The genetic material is a molecule called deoxyribonucleic acid (DNA). Each chromosome contains a single long strand of DNA that encodes the information needed to produce hundreds or even thousands of different proteins. Each species has a characteristic array of chromosomes that carries all the genes needed to produce that organism from a single cell. The relationship between the genetic makeup of an organism (the genotype) and the developmental effects of these genes (the phenotype) can be complex. It is, therefore, useful to begin with a simple overview of these processes. Here we introduce some of the key concepts of genetics using an illustrated guide that begins at the smallest unit of genetic organization within a nucleus and ends at the level of the population. Some important terms are shown in boldface type, and definitions are given in the Glossary.
· DNA is made up of subunits called nucleotides composed of a sugar (S), a phosphate group (P), and a nitrogenous base (B). There are four nucleotides that differ by the nucleotide base they contain: adenine (A), guanine (G), thymine (T), and cytosine (C). Genetic information is encoded in DNA by the sequence of these four bases.
nucleotide
· Nucleotides are linked by a bond between the sugar of one nucleotide and the phosphate group of the next.
two nucleotides (schematic representation)
· This produces a long chain that can be literally millions of nucleotides long.
a portion of one strand
· A single DNA molecule is composed of two such strands that join together by bonds between the nucleotide bases (A paired with T, and C paired with G). This forms a DNA double helix.
schematic of DNA and double helix
· The DNA is bound to structural proteins (histones that make up the nucleosome) that help pack the DNA in the nucleus and to regulatory proteins that turn genes on and off during development.
nucleosomes with DNA
· Each chromosome is made up of one long DNA molecule (one DNA double helix) and its associated proteins. The centromere is the attachment site for the spindle fiber that moves the chromosome during cell division.
· Along the length of this DNA molecule are the regions that code for the production of proteins. These regions (or genetic loci) are the genes. Each gene can be up to a thousand or more nucleotides long, and every chromosome carries as many as several thousand different genes. All of the genes on a given type of chromosome are thus linked on a single DNA molecule. This linked group of genes is called a linkage group. The linear order of genes on a chromosome can be mapped to produce a linkage map.
chromosome showing a map of five genes in linear order on its linkage group
· The body cells (somatic cells) of most organisms contain two copies of each type of chromosome (diploid). These are the homologous chromosomes. Since homologous chromosomes carry the same series of genes, they are members of the same linkage group. They can, however, differ in the form that a given gene takes (that is, normal or mutant). The different forms of a gene are called alleles.
two homologous chromosomes carrying alleles A and a
· When the nucleus prepares to divide, each DNA molecule replicates except for the centromere. This yields two identical copies of the DNA molecule bound at the centromere. At this stage, the two copies are called sister chromatids. Since they have not yet divided at the centromere, however, each unit is still considered a single chromosome.
two homologous chromosomes, each with two sister chromatids
· Chromosomes that carry different sets of genes are called nonhomologous chromosomes. Every species has its own characteristic number of different chromosomes (n). The total number of chromosomes in a somatic cell is, therefore, 2n. All the genes needed to produce that organism will be found somewhere on one of these n linkage groups. The total 2n genetic makeup is the genome. In the figure of the hypothetical cell, there are three pairs of nonhomologous chromosomes in the genome.
hypothetical cell with 2n = 6, as seen during nuclear division
· Mitosis is a type of nuclear division that yields two identical diploid cells (2n). Meiosis is a special type of nuclear division found in reproductive (germinal) tissue that yields gametes. Each gamete carries only one copy of each linkage group and has a haploid (n) number of chromosomes. The diploid chromosome number is re-created at fertilization when the haploid maternal set and the haploid paternal set fuse.
mitosis and meiosis
· If we focus our attention on one gene, the alleles on the two homologous chromosomes can either be the same (AA or aa = homozygous genotypes) or be different (Aa = heterozygous genotype). These separate (segregate)during meiosis to produce haploid gametes.
branching diagram to show haploid products
· The products of segregation and fertilization are highly predictable, giving rise to the basic rules of genetic transmission. Gregor Mendel set the foundation for this area of genetics.
Mendelian cross using Punnett square for two heterozygous parents
· A pedigree diagram shows genetic relationships from a series of different Mendelian crosses. Circles indicate females and squares indicate males.
a simple pedigree
· Most genes code for the production of proteins. One of the two strands (the template strand) of a DNA molecule is “read” (through transcription) to yield a molecule of messenger RNA (mRNA). This then binds with ribosomes, where it defines the sequence of amino acids needed to produce the correct polypeptide (protein). This is translation.
DNA 
mRNA protein
· Many proteins are enzymes, which catalyze specific biochemical steps. Thus, genes work by controlling the biochemical activities for growth and function of cells. In this way, the genome codes for all of the morphological, physiological, and behavioral characteristics (phenotypes) of an animal or plant.
biochemical pathway
· Some characteristics are the result of several genes and environmental factors working together. Their expression is measured on an appropriate scale (such as height in meters). These are quantitative traits (multifactorial or polygenic traits).
· The genetic makeup of an individual is the genome, whereas the total genetic makeup of all individuals in the population is the gene pool. The gene pool is described in terms of allele frequencies, where p is the frequency of the A allele and q is the frequency of the a allele. By using appropriate assumptions, the genetic makeup of individuals in the population can be predicted.
alleles in a hypothetical gene pool
· Hardy, Weinberg, and Castle established the foundation for population genetics by showing that allele frequencies remain in equilibrium unless acted upon by selection, migration, mutation, sampling error in small populations, or deviations from random mating. Population genetics is the study of changes in allele and genotype frequencies that occur when these factors act on animals or plants.
Punnett square with allele frequencies
Genetics is a dynamic and exciting field (but, of course, you would expect us to say something like that). But it can also be confusing, since there are so many levels at which you can look at inheritance and the use of genetic information. This introduction to genetic organization and scale is intended as a kind of outline to some key levels and processes. Although they overlap, we can readily see three main perspectives. First is the molecular level of DNA structure and coding (nucleotide—DNA—transcription—translation—protein product). Second are the rules of genetic transmission, which are based on probabilities of inheritance (segregation and independent assortment in meiosis—probabilities and genotypes—genes in families—genes in populations). Third is the way the genotype controls biochemical activities during development to produce the organism’s phenotype (proteins—enzyme control of biochemical pathways—gene interactions—development). This primer will investigate these areas of genetics individually. But it is always important to keep in mind that they are really just different ways of looking at the same thing: the coding, transmission, and use of information by cells.