allele: Although we all carry the same set of genes, they are not identical in all of us. An allele is a variant of a gene (or position in the genome) akin to an alternative spelling, or a typo. The US version of this book will contain the word behavior, whereas in the UK it is behaviour, which doesn’t change its meaning, whereas the single letter change in affect and effect alters meaning from “to change” to “result.” Both are examples of alleles.
amino acid: Small molecules that link together to form proteins. Each amino acid is encoded in DNA by a particular arrangement of three bases.
bases: Also called nucleotides, or referred to as letters; the individual components of DNA, the arrangement of which determines the genetic code. There are four bases, abbreviated to A, T, C, and G. The double helix in a sense is like a twisted ladder, the rungs of which are made of pairs of bases; A only ever pairs with T, C only ever with G. (In RNA, the T is replaced with a U.)
BCE: Before the Common Era. Synonymous with the term Before Christ, but science has replaced it, appropriately, with something universal.
CE: The Common Era, synonymous with the old-fashioned Anno Domini; e.g., the year this book was first published was 2016 CE.
chromosome: Long stretches of DNA that harbor genes. Species have a specific number of chromosomes. In humans it’s twenty-two pairs of autosomes, plus two sex chromosomes: two Xs if you are a woman, and an X and a Y if you are not.
codon: Three bases in a particular order encode a specific amino acid, which link together to form proteins. There are four bases, therefore sixty-four possible combinations if arranged into threes. But biology only uses twenty amino acids (plus a signal to end a protein, STOP), which means that the genetic code has redundancy in it: Several different codons can encode the same amino acid. A stretch of codons in DNA marks out a gene.
DNA: Deoxyribonucleic acid: the material of genetics, typically arranged into chromosomes. DNA is a script in which genes are written.
gene: In its simplest form, a gene is a sequence of DNA that encodes a working protein. This definition adheres to what is known as the “central dogma”: DNA makes RNA makes protein. Genes are composed of DNA, and these lie as part of chromosomes, like sentences in text. There is no more precise definition, but there are active genetic elements that do not follow this model. In the last few years, many bits of DNA that encode RNA that never becomes a protein have been identified. These may yet be classified as genes.
genetics: The study of genes, DNA, disease, inheritance, evolution, and a whole lot more.
genome: The entire genetic material of an organism, the sum total of its DNA. The Human Genome Project was a huge publicly funded collaborative scientific endeavor that concluded in the first few years of the twenty-first century. Its aim—which was delivered under budget and on time—was to provide a complete readout of a human genome as a database that we could go on to mine for information about how humans work, how we evolved, and what happens when we go wrong. The principal output was a reference genome, an average human from which we can compare our infinite variation.
genomics: The study of genomes, including genetics but much more as well. In studying genomes we look at more than simply the genes that have specific functions in an organism, and analyze the regulation of those genes, and the information stored in DNA.
genotype: Genotype refers to the version of the genes that you possess.
heterozygous/homozygous: These refer to the pair of each gene you have, one inherited from each parent. Heterozygous means you have two different versions of the same gene; homozygous means they are the same.
Mendelian/Mendel’s Laws: The basic rules of biological inheritance were set by Gregor Mendel in the nineteenth century, as a result of his experiments in which he bred thousands of pea plants and observed how various characteristics were passed from generation to generation. There are three broad rules, which can be summarized thus:
1. Organisms have two copies of each allele, one inherited from each parent.
2. Each trait is inherited independently of any other.
3. Some alleles are dominant over others, which are called recessive. If you inherit one dominant from one parent, and one recessive from the other, only the dominant will be expressed. To see a recessive trait, you must have inherited two recessive alleles, one from each parent—for example, with the alleles for red hair.
As is so often the case, these laws are more like rules, in that there are some notable exceptions. Biology is annoying like that.
mitochondria: Tiny compartments within cells that are primarily responsible for generating energy for that cell. They contain their own chromosome, a small loop of DNA separate from the vast majority contained within the nucleus of a cell. Mitochondrial DNA (which is sometimes written as mtDNA) is of great interest to geneticists and genealogists as it is only ever passed on from mother to child, and thus can exclusively chart a matrilineage.
phenotype: The phenotype is the physical manifestation of a gene or genotype. For example, a particular version of the gene MC1R (the genotype) will mean your phenotype will be a redhead.
protein: The primary functional biological molecules in living things. They are made up of simple molecules called amino acids assembled into long strings. These fold up to make a three-dimensional structure, and often assemble with other proteins in cells to enact their function. All life is made of, or by, proteins.
single nucleotide polymorphism: Many of the genetic differences between people are made up of individual letters of genetic code being different at specific points in the genome (see allele, above). When these are simply alterations in a single letter at a specific point, we call them single nucleotide polymorphisms, which is universally abbreviated to SNP (and pronounced “snip”). The words inquiry and enquiry would be an alphabetical equivalent of a SNP.