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Genes are the building blocks of your DNA, while alleles are the term for variants of the gene. Like “soda” refers to the carbonated beverage, soda can come in different flavors or brands.
What alleles (versions of a gene) you have in your DNA determines the majority of your physical traits. Read on to see how this works.
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An allele is a version of a gene that you inherit from one of your parents. Two alleles make a complete gene.1
Humans and other organisms that create offspring from two parents have genes that are made of two alleles.
One allele is inherited from each parent. For example, if your mother has brown eyes, you might inherit a brown eye allele. If your father has eyes of a different color, you might get a different allele.
Alleles are just versions of a gene. Both terms technically refer to the same series of proteins, but their usage in biology differs.1
You would use the term “gene” to describe sections of DNA or a DNA sequence. If you must differentiate between the form of a gene inherited from each parent, you would use “allele” to talk about the allelic variation.
“Mutation” is often misunderstood by popular culture. The word brings to mind creatures rapidly transforming or having strange physical characteristics. In genetics, mutation simply means the slight changes in genes and alleles as they are passed down.
As cells divide, they also make copies of DNA. Over the course of millions of cell divisions, slight adjustments will likely occur. Think of it as copying a page of text a million times by hand—your handwriting will be slightly different sometimes.
These variations are already considered mutations. Eventually, changes that accumulate on a chain of copied genes create a distinct allele. Mutations also have a role in natural selection and increasing the survival rate of a species.
DNA is the complete set of instructions for a creature’s cells. Because your alleles carry these DNA sequences, they tell cells how to divide, how much of each type of cell to make, and so much more.
These instructions determine your physical characteristics, such as height, skin color, hormone levels, and propensity toward certain diseases.
The differences between physical traits are dependent on what mutant alleles you have. Having alleles that tell your cells to grow black hair causes you to have black hair, and so on.1
Alleles are also classified as either dominant or recessive. For each allele pair you inherit from your parents, whether these alleles are dominant or recessive determines what trait you inherit from each parent.1
Dominant alleles ensure that the trait they are related to is expressed. The brown eye allele is dominant, so if you have it, you will likely have brown eyes.
Recessive alleles express their trait only when there is no dominant allele present. Blue eye alleles are recessive, so if you have both the brown and blue eye alleles, you will still have brown eyes.
An inheritance pattern determines whether a trait is passed down or not. It’s how you determine whether a genetic condition will be passed from one generation to the next.
There are three types:
Gene locus refers to a specific location along a gene or DNA sequence.2 When studying a particular gene, noting the gene locus helps scientists track specific alleles. It’s like saying “the hundredth line of code” or “page 176 of the book.”
When scientists examine chromosomes under a microscope, genes look like actual elongated shapes.
These shapes come in pairs—one from the biological mother and the other from the biological father. With a gene’s given genomic location, it is easier for scientists to visually examine alleles.
The Human Genome Project was an international research project that mapped out all of the genes in human DNA. This essentially means creating a complete list of all the gene locuses in human DNA. The project was started in 1990 and completed in 2003.3
This means that we now know the complete code for human DNA. This does not, however, mean that we can account for all different versions or variant forms of genes yet. It also does not mean that scientists have the ability to predict every trait an individual inherits.
Ongoing genetic research around diseases and biological function revolves around now finding connections between genes and their effects on the body.
Genotype refers to what versions of genes you have. Phenotypes refer to the physical characteristics an organism’s genotype produces.
The variant forms of your alleles make up your complete genotype. It is usually not the case that any single gene actually informs your eventual physical attributes. Instead, your whole genotype comes into play in a complicated manner.
For example, it is not a specific gene that controls skin pigmentation. Different alleles affect different things in your body that produce your skin color.
These could be alleles that determine how much melanin your body makes. These could be alleles that even affect your ability to metabolize sugar. In truth, over 150 alleles affect skin color.
An organism’s phenotype is any of such affected physical characteristics. You have a phenotype for your height, hair color, eye color, blood type, and more.
Multiple alleles interact to affect such things as your propensity for disease. Everything from your hormone production to your metabolism is affected by multiple alleles.
Genetic variations can be so drastic that they can cause genetic diseases. These genetic mutations are called “pathogenic,” meaning they are harmful instructions for your body.4
In some cases, a person may even have a missing or extra chromosome in their DNA which can severely affect their physical traits.4
There are three categories of genetic disorders:
Humans have 22 numbered gene pairs and one pair known as the sex-linked chromosomes. These are labeled the X chromosome and Y chromosome, and they contain the alleles that affect sexual characteristics.5
Biological males in humans have an X and a Y chromosome. Biological females have two X chromosomes instead. This also means that the offspring of a male and female parent will always inherit one X chromosome from their mother and either an X or Y chromosome from their father.5
Despite having two X chromosomes, biological females are less susceptible to having diseases with an X-linked inheritance pattern. This is because the second X chromosome can act as a backup set of instructions that make up for the deviation of the affected chromosome.5
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