Do Humans Have 48 or 46 Chromosomes?

Chromosomes are long, thread-like structures made up of DNA, present inside the nucleus of cells and carry genes. Genes are the basic unit of heredity and contain the instructions for the characteristics and traits that are passed from parents to their children.

DNA is made up of four chemical building blocks called nucleotide bases, which include:

1. Adenine
2. Cytosine
3. Guanine
4. Thymine

The order of these bases determines the genetic information that is carried by the DNA molecule. During cell division, the chromosomes are replicated and passed on to the daughter cells, ensuring that the genetic information is transmitted to the next generation.

There are two types of chromosomes:

1. Autosomes
   • Twenty-two pairs of autosomes in humans (a total of 44 autosomes).
   • Responsible for all characteristics of the person except determining an individual's sex and sexual characteristics.
   • Present in equal numbers in both sexes.
2. Allosomes
   • Are sex chromosomes that determine an individual's sex and are designated as either X or Y.
   • Women have two X chromosomes (XX) and men have one X and one Y chromosome (XY).
   • Responsible for the development and expression of sexual traits.

Changes in the number or structure of chromosomes can have significant effects on health and development and can cause a range of physical and cognitive abnormalities. These changes are called chromosomal abnormalities and can occur due to the following:

• Errors during cell division
• Exposure to certain environmental factors
• Inherited genetic mutations

One of the most common chromosomal abnormalities is aneuploidy. A condition in which an individual has an abnormal number of chromosomes in one set of chromosomes (haploid).

This can happen if there is a problem during meiosis, the process of cell division that occurs during the production of eggs and sperm. Aneuploidy can cause a range of health problems, depending on the specific chromosomal abnormality.

Other chromosomal abnormalities include structural changes, such as:

• Deletions: A part of a chromosome is missing.
• Duplications: A part of a chromosome is repeated.
• Translocations: A part of one chromosome is moved to another chromosome.

These structural abnormalities can cause health problems depending on the specific location of the change and affected genes.

Trisomy is a condition where an individual has an extra copy of one of the chromosomes in the genome, resulting in a total of 47 chromosomes instead of the typical 46.

Trisomy can cause physical and cognitive abnormalities. The severity of these abnormalities can vary widely depending on the specific chromosomal abnormality. The specific symptoms and complications of trisomy depend on the affected chromosome and the genes in that chromosome.

5 examples of trisomy conditions

1. Trisomy 21 (Down syndrome)
   • Most common chromosomal abnormality in humans and is caused by an extra copy of chromosome 21.
   • The characteristics include:
     • Small head
     • Short stature
     • Intellectual disability
2. Trisomy 18 (Edward’s syndrome)
   • Caused by an extra copy of chromosome 18.
   • The characteristics include:
     • Small head
     • A small jaw
     • Malformations of the heart and other organs
   • Has a high mortality rate, with most affected individuals surviving only a few months after birth.
3. Trisomy 13 (Patau syndrome)
   • Caused by an extra copy of chromosome 13.
   • The characteristics include:
     • Small head
     • Abnormal eyes and brain
     • Malformations of the heart and other organs
   • Has a high mortality rate, with most affected individuals surviving only a few months after birth.
4. Trisomy 9 (Ring chromosome 9 syndrome)
   • Caused by an extra copy of part of chromosome 9, which forms a ring shape.
   • The characteristics include:
     • Abnormalities of the head and face
     • Intellectual disability
5. Trisomy 16 (Mosaic trisomy 16)
   • Caused by an extra copy of chromosome 16 in some but not all cells in the body.
   • The characteristics include:
     • Abnormalities of the head and face
     • Intellectual disability

Having less than 46 chromosomes is a condition called monosomy, and it can cause a range of health problems depending on the specific chromosomal abnormality.

Monosomy occurs when an individual is missing one of the chromosomes in the genome, resulting in a total of 45 chromosomes instead of the typical 46.

Monosomy can cause mild to severe physical and cognitive abnormalities, affecting their health and development.

5 examples of monosomy conditions

1. Monosomy X (Turner’s syndrome)
   • Affects women and is caused by the absence of one of the two X chromosomes.
   • The characteristics include:
     • Short stature
     • Ovarian dysfunction
     • Cognitive abnormalities
     • Intellectual disability
2. Monosomy 1p36 (1p36 deletion syndrome)
   • Caused by the absence of a small part of chromosome 1.
   • The characteristics include:
     • Abnormalities of the head and face
     • Intellectual disability
3. Monosomy 7 (Williams syndrome)
   • Caused by the absence of a part of chromosome 7.
   • The characteristics include:
     • Abnormalities of the head and face
     • Cardiovascular problems
     • Intellectual disability
4. Monosomy 5p (Cri-du-chat syndrome)
   • Caused by the absence of a part of chromosome 5.
   • The characteristics include:
     • Small head
     • Abnormalities of the face and limbs
     • Intellectual disability
5. Monosomy 20p (Wolf-Hirschhorn syndrome)
   • Caused by the absence of a part of chromosome 20.
   • The characteristics include:
     • Small head
     • Abnormalities of the face and limbs
     • Intellectual disability

It is generally not possible to correct chromosomal abnormalities or change the number of chromosomes in an individual's genome.

Chromosomal abnormalities are caused by errors that occur during the formation of eggs and sperm or during the early development of the embryo. These errors cannot be reversed, and the genetic changes are permanent.

Treatment and other strategies can address specific symptoms or complications caused by these abnormalities.

In certain instances, it could be possible to treat symptoms of chromosomal abnormalities or mitigate their effects. However, the effectiveness of treatment options depends on the specific chromosomal abnormality and the severity of the symptoms.

For instance, Down syndrome cannot be cured, but treatment can help improve the quality of life. Physical, occupational, and speech therapy, along with medications, manage specific symptoms of Down syndrome. Education and support for individuals with Down syndrome and their families help significantly.

Other chromosomal abnormalities could be treated with medications or surgery. Some individuals with trisomy 18 or trisomy 13 may need surgery to repair heart defects or other physical abnormalities.

In cases of monosomy, hormone therapy or other medications address specific symptoms or complications. Women with Turner’s syndrome could be treated with growth hormone therapy to help them reach a more normal height. Estrogen replacement therapy can address ovarian dysfunction in women with Turner’s syndrome.