so forth. Eventually, after many rounds of cell division and growth, it becomes a baby. Generally, when cells divide, they copy the exact chromosome number and then split them into the new cells. Therefore, all the daughter cells have the same chromosome number as their parent cell.
As the zygote at the beginning of its development contains 46 chromosomes, all of its daughter cells (which make up the human body cells) will also have 46 chromosomes. Therefore, after two rounds of cell division following fertilization (which generates four cells), each cell will contain 46 chromosomes.
With every chromosome pair consisting of one chromosome from each parent, each offspring inherits a unique mix of genes from the mother and the father. As the organism grows and matures, it develops a unique blend of both parents’ traits, because its genome contains a mix of maternal and paternal genes.
When a human reaches sexual maturity, they start producing sperm or egg cells, or gametes. Each gamete will randomly keep one chromosome from each of the 23 chromosome pairs. The resulting organism’s sperm or egg cells will have 23 chromosomes selected at random.
Definition: Gametes
Gametes are an organism’s reproductive cells that contain half the genetic material of a normal body cell.
Other organisms have different chromosome numbers. For example, chimpanzees have 48 chromosomes arranged in 24 chromosome pairs. However, similar to humans at sexual maturity, their gametes have only half their body cells’ chromosomes. Therefore, their gametes have 24 chromosomes.
Many traits are inherited. This means that the characteristic encoded by a gene can be passed down from the parents to their children. However, some traits are not inherited but are rather acquired during an organism’s lifetime. Acquired traits result from interactions with the environment and cannot be passed down from the parents to their offspring.
For example, blood group, eye color, and natural hair color are all inherited traits. If a child has blue eyes, then their parents must both carry the gene for blue eyes. This child may pass down this gene for blue eyes to their children later in life.
On the other hand, piercings, scars, and tattoos are examples of acquired traits. For example, if a person develops a scar after scraping their knee playing rugby, this scar is the result of that person’s interaction with their environment, not their genetics. Therefore, this person will not pass down this scar to their children.
Example 3: Identifying Acquired Traits
A student is asked to describe themselves.
They write, “I have blue eyes and naturally brown hair. When I was younger, I tripped over and hurt myself, and now I have a scar on my arm.”
Which of the characteristics described above did this student not inherit from their parents?
- The scar on their arm
- Blue eyes
- Brown hair
- Blue eyes and brown hair
Answer
Organisms carry two different types of traits: inherited and acquired.
Inherited traits are encoded by a person’s genes. Genes are DNA sequences that encode protein products that result in a person’s characteristics. Inherited traits can be passed down from the parents to their children through genes, which carry the traits’ instructions. Eye color and hair color are both examples of inherited traits, making all the options that refer to blue eyes and brown hair incorrect.
On the other hand, some traits are acquired. Acquired traits are not encoded by a person’s genes and arise from interaction with one’s environment. As acquired traits are not encoded genetically, parents cannot pass down their acquired traits to their children. For example, a scar is a physical characteristic that results from an injury, not gene expression. A scar on a mother’s arm will not be passed down to her children. Therefore, the characteristic described by the student, which has not been inherited from their parents, is a scar on their arm.
Genes have different versions. Among these, some versions are dominant to others. When a gene version is dominant to another version, the dominant version is always expressed as a trait. The recessive version (the nondominant version) is usually hidden unless the organism has two recessive gene version copies. In that case, the organism expresses the trait encoded by the recessive version.
Definition: Trait
A trait is a variation of a characteristic.
Key Term: Dominant Trait
A dominant trait is the version of a trait that is always shown if the organism inherits the genetic information encoding that trait.
Key Term: Recessive Trait
A recessive trait is a version of a trait that is only shown if the genetic information that produces the dominant trait is absent.
For example, in humans, the freckles gene version is dominant to the no-freckles gene version. As illustrated in Figure 3, suppose a child inherits a freckles gene from their father and a no-freckles gene from their mother. In that case, the child will have freckles because their paternal freckles version is dominant to their maternal no-freckles version.
However, suppose the child inherited two no-freckles copies from both parents. In that case, the child will not have freckles because no dominant version is masking the recessive gene’s trait.
An important takeaway here is that even though the offspring carry genes from both parents, their traits are not always a mix of their parents’ features. A blue-eyed mother and a brown-eyed father do not have blue–brown-eyed children because the gene for brown eyes is dominant to the gene of blue eyes. Their children, therefore, will instead have brown eyes.
Example 4: Identifying Recessive Genes
The diagram provided shows how eye color can be inherited in humans. What term is given to the gene that produces blue eyes in this example?
- Submissive
- Inactive
- Recessive
- Passive
- Dominant
Answer
Genes are DNA sequences that encode the information necessary for making proteins. These proteins form the basis of physical traits, which are observable to the naked eye. For example, genes encode the instructions for the eye color that a person inherits. Genes are located on structures called chromosomes.
Every person has 46 chromosomes in each of their normal body cells. These 46 chromosomes are organized into 23 chromosome pairs. Each pair consists of one chromosome inherited from the person’s biological mother and one chromosome inherited from the biological father.
Additionally, every person carries two copies of each gene: one gene from their mother and one gene from their father. Each maternal gene is located on each maternal chromosome, and each paternal gene is located on each paternal chromosome.
Genes have different versions. Some gene versions are dominant to other gene versions. When an organism has a dominant gene version, the trait that this gene encodes masks the other gene version’s trait, even if it is present in the organism’s genome. Gene versions that are masked by dominant genes are called recessive genes.
The gene version for brown eyes is dominant. When someone carries a gene copy for brown eyes, they will always have brown eyes. Even if they also have the gene for blue eyes (a recessive gene) on their other chromosome, they will still have brown eyes because of their dominant gene.
Since the child in the question carries both the gene for blue eyes (on the left chromosome) and the gene for brown eyes (on the right chromosome) and they have brown eyes, then this means the gene for brown eyes must be dominant. Therefore, the gene for blue eyes must be recessive.
Inheritance is incredibly important to living organisms. It allows organisms to pass down genes that allow them to survive and reproduce.
Key Points
- The instructions for an organism’s characteristics are encoded in their DNA as sequences called genes, arranged into threadlike structures called chromosomes.
- Inheritance describes how children inherit a mix of genes and their encoded characteristics from their biological mother and father.
- Human body cells have 46 chromosomes that are found in the cell nucleus, whereas sex cells (sperm and egg cells), which are also known as gametes, carry only 23 chromosomes.
- When a father’s sperm fertilizes a mother’s egg, they each contribute 23 chromosomes, which combine to form a zygote with 46 chromosomes.
- Although organisms inherit some traits from their parents, they acquire other traits from their interactions with their environment.
- Some genes are dominant. When an organism has a dominant gene, it will always exhibit its corresponding trait. Some genes are recessive. An organism with a recessive gene will not exhibit the recessive trait if there is also a dominant gene present.
