The Role of Genetic Inheritance: DNA, Genes, and Heredity
Genetic inheritance, also known as the inheritance pattern, refers to the process of passing down traits from one generation to the other. We now know that one person’s genome—or the entire database of genes they possess, so to speak—comes from the mother and the father. To be specific, half of the genome is from the mother and the other half is from the father. To be even more accurate, the term genome refers to the collection of all of the genes a person has. Genes are made up of what is often referred to as our genetic blueprint, or DNA. Genes are located on chromosomes. In other words, chromosomes are sequences of a lot of genes that are, in turn, made up of DNA. To tie this back to the beginning, DNA is passed down from our mothers and fathers by means of genetic units, or genes. If this is all just a bit too confusing, do not fear—detailed explanations are following.
Every cell in our body contains chromosomes that hold the recipe for making living organisms. Chromosomes are made up of DNA strands. Most of our cells have 46 chromosomes or 23 chromosome pairs. Out of those 23, 22 are autosomes and one pair is made up of sex chromosomes. Females have two X-chromosomes and males have one X and one Y-chromosome. These chromosomes come in pairs because a single sperm and an egg uniting results in double the number of chromosomes, hence the pairs. Given that the chromosomes hold our most precious material, they are buried deep within the cell, in an organelle that is called the nucleus. There, they are protected by a double membrane. The chromosomal structure is unique, and it is largely responsible for all of the genetic material being able to fit within a small compartment of the cell. Chromosomes are very tightly packed strings of DNA that are supercoiled and wound around proteins called histones. This enables the long DNA structures to fit inside each cell’s nucleus.
Every single living organism has genes that exist in the body. Genes are units that are often regarded as instructions for the organism’s outward characteristics as well as its behavior such as survival and overall behavior in the environment. Genes are made up of DNA that instructs the cell to make larger polypeptides, also known as proteins, the building blocks for many things within the body. Long strands of DNA make several genes. From a different perspective, these long DNA strands are also chromosomes that contain all of the genes that are made up of DNA. Genes are very dynamic—they can be anywhere from a few hundred to more than 2 million DNA bases long.
Furthermore, there are different kinds of one gene, and these kinds of genes are called alleles. For example, the gene that determines the color of your hair has several alleles, each one for different hair color. One allele of each gene is inherited from each of the parents. Some alleles are dominant and others are recessive. These two terms refer to a gene’s ability to be visible on the outside as a trait, also called a phenotype. Dominant alleles are always visible, while recessive alleles are not. While only one dominant allele is enough to manifest itself as a dominant trait, two recessive alleles are needed for a recessive trait to be visible as part of the phenotype.
During the process of reproduction, DNA is passed down from the parents to their offspring. Fundamentally speaking, DNA is a very long chemical molecule—or polymer—that is made up of unit repeats that are held together and as such very much resemble beads on a necklace. DNA is made up of building blocks that are called nucleotides that have three parts: a phosphate group, a sugar group, and a nitrogen group. These nitrogen groups are the basis of DNA, and there are four of them, namely A (adenine), C (cytosine), G (guanine), and T (thymine). Among them, the C pairs with the G and the T pairs with A, forming really strong bonds. These are called covalent bonds, and they are responsible for holding together the two DNA strings together to make one DNA molecule that is also referred to as a double helix due to the helical structure that it makes. Also, rather than just thinking about the DNA helix as a structure, it helps to look at it as a language, made up of hundreds of millions of As, Cs, Gs, and Ts that convey instructions for how the cells should behave to ultimately determine who we are. The parts of the DNA that code for proteins are called genes. The other parts of our DNA serve as on/off switches that direct the cell when to turn a gene on or off. Finally, a very large portion of our DNA does not appear to have a specific function whatsoever.
Some consider the ability of living things to reproduce to be one of the most fundamental properties. “Heredity” is an umbrella term for the process of inheritance, or the passing down of genetic material from one generation to the next. This can include types of disease such as Huntington’s disease, cystic fibrosis, and breast cancer. Along the same lines, every single cell came from a preexisting one, a manifestation of genetic material that is replicated and passed from parent to progeny during meiosis, or cell division. The process of DNA replication and distribution from progeny to offspring is at the heart of all of biology. As such, it is easy to see how the elucidation of the process of DNA replication and transmission revolutionized our understanding of all living things and formed a baseline for the modern understanding of biology at the basic level.