that an organism receives one of these from each parent. Further, one of these can be dominant or recessive, which determines which characteristics are expressed. Mendel also realized that the inheritance of the gene of one trait is not affected by the inheritance of the gene for another trait. In the previous example illustrating the inheritance of color and height, those factors influencing color do not affect height, and vice versa. That is, the probability for each occurs separately. This fact is known as Mendel’s second law or the law of independent assortment.
Since Mendel’s time, we have learned a great deal concerning the process of inheritance. What he referred to as elements or units of information, we now call genes (see Figure 2.25). We also know that genes can have alternative forms, which we call alleles. Independent researchers, Walter Sutton and Theodore Boveri, in 1903 put forth a theory we now accept as fact, that genes are carried on chromosomes. We now know that each of the approximately 20,000 human genes occurs at a specific site, called a locus, on one of our 23 different pairs of chromosomes. As genetics progressed in the twentieth century, it became clear that it was necessary to go beyond the two laws suggested by Mendel to a more complex understanding of how traits are passed from generation to generation. For example, if two genes are located close to one another on the same chromosome, then the result is different from that predicted by Mendel’s second law.
Figure 2.25 Genetic Components Found in a Drop of Blood
Source: National Human Genome Research Institute.
What Do Genes Do?
Genes form the blueprint to describe what an organism is to become. Over our evolutionary history, a majority of human genes reflect little variation. This is why all humans have two eyes and one nose and one mouth. However, perhaps one fourth of all genes allow for variation. What makes things interesting is that the two genes of these pairs are usually slightly different. The technical name for the unique molecular form of the same gene is an allele. It has been estimated that of our approximately 20,000 genes, some 6,000 exist in different versions or alleles (Zimmer, 2001).
genes: the basic physical and functional unit of heredity, made up of DNA; act as instructions to make proteins
allele: the alternative molecular form of the same gene
homozygotes or homozygous: when a person has two copies of the same allele
heterozygotes or heterozygous: when a person has two different alleles at the same location
encode: to lay out the process by which a particular protein is made; this is the job of a gene
proteins: made up of amino chains from DNA, proteins do the work of the body and are involved in a variety of processes; functionally, proteins in the form of enzymes are able to make metabolic events speed up, whereas structural proteins are involved in building body parts
When a person has two copies of the same allele, they are said to be homozygotes or homozygous for that allele. If, on the other hand, they have two different alleles for a particular gene, they are said to be heterozygotes or heterozygous for those alleles. Given that the alleles that come from your mother may not result in exactly the same characteristics as those from your father, variation is possible. It is these variations that allow for the process of natural selection to have its effect.
The job of a gene is to lay out the process by which a particular protein is made. That is, each gene is able to encode a protein, influencing its production. Proteins, which do the work of the body, are involved in a variety of processes. Functionally, proteins in the form of enzymes are able to make metabolic events speed up, whereas structural proteins are involved in building body parts. Similar proteins in insects are involved in creating such structures as spider webs and butterfly wings. Proteins are diverse and complex and are found in the foods we eat as well as made by our cells from some 20 amino acids. Proteins serve as signals for changes in cell activity as illustrated by hormones. Proteins are also involved in health and disease as well as in development and aging.
Although the cells in the body carry the full set of genetic information, only a limited amount is expressed at any one time related to the function of the cell. That is to say, although a large variety of proteins could be produced at any one time, there is selectivity as to what is produced relative to internal and external conditions. Further, the location of the genes makes a difference in that cells in the brain produce different proteins from those in the muscles, or liver, or heart.
A gene is turned on (produces the protein) or turned off (does not produce the protein) relative to specific events. Just because a person has a specific gene does not mean that it will necessarily be expressed. The environment in which a person develops and lives plays an important role in gene expression. Even identical twins with the same genotype can display different phenotypes if their environmental conditions differ during their development. For example, if one was to grow up in a high mountain range and the other in a desert below sea level, important physiological differences such as lung capacity and function would be apparent. There are few factors other than blood type in terms of human processes that can be explained totally by genetic factors alone. It is equally true that few human processes can be explained totally by the environment.
DNA
With the discovery of the structure of DNA by Watson and Crick in 1953, specifying the method by which genetic material was copied became possible. Deoxyribonucleic acid (DNA) provides information necessary to produce proteins. Proteins can be viewed as a link between the genotype (complete genetic composition of an organism) and the phenotype (an organism’s observable characteristics). Moving the genotype to the phenotype initially begins in two steps. First, the information in DNA is encoded in ribonucleic acid (RNA). Second, this information in RNA determines the sequence of amino acids, which are the building blocks of proteins. Technically, the DNA synthesis of RNA is called transcription, whereas the step from RNA to protein is called translation. RNA is like DNA except its structure is a single strand, whereas DNA has a double strand. Once encoded, the RNA goes to a part of the cell capable of producing proteins. Proteins are produced by putting together amino acids.
deoxyribonucleic acid (DNA): a molecule that provides information necessary to produce proteins, which are involved in growth and functioning
genotype: the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment
phenotype: an organism’s observable characteristics
ribonucleic acid (RNA): DNA information is carried as RNA, which determines the sequence of amino acids, the building blocks of proteins; it is made up of single strands rather than the dual strands in DNA
To be more specific, DNA represents the chemical building blocks, or nucleotides, that store information. There are only four types of bases for this coding. DNA molecules are composed of two strands that twist together in a spiral manner. The strands consist of a sugar phosphate backbone to which the bases are attached. Each strand consists of four types of nucleotides that are the same except for one component, a nitrogen-containing base. The four bases are adenine, guanine, thymine, and cytosine. These are generally referred to as A, G, T, and C. To give you some sense of size, each full twist of the DNA
