What are genes?


What is a gene? Basic information and structure of DNA, chromosomes, genomes, and genes, as well as chromosomal aberrations and genetic abnormalities will be explained.

Basic information and structure of DNA/chromosomes/genome/genes


All life has DNA.
DNA is present in animals and plants, as well as in viruses, bacteria, and microorganisms that can only be seen through microscopes.
For example, DNA is present in all 60 trillion human cells.
The cells, organs, and organs of the body are built based on the amount of information in the DNA.

DNA is a substance with a double helical structure in which four types of bases, called adenine (A), thymine (T), guanine (G), and cytosine (C), are arranged facing each other.
The sequence of these four substances is called a nucleotide sequence.

In DNA, A and T and C and G are always arranged in fixed pairs on two helical tapes facing each other.
For this reason, one tape is always enough to make the other.
Organisms use this principle to copy the DNA of parents to their children.

DNA in cells, which has a mechanism by which the same characteristics are inherited from cell to cell and from parent to child, plays the role of "blueprint of the body (life)," that is, the information that is the basis of heredity.
This is why parents and children often share similar characteristics.


Chromosomes are substances that transmit genetic information from cell to cell and from one generation to the next, and are responsible for regulating the differentiation and function of each cell.
Chromosomes are structures formed when DNA, the body of heredity, is wrapped around proteins called histones.

DNA wraps around histones to take on the structure of chromosomes, making DNA less susceptible to breakage.
Also, without chromosome structure, chromosomes would not be correctly distributed as genetic information during cell division.

The number and types of chromosomes vary from organism to organism, but humans inherit 23 chromosomes from their father and 23 from their mother, so humans have a total of 46 chromosomes.

Also related to chromosomes are the terms homologous chromosomes, bivalent chromosomes, sex chromosomes, and autosomes.

Homologous chromosomes
Homologous chromosomes are a term used to refer to a set of two chromosomes that are similar in shape to each other and have similar regions of genetic information.
The term refers to chromosomes of the same type and size, regardless of whether they are of so-called paternal or maternal origin.
These homologous chromosomes bind to each other, but chromosomes that differ in shape or genetic information do not bind to each other.

Bivalent chromosome
Bivalent chromosomes are different from normal chromosomes in that they are found during meiosis (cell division that takes place when cells involved in sexual reproduction are produced).
Bivalent chromosomes serve to efficiently produce gametes (cells involved in sexual reproduction).

sex chromosome
Sex chromosomes are those chromosomes that function to determine sex.
Humans have X and Y chromosomes as sex chromosomes.
There are 46 chromosomes in each cell, 44 of which are autosomes and the remaining two are sex-determining chromosomes.

autosome (non-sex chromosome)
Autosomes are all chromosomes other than sex chromosomes in a cell.
Autosomes carry genes that determine all genetic traits except sex.
Unlike sex chromosomes, it does not contain genes related to sex determination.


The sequence of the four bases that make up DNA, adenine (A), thymine (T), guanine (G), and cytosine (C), is the genetic information that is passed down from parent to child to create human individuality, and this entire genetic information is called the genome.

From head to toe, everything about it is shaped by genes.
The genome is a set of genes that determines the properties of the entire body.

To use a simple plastic model analogy, the gene is the blueprint for each individual part, and the genome is the blueprint for the entire plastic model.

In the case of humans, it is called the human genome, which contains a variety of information.
Some of these areas are of little significance, while others record information that is important for making human body parts.

For example, the quality of the hair (straight or coarse), the size of the earlobe, and even the wetness of the earwax are part of the genetic information.

All of this information for shaping life is collectively called the genome.


Genes are DNA with genetic information.
Parents and children are not exactly the same, but they can be very similar or somewhat alike. This phenomenon is called heredity. In this case, each piece of genetic information actually passed down from parent to child is a gene.

Genes, like DNA, are present in all living organisms, and humans are said to have about 22,000 genes.

The reality of genes is genetic information written by DNA.
Genetic information written in genes refers to protein information.

To be precise, the gene contains information about the amino acid sequence that makes a protein, and in what order the amino acids can be connected to form a particular amino acid.

Proteins are necessary for shaping living things and sustaining life.
Digestive enzymes such as amylase and pepsin, which are necessary for digestion, are also proteins, as is hemoglobin, which carries oxygen throughout the body.
Proteins are so important material and information that they are recorded as genetic information in genes.

Chromosomal and genetic abnormalities

chromosomal abnormality

Chromosome aberrations can cause a variety of diseases due to chromosomal mutations.
Chromosomal abnormalities are said to occur in 0.7% of newborns and can be broadly divided into two categories: structural and numerical.

The caveat here is that chromosomes are inherited from both parents, but not necessarily inherited.
Even if the chromosomes of both parents are normal, mutations can occur during various processes, such as during the formation of eggs and sperm, and during the formation of fertilized eggs.

Because we hear the term "hereditary disease" so often, some may assume that chromosomal abnormalities are inherited, but we must remember that this is not always the case.
However, it is important to know that there is no possibility of inheritance.

genetic abnormality

Chromosomal aberrations are large mutations in the structure of a gene, whereas genetic abnormalities can be single gene diseases caused by mutations in a single gene, or multifactorial genes caused by mutations in several genes.

