In any given species, the genetic makeup is uniform across a population. The attributes that differentiate one individual from another arise from variations in the genome. These variations are called genetic mutations. These mutations may have a negative or positive effect depending on the gene that is altered. This BiologyWise article provides an understanding of mutations and their effects.
Alike, yet unique!
In humans, 99.9% of the genome is exactly the same in all individuals. The differences among the individuals arise due to variations in merely 0.1% of the whole genome.
To understand what a genetic mutation is, one must first understand the concept of genes. A gene is a hereditary unit that consists of sequences of DNA that code for various molecules, which eventually help define an organism and regulate all processes associated with it. There are close to 30,000 genes in the human genome. Each gene consists of a juxtaposition of the four basic building blocks of DNA: Adenine (A), Guanine (G), Thymine (T), and Cytosine (C). These bases, present in various combinations, are responsible for coding all the various molecules that form an organism and help it function. Any alteration in the set sequence of a gene can be called a genetic mutation. The alteration may sometimes lead to a desirable change or to an undesirable change. In some cases, no change might be observed despite a genetic mutation.
Some Common Genetic Mutations in Humans
The color of human hair is a multifactorial trait that generally depends on the interaction between the eumelanin and pheomelanin pigments. These pigments are coded for by different forms of the MC1R gene. The expression of this gene, however, is controlled by various other genes. The melanocortin 1 receptor (MC1R) gene in its normal form produces pheomelanin and converts it into eumelanin. In the event of mutation, the amount of pheomelanin builds up, and is not converted into eumelanin.
Pheomelanin produces colors from orange to yellow, while eumelanin is responsible for the black and brown pigments, and also for determining the darkness of the hair color. Blonde hair has a very low percentage of eumelanin, whereas brown hair has brown eumelanin in a large percentage. Black eumelanin leads to the presence of black hair. Pheomelanin is comparatively more stable, and hence breaks down very slowly when oxidized. As it decomposes, the color changes from red to orange to yellow, and finally to white. The interactions between these pigments, and their presence in varying concentration lead to the occurrence of natural hair colors like black, brown, red, blonde, and white.
Example: Red Hair
In the case of red hair color, due to the malfunction of the MC1R gene, the pheomelanin in the hair follicles, builds up and is not converted into eumelanin. The stable and reddish nature of the pheomelanin molecules cause the resultant red hair color. The inheritance of red hair color depends on the hair color of the parents. If both parents are red-haired, then the progeny also has red hair. However, in cases where one parent has a different hair color, the non-mutated gene copy from that parent may correct or compensate for the mutated gene copy from the other parent, resulting in progeny that may or may not have red hair.
The color of the eyes is a physical trait of humans that is determined by more than one gene. All humans originally had brown eyes. Somewhere along the course of evolution, different eye colors emerged due to genetic mutations. Since then, these mutations have been passed down through generations. A research paper in the Journal of Human Genetics provides evidence that close to 16 genes may be responsible for eye color, and the way these interact with each other determine the specific shade of color. However, the two main genes involved in this process are the OCA2 and the HERC2 genes located on chromosome 15. The OCA2 gene is responsible for production of melanin, a molecule that imparts color. The absence of this molecule leads to albinism. The HERC2 gene regulates the expression of the OCA2 gene. A specific mutation in the HERC2 gene along with single nucleotide polymorphisms (SNPs) in the OCA2 gene causes the development of a spectrum of different eye colors in humans. The condition of heterochromia, where the two eyes have different coloration, is also due to the interaction of the OCA2 and HERC2 genes.
Example: Blue Eyes
The occurrence of blue eyes is a recessive trait. The brown or black eye color is usually dominant over it. It is caused due to specific interactions between the OCA2 and HERC2 gene. The progeny only has blue eyes if both parents also have a blue eye color. If one of the parents has a different eye color, that color is usually seen in the progeny. However, there have been rare cases reported where this recessive trait skipped generations in family lineages, often occurring in alternate generations, irrespective of the eye colors of the parents.
While the occurrence of freckles is not dependent on genetic factors, their abundance is due to genetic mutations in the MC1R gene. Melanocytes are specialized cells in our skin, which produce the pigment melanin. Melanin protects our DNA from the harmful ultraviolet (UV) radiations given off by the sun. Generally melanocytes are evenly spread throughout the skin, causing the skin to tan due to melanin production. In individuals with mutated MC1R gene, these melanocytes are present in clumps across the skin, and on exposure to sunlight lead to the occurrence of freckles. Since the gene is mutated, the melanin produced exhibits a shade of red.
It is yet another example of multifactorial traits. Additionally, it also exhibits variable penetrance, which in simple words implies that this trait does not follow a strict pattern of inheritance. If a parent has a cleft chin, the offspring may or may not exhibit the same. This is because this trait essentially occurs due to the incomplete fusion of the jawbone during the process of embryogenesis. Modifier genes may be activated during the process, leading to a correction of the error and causing the progeny to show the absence of the cleft. However, if those genes are not activated, the error remains uncorrected, and the fetus is born with a cleft chin.
This particular phenotype is also termed distichiasis. It is the condition where eyelids show presence of a double row of eyelashes. Hair follicles replace the oil glands present on the waterline along the eyelids. This occurs due to a mutation in the FOXC2 gene, which is responsible for most of the tissue development during embryogenesis. Extensive mutations in this gene may cause lymphedema and failure of the lymphatic system.
