Everyone’s genes are arranged along the chromosomes in the same order. However, when we look at the DNA molecules that make up each gene, there are some differences. These different versions of the gene are called alleles. Most genes code for proteins. Differences in DNA can cause differences in the protein products encoded by the alleles. Proteins can perform their tasks in different ways, which can lead to differences in phenotypic characteristics, e.g. different pigmentation or eye color. Whether we have blue or brown eyes depends on the alleles passed down by our parents.
What are alleles?
Alleles are different versions of the same gene with slight modifications in the DNA sequence. They occur at a specific locus (a specific area of the chromosome occupied by a gene; plural loci) on a given homologous chromosome. Two alleles can exist at one particular loci, one encoding a cytosine base and one encoding a thymine base. People inherit two copies of their genome – one from each parent. This is why humans are also referred to as diploid organisms.
The unique combination of alleles in the genome is known as the genotype. These gene variants still code for the same trait (i.e. eye color) but differ in the way the trait is expressed (organism phenotype). In most cases, there is no single locus whose alleles determine how a trait is expressed. Consider eye color, that is, blue, green, brown and hazel eyes are encoded by a unique set of alleles at each genetic loci. A more in-depth look reveals that there are around 16 different genes responsible for eye color, although 2 of these 16 genes have the greatest impact.
If the paired alleles are the same, the genotype of the organism is said to be homozygous for the trait; if they are different, the genotype of the organism is heterozygous. The dominant allele will replace the features of the recessive allele in the heterozygous pair. However, alleles may be codominant for some traits, ie they do not act as dominant or recessive. An example is the human blood group system ABO; people with blood type AB have one allele for A and one for B (people without any group are not type O).
The greater the number of potential alleles, the greater the variation in the hereditary trait. This combination of genes and gene variants underlies human genetic diversity and is the reason no two people are alike.
Mutations, interbreeding, and environmental conditions selectively alter the frequency of phenotypes (and therefore alleles) in a population. For example, alleles carried by high-performance individuals (meaning that they successfully reproduce and pass their genes on to their offspring) are more likely to survive in the population than alleles carried by less-fit individuals that are gradually “lost” from the population.
See also: The most common genetic diseases. How are they made?
Alleles – mechanism of action
It is the alleles that determine what traits we inherit from our parents. The way in which alleles are paired is known as inheritance patterns, which make up all variations of a person’s genetic makeup. Alleles can be said to contain at least two sets of instructions for each gene, so the organism must determine which set to choose, or what trait to express.
This can be easily explained with the example of eye color. The color of our eyes is a matter of genes, or more precisely, of alleles. Genes determine traits such as height, eye color, and hair color. However, a single trait is usually determined by many genes, not just one. For example, height alone is determined by more than 400 genes. Humans have two alleles in the same place on the chromosome (they are very long strands of DNA wrapped around special proteins called histones). Humans have 46 chromosomes; each parent donates 23 of these chromosomes. Accordingly, the expression of any given feature would depend on two sources of information. The two sources are the paternal allele and the maternal allele.
Various combinations of alleles produce brown, blue, green or hazel (hazel) eye colors. Okay, but what next? We can talk about two cases.
First, if each parent passes identical alleles to the eye color gene, we are talking about a homozygous genotype (genotype is all genes transferred to an individual by his parents). Then the instructions given by the alleles will be the same, so the same eye color as the parents will have the child. Second, if each parent brings different alleles to the gene, we are talking about a heterozygous genotype, meaning the instructions will not match and the organism will have to move towards the stronger (or dominant) allele.
Genotypes are all genes passed on to a child by their parents. However, not all genes are translated into visible features. A set of physical characteristics that a person possesses is called a phenotype. The phenotype consists only of expressed genes.
It should be added here that the allele can be dominant or recessive. Dominant alleles express a trait even if there is only one copy. Recessive alleles can only express themselves when there are two copies (one from each parent). It also follows that dominant alleles take precedence over recessive alleles. And just to clarify, blue eyes are a recessive trait, so it usually only occurs when the blue eye alleles are the same in both parents. Brown eyes, on the other hand, are considered the dominant trait, so it is enough for the allele from only one parent to have it for the child to have brown eyes.
Alleles – functions
Alleles, along with DNA and genes, play a large role in determining heritable traits.
Dna, or deoxyribonucleic acid, is hereditary material that humans and other living organisms receive from each parent. DNA is basically responsible for carrying all the necessary genetic information in the cells of the body. Half of a person’s DNA comes from the mother and the other half from the father. DNA itself is made up of small pieces called genes.
Genes code for proteins or parts of proteins that affect the immune system, skin pigmentation, hormone production, and eye color. They are transcribed into RNA molecules which are then translated into proteins. DNA segments designated as genes consist of both coding and non-coding regions. Coding regions, also called exons, are stretches that are transcribed into the final protein. Non-coding regions or introns are not transcribed, but are believed to perform many other functions, such as the regulation of transcription. Experts estimate that humans have approximately 20 to 000 genes. From the wheel, the set of genes forms the genotype. Each person has a unique genotype, which explains the enormous variety of human appearance and biology.
The version of each gene that the parent passes on to their child is known as an allele. Alleles are found on chromosomes, which are the structures that store our genes. In particular, alleles affect how our body’s cells work, determining traits such as skin pigmentation, hair and eye color, height, blood type, and more.
Also read: How does the immune system work?
Alleles and mutations
Genetics is a complex field that, despite extensive research, remains a mystery to scientists. While it is possible to predict quite accurately what eye or hair color a child may have from the combination of alleles, it is not always possible to predict with absolute certainty which features will emerge. This is because of genetic combinations that also depend on hidden or recessive alleles that each parent may have.
It was once assumed that a single, simple inheritance pattern would produce a person’s eye color, but today it is known that dominant traits such as brown eyes can be the result of many different combinations of alleles. Moreover, these traits may disappear in one generation and reappear later. Therefore, the way alleles help determine eye color and other traits is complex, so genetic variations can sometimes produce unexpected results.