Basics of Human Biology & Genetics

By Dr. Tara Sander Lee, Ph.D. Biochemistry; Charlotte Lozier Institute

If you’ve received a diagnosis of a genetic disorder for your child, you might find the medical terminology confusing. This is a brief explanation of human genetics to help you better understand the terminology and how genetics plays a role in your child’s condition. For more information on human development from fertilization to birth, please visit the Voyage of Life.

At the moment of conception on day 1, when a woman’s egg is fertilized by a man’s sperm to create a single-cell embryo, key steps are initiated that begin development of a new human life. This new human being contains the most basic, fundamental molecules it needs to grow, mature, and function, from the very beginning, including a very important molecule called deoxyribonucleic acid (or DNA).1   Every human being consists of DNA, which is a unique genetic code in all living things and is passed on from generation to generation, fashioning distinctive characteristics and distinct traits to each living thing. DNA is packaged in the form of chromosomes.

Human gametes (the sperm and egg) each contain half the number of chromosomes.  The union of the male and female DNA during fertilization (sperm-egg fusion) restores the complete number of chromosomes needed to create a new human being with a complete set of DNA for that individual’s entire life. This genetic information is tightly packaged and arranged on chromosomes (numbered 1-22, plus 2 sex chromosomes X and/or Y) and stored in the nucleus of each human cell.  

The complete human genome is present at conception in the earliest single-cell embryo, called a zygote. As the cells divide, the DNA is copied into every new cell. For more information, see DNA and Genetic Variation: Dive Deeper at The Voyage of Life. DNA consists of four different nucleotides (A, G, C, T) that join together on a single strand and make a sequence. DNA is normally found as a double-stranded molecule, in which two separate DNA strands are wound around each other to form a double helix and each nucleotide of one strand forms a base pair with the nucleotide of the opposite strand.  

In the same way that 26 letters of the alphabet combine to create a countless number of words and sentences for communicating, so it is with DNA. The four nucleotides combine in different ways to create various sequences, called genes, which are fundamental units of genetic information that provide specific instructions for a particular property or function within the cell. There are approximately three billion base pairs of DNA sequence in each human diploid cell.2 No two humans ever have been or will be genetically the same, even in the case of twins.  

Sometimes children have a missing or extra chromosome, known as aneuploidy. Trisomy disorders, such as trisomy 13, 18, and 21 (Down syndrome) are examples of a genetic disorder caused by the presence of an extra copy of chromosome (See Down Syndrome: Dive Deeper at The Voyage of Life.). It is also possible for children to have a small part of a chromosome that is missing some DNA or has extra copies of the DNA code. The genetic disorder 22q11.2 deletion syndrome is an example of a missing copy of DNA. 

Chromosomal disorders follow a naming convention. For instance, the disorder 22q11.2del syndrome indicates:

22 =  chromosome affected

q = long arm of the chromosome (p is the short arm)

11.2 = affected DNA segment (band) on the chromosome

del =  DNA code is missing (deleted)

There are many different types of DNA variants such as deletions, duplications, insertion, etc. For more detailed information, please see this link: What Kinds of Gene Variants are Possible? Some children also can have a very small change (or mutation) in their DNA that only affects one nucleotide but can have detrimental effects if the DNA change is located in a critical region of a gene necessary for cell function.  

Cystic fibrosis is an example of a single mutation gene (also called monogenetic) disorder. These mutations may be inherited from one or both parents. A dominant genetic disorder needs only one mutated gene from either parent to have an affected child, while a recessive disorder such as Cystic Fibrosis requires two gene mutations (one from each parent). 

Genetic disorders are not caused by any action or lack of action by parents before conception of their child. Some genetic disorders are inherited from the parents, but many are not, and occur sporadically. For some chromosomal disorders, such as Down Syndrome, the risk is higher in women of advanced maternal age (over 35 years of age).

Most genetic anomalies occur at conception when the man’s sperm fuses with a woman’s egg to form a single-cell embryo. As a result, the genetic disorder is typically present in every cell of a person’s body. Sometimes, a condition called mosaicism occurs, where the disorder is not present in every cell. 

Some genetic disorders can be diagnosed, and sometimes even treated, before birth. To learn more, visit the sections on Understanding test results and Fetal Treatment Options.

Sources:

  1. Maureen L. Condic, “When Does Human Life Begin? The Scientific Evidence and Terminology Revisited,” University of St. Thomas Journal of Law and Public Policy 8.1 (2013): 44-81; and idem, “A Scientific View of When Life Begins” (June 11, 2014), Charlotte Lozier Institute, https://lozierinstitute.org/a-scientific-view-of-when-life-begins/ (accessed November 22, 2020).
  2. International Human Genome Sequencing Consortium, “Finishing the Euchromatic Sequence of the Human Genome,” Nature 431 (2004): 931-45, https://doi.org/10.1038/nature03001