Every living organism carries genetic information in the genome, a biological blueprint of sorts. The human genome is a complete set of DNA sequences encoded within 23 chromosome pairs. There are 46 chromosomes per genome, in total.
The difference between the human genome and that of our closest relative, the chimpanzee, is somewhere around 4 percent. The highest difference in genomes between two human beings is around 0.1 percent. This may seem like a tiny variation on paper, but in physical terms, these differences can be drastic. The genome influences many of our biological factors, such as height, appearance, and predispositions to disease.
The Human Genome Project and its effect on the world of science
The Human Genome Project published the first complete sequence of individual human genomes on February 12, 2001. Thousands of human genomes were completely sequenced over the next decade. The resulting data has revolutionized science, allowing researchers to truly understand the blueprint for building a human being. This knowledge has had a major impact on medicine, biotechnology, and bioinformatics.
Diploid and haploid genomes
Of our 23 pairs of chromosomes per genome, 22 of those pairs are found in both genders, with one pair (sex chromosomes) present as either XX (female) or XY (male).
Most of the body’s cells, except for male sperm and female ova, contain a full, identical set of 46 chromosomes in their nucleus, consisting of approximately 6 billion base pairs of DNA per cell. These are known as diploid genomes.
Haploid genomes are found in reproductive cells (i.e., sperm and ova). Haploid human genomes contain 3 billion base pairs and are composed of 23 chromosomes. Each haploid genome is a half-set of the genetic information required to make a human. It is the genetic contribution passed onto a child by one parent. At conception, this half-set of 23 chromosomes from one parent is combined with 23 chromosomes from the other to form a complete set of 46 chromosomes—6 billion base pairs of DNA—in the developing fetus.
Chromosomal abnormalities can be both structural and numerical. Anything less than a set of 46 chromosomes represents a deviation from the normal amount of genetic material present and can result in health and developmental problems. Where a structural change is present, the significance of the change and the severity of the condition depends on which chromosome has been altered. The type of problem and its effects can vary greatly from one person to another, even when the same genetic abnormality is present.
Aneuploidy occurs when the genome contains less or more than the normal 46 chromosomes. A change in the number of cells can occur during the formation of reproductive cells, in early fetal development, or, sometimes, after birth. It can affect growth, development, and body function.
Trisomy, a common form of aneuploidy, is the presence of an extra chromosome in cells. People with this condition carry three copies of a particular chromosome in their genomes, instead of the usual two. Down syndrome typically presents three copies of chromosome 21 in each genome, resulting in a total of 47 chromosomes instead of the usual 46.
Monosomy is a form of aneuploidy where a person has one copy of a chromosome instead of the usual two. Turner syndrome presents itself in women with just one copy of the X chromosome in each cell, resulting in 45 cells instead of 46.
Other types of chromosome disorders include:
- Edwards syndrome: an extra chromosome 18
- Klinefelter syndrome: an extra X chromosome
- Patau syndrome: an extra chromosome 13
Detection of genetic disorders and genome sequencing
Different forms of the same gene are known as alleles. One allele for every gene is inherited from your father and one allele for each gene is inherited from your mother.
Changes in the nucleotide sequence from one allele to another are caused by a mutation. This faulty gene can be transmitted to the next generation, making genetic disorders hereditary.
Chromosome analysis, sometimes known as cytogenetic testing or karyotyping, detects chromosome abnormalities to help diagnose genetic diseases. Doctors analyze a sample of blood or tissue, or, for an unborn child, a sample of amniotic fluid or chorionic villus.
Genome sequencing is considered by many to be medicine’s single biggest breakthrough, as it enables health practitioners to quickly and more effectively diagnose and treat critically ill newborns with rare genetic disorders, improving medical outcomes and ultimately saving lives.