Single-gene disorders are further classified into autosomal dominant, autosomal recessive, and X-linked recessive inheritance.
Dominant inheritance is a form of inheritance that occurs when one of a pair of genes has the abnormal gene, while recessive inheritance is a form of inheritance that occurs only when both of a pair of genes have the abnormal gene.

This classification is made because the difference in the incidence between men and women occurs depending on whether the chromosomes to which the genes are assigned are autosomes or sex chromosomes (X chromosomes).

There can be both genetic and environmental factors in the cause of genetic abnormalities.

In the case of genetic factors, with dominant inheritance, one (or both) of the child's parents is also a patient, and with recessive inheritance, both parents of the child are carriers of the mutated gene.

Environmental factors include a variety of radiation, chemicals, and other factors, often with unknown causes.

Pediatric Chromosomal Aberration Diseases and Their Types

As described in the previous section, chromosomal aberration diseases can be divided into structural and numerical aberrations.
A structural anomaly is a single abnormality in which the structure itself is changed within a single chromosome, such as when part of a chromosome is broken off and attached to a different chromosome or turned upside down.

On the other hand, a numerical abnormality is a condition in which the number of chromosomes is either too few or too many.
There are 23 pairs of chromosomes in a single cell, and each pair consists of two chromosomes, one from the mother and one from the father.
In short, the correct state of affairs is that a single cell has 46 chromosomes.

However, the process of division may result in three or even a single line where two lines are present.

The condition in which only one is present is called monosomy, and conversely, the condition in which three are present is called trisomy.
As mentioned briefly on the NIPT page, for example, the condition of having only one X chromosome is called Turner's syndrome.
The condition of sex chromosome trisomy, in which one X chromosome is too many, XXY, is called Klinefelter's syndrome.

In autosomes, chromosomes 13, 18, and 21 are known to be prone to trisomy.
13 and 18 trisomies actually often result in spontaneous abortion, and even in the unlikely event of no miscarriage, more than half of them die within the first week of life, and no treatment has been established at present.
Note that 21 trisomy is widely known as Down syndrome.

What kind of disease is Down syndrome?

Down syndrome is one of the most commonly heard of chromosomal abnormalities.
Many people have some idea of what it is, but not many know in detail what kind of disease it is and what symptoms it causes.

Down syndrome is a very common condition called "standard type," in which three chromosomes are present instead of the usual two in the 21st chromosome.
Let us first explain what a standard type is.

standard type

The aforementioned standard type accounts for 90-95% of Down syndrome.
In the standard type, the chromosomes of the mother and father are unequally separated during gametogenesis, resulting in three chromosomes 21 in the child.
In this case, the chromosomes of both parents are normal in most cases.

translocation (genetics)

The translocation type accounts for about 5% of all cases and is a down syndrome.
In this type, one of the parents' chromosome 21 is attached to another chromosome, resulting in a partial trisomy.
In this case, one of the parents carries the translocated chromosome.


Mosaic type is a very rare pattern of Down syndrome, accounting for only a few percent of all cases.
The human body is composed of many cells, but in mosaicism, both cells with normal chromosome 21 and cells with 21 trisomy are mixed together.
In the mosaic type, as in the standard type, the chromosomes of both parents are normal.

Some people assume that Down syndrome is inherited, but when we sort it out in this way, we find that only the translocation type of Down syndrome is inherited from both parents, which is only about 5% of all cases.

Symptoms of Down syndrome

In the case of Down syndrome, three main symptoms are known to occur.


A child with Down syndrome will have a distinctive appearance and facial features.
The head is rather small and the occiput is precipitous.
In addition, both eyes are somewhat distant and upturned, and the tongue is large and forward, leaving the mouth open.

developmental disorder

The disease Down syndrome causes delayed muscle development and language development due to inadequate external stimulation at an early age when the child begins to take an interest in the outside world.
Because of their weak musculature, they may appear less active or docile.
They often speak without clarity, emphasize only the end of words, or speak in a flat manner with no inflection.

Intellectual developmental disabilities are also a symptom of Down syndrome, but these are quite broad and it is difficult to say that a person has an intellectual disability in a single sentence.
Some are able to perform daily activities on their own, some can drive a car normally, and some have graduated from college.
On the other hand, there are those whose intellectual development is so impaired that they cannot lead their daily lives on their own.

complications (in an illness)

Down syndrome can cause complications across many departments, including cardiology, gastroenterology, otolaryngology, orthopedics, and hematology.

Endocardial, atrial septal, and ventricular septal defects, in which the walls dividing the heart into four chambers are not fully formed, are examples of congenital heart disease conditions.
Congenital heart disease is present in about 50% of Down syndrome cases.

Down syndrome also makes people more susceptible to leukemia, a cancer of the blood, and the incidence is 10 to 20 times higher than in those without Down syndrome.
Leukemia is diagnosed by blood tests in early childhood that show increased white blood cell counts, anemia, and thrombocytopenia, while Down syndrome has been shown to be associated with less frequent development of solid tumors.