This trait is exhibited when genetic defects lead to shortened facial muscles, and cause the formation of indentations when an individual smiles. The aberrant muscles are caused due to defects caused in the subcutaneous connective tissue during the course of development of the embryo. The exact genetic cause for this is as yet unidentified, but geneticists believe that it may be a dominant trait caused due to a single gene mutation. The inheritance of this trait is variable. If both parents have this quality, then it is inherited by the child. But in the case of only one parent exhibiting dimples, the child may or may not show its presence.
Types of Genetic Mutations
Mutations may result from a damage to the genome (exogenous or endogenous), faulty repair of damaged DNA, insertion and deletion of individual bases or of short sequences of DNA, or from errors that have occurred during replication of the genome. They are classified and named based on the cause and effect of the mutation.
Mutations Causing Structural Changes
Point mutations: These mutations are characterized by changes in a single nucleotide in a gene. There are 4 types of point mutations: deamination, depurination, transition, and transversion.
- Deamination: This mutation arises when a base undergoes hydrolysis and the amine group is displaced from the base, transforming it into an atypical base with a keto group. For example, deamination of cytosine would yield a uracil base and that of adenine will yield a hypoxanthine base.
- Depurination: This mutation occurs when there is loss of a purine base from a sequence, giving rise to an apurinic site. The purine bases are adenine and guanine.
- Transition: In this type of mutation, a purine base is replaced by another purine base or a pyrimidine base is replaced by another pyrimidine base. Adenine and guanine are purines, and pyrimidines include cytosine and thymine.
- Transversion: This mutation involves a purine being replaced by a pyrimidine and a pyrimidine replaced by a purine. This type of mutation is less common as compared to transition mutations.
If these mutations occur in the coding regions of the genes, they are further classified based on the type of effect seen in the resultant molecule. They are of 4 types.
- Frame-shift mutations: Nucleotides are read by transcribed and translated by grouping them into groups of three, called codons. Each codon codes of a unique amino acid. The nucleotides are read as codons by the open reading frame. In the event of a deletion or insertion, the grouping is disrupted and leads to a different grouping that shifts the reading frame, causing a frame-shift mutation.
- Silent Mutations: The altered base causes a change in the amino acid codon, but the amino acid remains the same despite the change. For example: the codon of the amino acid lysine is TTC, if due to a transition mutation the codon is changed to TTT, it will be a silent mutation as the codon TTT also coded for lysine.
- Missense mutations: The altered base causes the amino acid codon to change such that the codon now codes for a different amino acid. For example, if the lysine codon TTC is changed to TCC codon which codes for arginine, it will result in a missense mutation.
- Nonsense mutation: These mutations occur when the altered base changes the codon to a stop codon that terminates the amino acid chain and could lead to a truncated protein. For example, if the TTC lysine codon is changed to ATC stop codon, the protein synthesis machinery will terminate and lead to a incomplete protein.
The addition of extra nucleotides to a sequence is called an insertion mutation. There can be an insertion of a single nucleotide, as well as a sequence of nucleotides. Such sequences are called transposable elements and they are characterized by their ability to change their position in the genome.
The removal of a single nucleotide or a sequence of nucleotides (transposable elements) is called a deletion mutation.
This occur when a multiple amplified copies of a gene are found, leading to overproduction of a molecule.
This involves the change in position of a set of genes from one chromosome to another non-homologous chromosome.
When the sequence of genes on a chromosome is disrupted and the sequence is reversed, the resulting change in orientation of genes is called an inversion.
Loss of heterozygosity
Every gene occurs in a set of two copies. The two copies are called alleles of the gene. An individual may possess the identical alleles (homozygous) or two different alleles (heterozygous) of the same gene. When there is a loss of one allele of the gene in a heterozygous individual, the condition is called loss of heterozygosity.
Mutations Affecting Function
Loss of function- If a mutation in the gene causes the gene to lose its normal function, the mutation is called a loss-of function mutation. In cancer, most tumor suppressor genes undergo this type of mutation.
Gain of function- When a mutation in a gene leads to the gain of a new and abnormal function, that mutation is called a gain of function mutation. In cancer, the oncogenes undergo this mutation and start rapidly regenerating.
Dominant negative mutation- In this type of gene mutation, the product of the altered allele of a gene acts as an antagonist towards the product of the normal wild-type allele of that same gene. The altered product does not let the normal product carry out its original function. This occurs in genes like p53, ATM, etc.
Lethal mutation- When a mutation in a specific gene leads to a halt in the development or even death of an organism, that mutation is called a lethal mutation.
Mutations Affecting Inheritance Pattern
Heterozygous mutations- This occurs when there is a mutation in only one allele of the gene.
Homozygous mutations- This involves the occurrence of the same mutations on both the alleles of a gene.
Compound heterozygous mutations- This condition arises when both alleles exhibit different mutations.
These examples gives one an understanding of the complex and intricate nature of genetic mutations. While most of the mutations are harmless and responsible for the physical variation across a species, some mutations do cause life-altering health conditions. Scientists are still working hard to decipher the function of each particular gene and the interactions between them.