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Genetics

DNA (deoxyribonucleic acid) is a biomolecule that holds the blueprint for how living organisms are built. DNA is made out of two long, twisted strands that contain complementary genetic information (like a picture and its negative). A gene is a segment of DNA that is passed down from parents to children and confers a trait to the offspring. Genes are organized and packaged in units called “chromosomes.” Humans have 23 pairs of chromosomes. One set of chromosomes comes from a person’s mother and the other set of chromosomes comes from the father. ⁽¹¹⁾

A chromosome is an organized package of DNA found in the nucleus of the cell. Different organisms have different numbers of chromosomes. Humans have 23 pairs of chromosomes — 22 pairs of numbered chromosomes, called autosomes, and one pair of sex chromosomes, X and Y. As you probably already know, males have XY sex chromosomes and females have XX sex chromosomes. Each parent contributes one chromosome to each pair so offsprings get half of their chromosomes from their mother and half from their father. ⁽¹²⁾

Four Types of Bases

ACGT is an acronym for the four types of bases found in a DNA molecule:

• Adenine (A)
• Cytosine (C)
• Guanine (G)
• Thymine (T)

A DNA molecule consists of two strands wound around each other, with each strand held together by bonds between the bases. Adenine pairs with thymine, and cytosine pairs with guanine. The sequence of bases in a portion of a DNA molecule, called a gene, carries the instructions needed to assemble a protein. ⁽¹³⁾

Double Helix

Double helix is the description of the structure of a DNA molecule. A DNA molecule consists of two strands that wind around each other like a twisted ladder. Each strand has a backbone made of alternating groups of sugar (deoxyribose) and phosphate groups. Attached to each sugar is one of four bases: adenine (A), cytosine (C), guanine (G), or thymine (T). The two strands are held together by bonds between the bases, adenine forming a base pair with thymine, and cytosine forming a base pair with guanine. ⁽¹⁴⁾

DNA replication is the process by which a molecule of DNA is duplicated. When a cell divides, it must first duplicate its genome so that each daughter cell winds up with a complete set of chromosomes. ⁽¹⁵⁾

Inheritance Patterns

Heredity refers to the genetic transmission of traits from parents to offspring. ⁽¹⁶⁾ A phenotype is an individual’s observable traits, such as height, eye color, and blood type. The genetic contribution to the phenotype is called the genotype. Some traits are largely determined by the genotype, while other traits are largely determined by environmental factors. ⁽¹⁷⁾Heredity helps explain why children tend to resemble their parents, as well as how a genetic disease runs in a family. Some genetic conditions are caused by mutations in a single gene. These conditions are usually inherited in one of several straightforward patterns, including autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, codominant, and mitochondrial inheritance patterns. Complex disorders and multifactorial disorders are caused by a combination of genetic and environmental factors. These disorders may cluster in families, but do not have a clear-cut pattern of inheritance. ⁽¹⁶⁾ We will explain these patterns, but first, let’s explore why it is important to understand inheritance patterns. ⁽¹⁾

Understanding inheritance patterns can allow you to use a family history to determine your risk for various diseases. ⁽¹⁾A family history is a record of medical information about an individual and their biological family. Human genetic data is becoming more prevalent and easy to obtain. Increasingly, these data are being used to identify individuals who are at increased risk for developing genetic disorders, such as heart disease, breast cancer, and prostate cancer. ⁽¹⁷⁾

Mendelian Inheritance

Mendelian inheritance refers to patterns of inheritance that are characteristic of organisms that reproduce sexually. The Austrian monk Gregor Mendel performed thousands of crosses with garden peas at his monastery during the middle of the 19th century. Mendel explained his results by describing two laws of inheritance that introduced the idea of dominant and recessive genes. This is the types of genetics that you learned about in your high school biology class. ⁽¹⁸⁾

Mitochondrial DNA Inheritance

While the majority of your genes reside within the nucleus of nearly every cell of your body (red blood cells are the only exception), a small number of genes can be found outside the nucleus in the mitochondria (Centre for Genetics Education, 2015). If you can recall from your high school or college biology course, the mitochondria are powerhouses for cells. Through the production of ATP (adenosine triphosphate), the mitochondria produce energy for cells so that organs may function properly. While your genes in the nucleus load onto the chromosomes, the genes Mitochondrial DNA is the small circular chromosome found inside mitochondria. The mitochondria are organelles found in cells that are the sites of energy production. The mitochondria, and thus mitochondrial DNA, are passed from mother to offspring. This is referred to as mitochondrial DNA inheritance. ⁽¹⁹⁾

While the typical nucleus has around 20,000 protein encoding genes, the mitochondria have only 37. Here is the amazing part: It pretty much only has the DNA from your mother! Whereas in Mendelian inheritance, half of the DNA comes from your mother (egg) and half from your father (sperm); the father’s mitochondrial DNA is not passed on. Recent studies with worms suggest that there is some sort of self-destruct mechanism in the father’s sperm to destroy the paternal mitochondrial DNA during conception. Research shows that when this mechanism is delayed or inhibited, there is a higher mortality rate. This hints that if the paternal mitochondrial DNA is passed on to the offspring, it is often incompatible with life (Centre for Genetics Education, 2015; Sato & Sato, 2013; Yin, 2016).