Genetic testing as we know it is a product of the modern age. However, humanity has been practicing genetic analysis in one form or another for thousands of years: breeding livestock, cultivating crops, and noticing that children tend to resemble their parents. Whether it’s colour, height, shape or some other characteristic, the fact that parents pass down certain physical traits to their offspring – is quite apparent. However, the specific mechanics through which that happens haven’t always been well-known.
This began to change in the 19th century, particularly with the famous experiments of Gregor Mendel. Mendel, an Augustinian monk and scientist, carried out a series of experiments with pea plants, cross-breeding them to discover which traits (height of the plants, colour of the flowers, texture of the peas, etc.) would be passed down. His findings on dominant and recessive traits, and his discovery that many of these traits are passed down independently of each other, would become the basis for the modern science of genetics.
Basic genetics
Central to the modern understanding of genetics is our knowledge of deoxyribonucleic acid, better known as DNA. Found in the cells of all living organisms, DNA is a long chain of a molecule, wound in a distinctive double-helix shape. Each “link” of the chain is a smaller molecule, known as a nucleotide. There are four possible nucleotides, each designated with a letter: A, T, C and G. The different possible combinations of these nucleotides mean that, effectively, DNA is a long string of “code”. It is this code that forms the instructions for cells to work: the genes. An organism’s total collection of DNA, known as its genome, effectively contains all of instructions that are needed for it to develop – instructions that are passed down to future generations.
Chromosomes and recombination
An animal’s genome is divided into several separate, smaller units known as chromosomes. If you look at the entire genome as a series of instructions, each chromosome can be seen as an individual “book” or “volume” of those instructions. Each chromosome contains a specific collection of genes, and in many animals, biologists have assigned each chromosome a specific number, making them easier to categorize.
An animal has two copies of each chromosome: one inherited from its father, the other from its mother. During reproduction, each parent randomly contributes one of its two copies to pass down to the newly-formed offspring. This means that, for each chromosome, there are up to four different chromosomal combinations that the offspring could possibly inherit.
Mutations
Simply put, a mutation is an “error” in a creature’s genetic code, caused by a change in the letters of its DNA. Mutations happen all the time in nature, and can be produced by a variety of causes, including ultraviolet light, chemical exposure, and even spontaneous “typos” that occur while the cell copies its DNA. Most of these mutations are immediately repaired by the cell’s internal mechanisms. Sometimes, however, a mutation is preserved, and is passed down to a creature’s offspring.
To many people’s understanding, the word “mutation” carries entirely negative associations, being associated with disease and general “unhealthiness”. However, a mutation does not necessarily have a negative effect. Features such as unusual eye or coat colours, longer hair, and a different body shape or size are also the result of particular mutations. Others even have no effect at all, but can be used as markers for tracing a particular animal’s ancestry.
The location on a chromosome where a mutation related to a given trait may occur is known as an allele. For example, an animal that has a mutated gene on one chromosome, and a healthy version of that gene on the other, has one mutant allele and one normal allele. Both alleles together form the animal’s genotype for that particular trait. The actual, physical trait that results from that genotype (e.g. a disease, a coat colour, etc.) is known as the phenotype.
Modes of Inheritance
“Heterozygous” and “homozygous” are terms that are used a great deal in genetics. These terms are used to refer to the number of copies of a particular allele that an animal has. If the animal has two identical copies of a particular allele, it is homozygous. A homozygous animal will pass that allele down to all of its offspring. If it has two different versions of that allele, it is instead heterozygous.
Depending on the specific mutation in question, a given phenotype might appear only if the animal is homozygous for it, or if it is either homozygous or heterozygous. Some traits are also more complicated, and might require the presence of several different mutations to appear. The exact manner in which a given trait is inherited is known as the mode of inheritance. These modes are discussed here.
