What are Genes?

Basic Information and Structure of DNA/Chromosomes/Genomes/Genes

DNA

All living organisms have DNA. DNA exists not only in animals and plants but also in microscopic entities like viruses, bacteria, and microorganisms that can only be seen through microscopes. For instance, every one of the approximately 60 trillion cells in the human body contains DNA. The cells, organs, and tissues of the body are created based on the information contained in DNA.

DNA is a substance with a double helix structure formed by four types of nucleotides called adenine (A), thymine (T), guanine (G), and cytosine (C) arranged facing each other. The sequence of these four substances is called the base sequence.

In DNA, A always pairs with T, and C always pairs with G along the two helical tapes facing each other. Therefore, if one tape exists, it can always make the other tape. Living organisms use this principle to copy DNA from parents to offspring.

DNA within cells, which carries the information for heredity, serves as the "blueprint of life" or the genetic information that determines the characteristics of an organism's body, organs, and tissues. This is why parents and children share similar traits.

Chromosomes

Chromosomes are substances that transmit genetic information from cell to cell and from generation to generation, regulating the differentiation and function of each cell. Chromosomes refer to structures formed by DNA, the essence of heredity, wrapped around proteins called histones.

By winding around histones, DNA forms the structure of chromosomes, making it less susceptible to damage. Moreover, without the structure of chromosomes, genetic information would not be correctly distributed during cell division.

The number and types of chromosomes vary among organisms, but humans inherit 23 chromosomes from each parent, resulting in a total of 46 chromosomes in humans.

Additionally, there are terms related to chromosomes such as homologous chromosomes, bivalent chromosomes, sex chromosomes, and autosomes.

- Homologous chromosomes: Homologous chromosomes refer to a pair of chromosomes that have similar shapes and carry genetic information in similar regions. They are chromosomes of the same type and size, albeit with differences in parental origin. These homologous chromosomes pair, while chromosomes with different shapes or genetic information do not pair.

- Bivalent chromosomes: Bivalent chromosomes are chromosomes observed during meiosis (cell division involved in sexual reproduction). Bivalent chromosomes help efficiently produce gametes (reproductive cells).

- Sex chromosomes: Sex chromosomes are chromosomes that determine sex. In humans, sex chromosomes are referred to as X chromosomes and Y chromosomes. Of the 46 chromosomes in a cell, 44 are autosomes, and the remaining two are sex chromosomes that determine gender.

- Autosomes: Autosomes refer to all chromosomes in a cell except sex chromosomes. Autosomes carry genes that determine all genetic traits except sex. Unlike sex chromosomes, they do not contain genes related to sex determination.

Genomes

While genes, composed of the sequence of four nucleotides—adenine (A), thymine (T), guanine (G), and cytosine (C)—inherit the individuality of humans from parent to offspring, the entire set of this genetic information is called the genome.

From head to toe, everything is determined by genes. The genome is the complete set of genes that determine the characteristics of the entire body.

Let's try to make it clearer by using a plastic model analogy: if genes are blueprints for individual parts, then the genome is the blueprint for the entire plastic model.

In the case of humans, it's called the human genome, which contains various information. Some regions may have little to no significance, while others record crucial information for building parts of the human body.

For example, everything from hair texture (straight or curly) to earlobe size and earwax moisture level is part of genetic information.

All this information, which shapes life, is collectively referred to as the genome.

Genes

Genes are pieces of DNA that carry genetic information. While parents and children are not exactly alike, they often share similarities, known as heredity. Each inherited piece of genetic information is a gene.

Genes, like DNA, exist in all living organisms. It's estimated that humans have about 22,000 genes.

Genes' essence is the genetic information written by DNA. The information contained in genes refers to information about proteins. More precisely, genes contain information about the sequence of amino acids that make up proteins. This sequence indicates how amino acids should be arranged to form specific proteins.

Proteins are essential substances/information that shape organisms and maintain life. Digestive enzymes such as amylase and pepsin, necessary for digestion, are proteins. Hemoglobin, which transports oxygen throughout the body, is also a type of protein. Proteins are crucial substances/information, so they are recorded as genetic information in genes.

Chromosomal and Genetic Abnormalities

Chromosomal Abnormalities

Chromosomal abnormalities result in various diseases due to mutations in chromosomes. Chromosomal abnormalities occur in about 0.7% of newborns, and they can be broadly categorized into structural abnormalities and numerical abnormalities.

It's essential to note that while chromosomes are inherited from both parents, mutations do not always inherit. Mutations can occur in various processes, such as when forming gametes (cells involved in sexual reproduction) or when forming zygotes (cells involved in fertilization).

Some people might think that chromosomal abnormalities always inherit because they often hear the term "genetic diseases." However, it's crucial to remember that this is not always the case. However, it's crucial to remember that this is not always the case. Nonetheless, the possibility of inheritance is not zero, so it's essential to be aware of that as well.

