The term gene pool refers to the total sum of genetic information present in a population at any given time. A gene pool can be assigned to any set group or population. This is true for plants, animals, and humans alike. Each gene pool contains all of the inherited information for all of the traits of the members of the population.
Genetic information, in the form of deoxyribonucleic acid (DNA ), is passed down from generation to generation. DNA tells a person's body how to work and how to grow. It provides instructions that assign features to each individual, such as giving one person brown hair and another person blonde hair, and one person brown eyes and another person green eyes.
DNA is much like a linear string, with individual segments along the string known as genes. Genes provide the specific directions for the body. Each gene is a segment of DNA, and sequencing of the four base molecules of DNA create the gene. Variations in the sequence account for variations in genes. A gene is the equivalent of an allele, and each particular gene is found on the same chromosome in each individual. The long, linear strings of DNA are arranged into smaller packages known as chromosomes . In general, there are 46 chromosomes in each cell of a person's body. The 46 chromosomes can be matched into 23 pairs. One of each pair is inherited from the mother's egg and one of each pair is inherited from the father's sperm. Most animals, including humans, contain two copies of each chromosome and likewise two copies of each gene. Each individual receives one allele from each parent because they receive one of each of the 23 chromosomes from each parent.
Although each person has 46 chromosomes, the DNA that makes up those chromosomes is slightly different from individual to individual. It is this variation within specific genes that gives the diversity observed throughout populations around the world.
Different versions of the same gene are referred to as alleles. Blood types are examples of alleles. In humans there are several different blood types, including A, B, O, and AB. These arise by various combinations of the three blood-type alleles; the A-allele, the B-allele, and the O-allele. The specific blood type a person has depends on the exact blood type alleles they inherited from their parents. For example, a person may inherit two O-alleles, in which case they would have type O blood, or they may inherit an A- and a B-allele, in which case they would have type AB blood, and so on.
Population genetics is the study of genetic variation within a population. This includes the subtle changes in DNA sequences and the frequencies of these different forms. Changes within the DNA sequences may arise through several pathways. Mechanisms commonly studied by population geneticists include mutation, natural selection, and genetic drift.
Mutations are changes within the DNA sequence that alter the original directions encoded within DNA. Mutation may result from damage to DNA, or a mistake in the replication of DNA resulting in a sequence change. The majority of mutations arise by chance, although some may be caused by environmental factors, such as toxins that penetrate the cells of the body and attack the DNA. Natural selection is the difference in mortality (death rates) and fertility (birth rates) between different genetic types. The interplay of the expressed phenotype and the environment influences natural selection. If the phenotype is favorable, the individual survives and perpetuates his or her genetic profile in the gene pool. Genetic drift is a process by which the frequencies of specific alleles change, by chance, within a population.
Each gene pool accounts for all of the alleles for all of the traits of the members of a population. Within a population, different alleles will occur at different frequencies. For instance, approximately 44% of the population has type O blood, 42% of the population has type A blood, 10% of the population has type B blood, and 4% of the population has type AB blood. The percentages of each blood type are directly related to the frequency of each blood type allele. The more frequent the A-allele, the more frequent type A blood would be seen in the population.
The gene frequency of an allele is equal to the number of times the allele occurs compared to the total number of alleles for that trait.
Gene frequency equals the number of a specific type of allele, or the total number of alleles in the gene pool
DNA changes and genetic disorders
Genetic disorders are caused by changes in the DNA sequence. In general, there is a non-disease causing allele and a disease-causing allele. Some genetic disorders arise by sporadic mutations in the DNA sequence. Others are inherited from one or both of the parents.
There are several different inheritance patterns associated with genetic disorders. Autosomal dominant and autosomal recessive are two of the most common. Chromosomes come in pairs, one from the egg and one from the sperm. Autosomal dominant disorders require that a person inherit only one disease-causing allele in order to be affected. Even though the corresponding gene on the other chromosome in the pair may be the non-disease-causing allele, having one disease-causing allele is enough to cause the disorder to be present. Autosomal recessive disorders require that a person inherit two disease-causing alleles, one on each chromosome of the pair, for the individual to be affected. If a person inherits only one disease-causing allele of a recessive disorder they are called a carrier. Carriers are not affected by disease; however, they carry the possibility of passing that disease on to a future child.
The frequency of disease-causing and non-diseasecausing alleles along with the frequency of affected individuals, carriers, and unaffected individuals are related within a mathematical equation known as the Hardy-Weinberg equation.
The equation itself is written as p2=2pq=q2=1. For autosomal recessive disorders, p2 represents the people within the population that have two non-disease-causing alleles (unaffected), 2pq represents the people within the population with one disease-causing allele and one nondisease-causing allele (carriers), and q2 represents the people within the population that have two disease-causing alleles (affected). Because the Hardy-Weinberg equation deals with allele frequencies, the equation p + q = 1 may also be used. In this case, p represents the frequency of the non-disease-causing allele within the population and q represents the frequency of the disease-causing allele within the population.
The Hardy-Weinberg equation is based on the work of Drs. Hardy and Weinberg. Independently, they suggested that there should exist an equilibrium, or balance, between different allele frequencies. They devised a list of conditions that must be true for this balance, known as the Hardy-Weinberg equilibrium, to occur. These include:
- no evolutionary forces acting upon the population
- the population is "infinitely" large (meaning it is so large that it may be assumed to be infinitely large)
- individuals have two copies of each gene
- there is random mating between individuals within the group
- the frequencies of the alleles are the same in both males and females
- generations are non-overlapping
The Hardy-Weinberg equation has several applications including use by population geneticists to study the characteristics of certain populations and use by genetic counselors to calculate recurrence risks for individual families affected by genetic disease.
There are several projects underway at this time in an effort to further understand the gene pool, population genetics, and the human genome. The Human Genome Diversity Project (HGDP) is an international project that seeks to understand the diversity and unity of the entire human species.
The Human Genome Project , a separate venture from HGDP, made the news in 2000 when scientists announced they had elucidated a working draft of the human genome sequence.
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Java O. Solis, MS