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Dominant Vs Recessive Genes

In this article, we will take a close look at the concepts of dominant and recessive genes. We will explore how these genes function, their role in inheritance, and their impact on various genetic disorders. We will also discuss examples of traits and diseases associated with these genes.
Jakub Gwiazdecki

Jakub Gwiazdecki

Fifth year medical student at the Medical Faculty of Comenius University in Bratislava.

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Overview

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What are the Differences Between Dominant and Recessive Genes?

The terms dominant and recessive describe the genetic probability of inheriting a particular gene. Every gene has two copies, which are named alleles. One comes from the side of the mother, the other from the side of the side of the father. Normally, when one of the genes is dominant and the other recessive, the first will be pronounced [1].

The expression of the alleles brings physically visible traits, like eye or hair color. However, an allele can also encode pathological mutations.

The main difference between dominant and recessive genes is when they are expressed. A dominant gene will always express its dominant trait. The recessive traits need two recessive alleles.

Based on the expression difference, it is possible to determine whether an allele (a gene copy) is dominant or recessive [2].

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Dominant and recessive genes are inherited from parents to offspring. Each parent contributes one allele for each gene. If a gene is dominant, only one copy is needed from either parent for the trait to be expressed. If a gene is recessive, both parents must contribute the gene for the trait to be expressed. For example, if both parents carry one copy of a gene for an autosomal recessive condition, their child has a 25% chance of inheriting the condition, a 50% chance of being a carrier, and a 25% chance of not having the condition or being a carrier [1].

Dominant and recessive traits are determined by how alleles, or versions of a gene, interact in a heterozygous individual. In complete dominance, the dominant allele completely masks the recessive one. For example, the allele for brown eyes is dominant over the one for blue eyes, so an individual with one of each will have brown eyes [1, 2]. In incomplete dominance, the dominant allele doesn't completely mask the recessive one, resulting in a blend of traits. An example is hair texture, where an individual with one allele for curly hair and one for straight hair will have wavy hair [1, 2]. In codominance, both alleles are equally expressed, like in the AB blood type [1, 2].

To see an example of how the dominant and recessive genes work, let's consider cystic fibrosis.

Cystic fibrosis is a monogenic, autosomal recessive disease. The expression of this disease depends on one gene, which is found on chromosome 7.

Because cystic fibrosis is a recessive disease, both alleles have to be affected by the mutation. When one of the copies is a healthy dominant gene, the disease will not occur. All the elements encoded by this gene will be expressed in the dominant copy. The defective recessive copy will not be used for protein production [3].

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Cystic fibrosis (CF) is primarily caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. The CFTR gene encodes a protein that regulates the movement of chloride ions in and out of cells. Mutations in this gene lead to the production of a defective CFTR protein, which affects the function of various organs, including the lungs, pancreas, and digestive system. CF is an autosomal recessive disorder, meaning that individuals must inherit two copies of the mutated CFTR gene (one from each parent) to develop the disease [1, 2].

Cystic fibrosis (CF) is inherited in an autosomal recessive manner, which means that both parents must carry a mutated copy of the cystic fibrosis transmembrane conductance regulator (CFTR) gene for their child to have CF. If both parents are carriers, there is a 25% chance that their child will have CF, a 50% chance that the child will be a carrier like the parents, and a 25% chance that the child will neither have CF nor be a carrier. CF is caused by mutations in the CFTR gene, which affects the regulation of chloride and bicarbonate ions in cells lining various organs [1, 2].

It's important to remember that most of the mutations are passed down recessively. Often, when a mutation happens, it leads to a loss of genetic function. The loss is then compensated by the healthy allele [4, 5]. This is the mechanism that makes the mutated genes recessive.

How Can Your Child Inherit a Dominant or Recessive Disease?

Genetic disorders can be inherited in two main ways: through dominant or recessive genes. The pattern of inheritance follows simple rules. There are three possible configurations of alleles [6].

First, a dominant homozygous configuration. In this case, a person will have two dominant alleles.

Next is a dominant heterozygous. This person will have one dominant and one recessive allele.

The third case is recessive homozygosity. Here, both the alleles are recessive, and only a recessive disease can be transmitted.

Whether your child will inherit the disease or not depends on the genes of you and your partner. To be sure, both of you have to undergo genetic tests.

Especially when one of you suffers from a recessive disease, it is worth checking the genes of the other. In this way, the probability of your child inheriting the disease can be estimated, using a Punnett Square.

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Genetic testing for couples serves multiple purposes. It can identify couples at increased risk of having a child affected with an autosomal recessive or X-linked disorder, facilitating informed reproductive decision-making [1]. It can also screen for carrier status of several diseases, helping couples understand their risk of transmitting genetic conditions to their offspring [2]. Additionally, preimplantation genetic testing (PGT) can prevent chromosomal abnormalities and other genetic disorders in embryos, aiding in the successful conception of a healthy child [4, 3].

Punnett Squares are used to estimate disease inheritance by illustrating the possible combinations of parental alleles. For instance, in autosomal recessive diseases, each parent has a 50% chance of passing on the disease allele. The multiplication rule of probability is applied, resulting in a 25% chance (50% x 50%) that a child will inherit the disease if both parents are carriers [1].

Dominant Genes Diseases

Dominant gene diseases are conditions that occur when a mutation in a single copy of a gene is enough to cause a disease. Here are some examples of autosomal diseases:

  • Huntington's disease [7]
  • Marfan syndrome [7]
  • Familial hypercholesterolemia [7]
  • Dominant Dystrophic Epidermolysis Bullosa [8]
  • Myotonic dystrophy types 1 and 2 [9]
  • Polycystic kidney disease [10].

Some of the mutations in alleles can also be found on the X chromosomes. For instance, X-linked diseases include:

  • Microphthalmia with Linear Skin Defects (MLS) Syndrome [11]
  • Aicardi Syndrome [11]
  • Goltz Syndrome (focal dermal hypoplasia) [11]
  • Incontinentia Pigmenti (Bloch-Sulzberger Syndrome)[12]
  • Oral-Facial-Digital Syndrome I [12]
  • X-linked Hypophosphatemic Rickets [13]
  • Fragile X Syndrome [14]
  • Rett Syndrome [15].

Recessive Genes Diseases

Recessive gene diseases are conditions that occur when an individual inherits two copies of an abnormal gene, one from each parent. Examples of autosomal recessive diseases include:

  • Cystic fibrosis [16]
  • Sickle cell anemia [10]
  • Tay-Sachs disease [10]
  • Homocystinuria [10]
  • Gaucher's disease [10].

Examples of X-linked chromosome recessive diseases are:

  • Duchenne Muscular Dystrophy [17]
  • X-linked recessive ichthyosis [18]
  • X-linked agammaglobulinemia (XLA) [19]
  • X-linked severe combined immunodeficiency [19]
  • X-linked mental retardation [20]
  • X-linked disorders with cerebellar dysgenesis [21].

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Jakub Gwiazdecki

Jakub Gwiazdecki

Jakub is in his fifth year as a medical student at Comenius University in Bratislava, Slovakia. He has special interested in cardiology and in patient-centered medicine. His love for heart health isn't just book-smarts; he wants to know how it works, what it means for our feelings, and how key it is for health and happiness. Jakub thinks real good health care comes from always putting the patient at the centre, treating each person as a whole.