Genetic Abnormalities

Next, let's talk about genetic abnormalities. While chromosomal abnormalities involve significant changes in the structure of the chromosomes themselves, genetic abnormalities involve mutations in a single gene resulting in monogenic disorders or mutations in several genes resulting in polygenic disorders.

Monogenic disorders are further classified into autosomal dominant inheritance, autosomal recessive inheritance, and X-linked recessive inheritance. Dominant inheritance means that if one of a pair of genes has the abnormal gene, the disease will occur, while recessive inheritance means the disease will only occur if both genes of a pair have the abnormal gene.

The distinction between autosomal and sex-linked chromosomes determines differences in the occurrence between males and females, leading to this classification.

Both genetic and environmental factors can cause genetic abnormalities. In cases of genetic factors, if one of the parents (or both) is a patient in dominant inheritance, and if both parents are carriers of the mutant gene in recessive inheritance, the child will be affected. Environmental factors include various things such as radiation and chemicals, and many cases remain unknown.

Pediatric Chromosomal Abnormalities and Their Types

As described earlier, chromosomal abnormalities can be categorized into structural abnormalities and numerical abnormalities. Structural abnormalities refer to single abnormalities where part of a chromosome is broken and attached to a different chromosome or flipped within the same chromosome.

Numerical abnormalities, on the other hand, refer to situations where the number of chromosomes is either too few or too many. While there should be 23 pairs of chromosomes in each cell, consisting of one pair each from the mother and father, during the process of division, sometimes there can be three or just one instead of two.

Monosomy refers to having only one instead of two, whereas trisomy refers to having three instead of two. As briefly mentioned on the NIPT page, Turner syndrome occurs when there's only one X chromosome. Klinefelter syndrome, on the other hand, occurs when there's an extra X chromosome, resulting in XXY.

It's known that trisomy is more likely to occur on chromosomes 13, 18, and 21. In the case of trisomy 13 and 18, they often result in miscarriage, and even if miscarriage does not occur, more than half of the newborns die within the first week, with no established treatment so far. Trisomy 21, also known as Down syndrome, is widely recognized.

What is Down Syndrome?

Down syndrome is one of the most commonly heard diseases among chromosomal abnormalities. While many people can somewhat imagine what it is , few know in detail what kind of disease it is and what symptoms it presents.

Down syndrome is a disease where there's an extra chromosome 21, which under normal circumstances, should have two chromosomes, not three, hence the term "trisomy."

Standard Type

The aforementioned standard type accounts for 90-95% of Down syndrome cases. In this type, the child has three copies of chromosome 21 instead of the usual two, as a result of unequal separation of the mother's and father's chromosomes during gamete formation. In most cases, the parents' chromosomes are normal.

Translocation Type

The translocation type comprises about 5% of Down syndrome cases. In this type, one of the parents' chromosome 21 attaches to another chromosome, resulting in partial trisomy. In this case, one of the parents carries a translocated chromosome.

Mosaic Type

The mosaic type is very rare, accounting for only a few percent of Down syndrome cases. In this pattern, some cells have a normal pair of chromosome 21, while others have trisomy 21. Like the standard type, parents' chromosomes are normal in mosaic type cases.

Some people may believe that Down syndrome is hereditary, but when we organize it like this, we realize that Down syndrome is only inherited from the parents in the case of the translocation type, which accounts for about 5% of cases.

Symptoms of Down Syndrome

There are mainly three symptoms associated with Down syndrome.

Physical Features

Children with Down syndrome exhibit distinctive physical features. They often have a slightly smaller head, with a flattened back of the head. Additionally, their eyes may be somewhat spaced apart and slanted, while their tongues may be larger and protrude forward, causing their mouths to remain open.

Developmental Disorders

Due to insufficient external stimuli during early childhood, children with Down syndrome experience delays in muscle development and language acquisition. Because of weak muscles, they may appear less active or have a subdued demeanor. Their speech may not be clear, often emphasizing only the ends of words or speaking in a flat tone.

Intellectual developmental disorders are also a symptom of Down syndrome, but this varies widely, making it difficult to generalize. Some individuals with Down syndrome can perform daily activities independently, drive cars normally, or graduate from university. On the other hand, some have severe intellectual disabilities and cannot manage daily life independently.

Complications

Down syndrome can lead to complications across many medical fields, including cardiology, gastroenterology, otolaryngology, orthopedics, and hematology.

Examples of congenital heart diseases include atrial septal defects, ventricular septal defects, and atrioventricular septal defects. Congenital heart diseases occur in about 50% of individuals with Down syndrome.

Additionally, individuals with Down syndrome are more prone to leukemia, a type of blood cancer, with a 10-20 times higher incidence rate compared to those without Down syndrome. Leukemia is diagnosed in infancy through blood tests showing increased white blood cell count, anemia, and decreased platelet count. However, the frequency of solid tumors is lower in individuals with Down syndrome.