Genetics

Genetics

Genetics is the study of the mode and mechanism of the transmission of heritable information. Heredity is the passing of a trait from one generation to the next. The heritable information of an organism is contained in its DNA, and the DNA an organism has is called its genome. DNA passes from cell to cell by cell division and from parent to offspring by reproduction.

The actual unit of inheritance is the gene, a region of DNA that codes for one trait. The sequence of DNA makes up the genotype of an individual. A genotype can be for one single gene, for the entire genome of an individual, or anywhere in between. The physical location of a gene on a chromosome is called a locus. The particular copy of a gene at each locus is called an allele. For example, the gene for eye color occurs at one locus and has different alleles that code for blue or brown or green, etc. Diploid eukaryotes have pairs of chromosomes. Therefore, individuals have two copies of each gene, one copy on each chromosome in the pair. The geno-type of a diploid organism for one single gene is the pair of alleles for that locus. So the genotype for eye color is composed of two alleles, one on each chromosome in the same location. Alleles interact with each other when they are expressed. This interaction is referred to as dominance. Sometimes one allele hides the other allele. Other times the alleles are both expressed equally. There can also be complicated interactions between alleles and the environment in expressing a trait.

How a gene is actually manifested into a physical structure is the phenotype of an individual. The phenotype is the outward appearance of an organism, the reactivity of a digestive enzyme, or even the presence or absence of a disease. The phenotype of an individual is important because it is what natural selection works on. The genotype determines the phenotype of a trait. Since there are two alleles for each locus and alleles can interact, different combinations of alleles produce different traits, in other words, different genotypes produce different phenotypes. The genotype is the underlying genetic basis of a phenotype.

Inheritance Through Reproduction Produces Genetic Variation

The difference among genotypes is referred to as genetic variation. There is genetic variation at one gene when different individuals have different combinations of alleles. Genetic variation also refers to the combination of alleles at different genes. Different phenotypes reflect underlying genetic variation. People with blonde hair and blue eyes have a different genotype and phenotype than people with brown hair and brown eyes. This difference is genetic variation.

In asexual reproduction, the parent and offspring have identical DNA. Mitosis is one form of cell division that produces daughter cells that are identical to the mother cell. Asexual reproduction results in clones, organisms that are identical to each other genetically.

Sexual reproduction produces offspring that are the combination of the genetic makeup of two individuals. In humans, a baby gets half of its genetic material from its mother and half from its father. Gametes, the sperm and egg, contain only half the genome of an individual; only one of the pair of chromosomes are in each gamete. Reducing the genetic material by half is accomplished through meiosis, cell division that produces gametes. Since gametes have only one copy of all chromosomes when they join to form a zygote, the zygote has two copies of each chromosome like its parents.

Organisms that are clones inherit the genotype of their parent since they are genetically identical. In sexually reproducing organisms, the identical genotype of an individual cannot be inherited since each offspring's DNA is made up from one half of the mother's DNA and one half of the father's. In the same way, a phenotype cannot be inherited because it is derived from the genotype. Only genes are inherited. When sexual reproduction occurs, genotypes are split up and new genotypes are formed, making sexual reproduction an important source of genetic variation for evolution.

In his pea experiments, Gregor Mendel observed that each gamete an individual makes is unique. The two processes that make gametes unique are the law of segregation and the independent assortment of homologous chromosomes. In normal meiosis, each gamete ends up with one copy of each chromosome. The law of segregation describes the process of the separation of the two alleles at the same locus on a pair of chromosomes into separate gametes. Independent assortment is when the two chromosomes in a pair are randomly distributed during meiosis into the four gametes. Each time four haploid gametes are produced from one parent cell, each gamete has a different combination of one set of chromosomes. Independent assortment is another important source of genetic variation.

Recombination redistributes combinations of alleles of different genes. During meiosis, crossing over happens among the tetrad of chromatids during prophase I. Bits and pieces of homologous chromatids are swapped among chromatids at the chiasmata during crossing over. This means that different alleles for the same gene are being swapped. The result is that for different genes, different alleles are now being combined. For example, suppose the gene for pea-coat texture is on the same chromosome as the gene for pea-coat color. On one chromosome, the allele for round peas is present with the allele for yellow peas. On the homologous chromosome, the combination is the allele for wrinkled peas and for green peas. Recombination through crossing-over events can produce a gamete that has one chromosome with the allele for round peas with the allele for green peas. It could also produce a gamete that has a chromosome with an allele for wrinkled peas with an allele for yellow peas. Of course it is possible to get the parental combinations in gametes as well. Recombination is another very important source of genetic variation.

Genetic mutation is the ultimate source of genetic variation. When DNA is replicated during cell division, mistakes are made at very low levels in copying and dividing chromosomes. These mistakes can lead to changes in the DNA called genetic mutations. Mutations can be in the sequence of the DNA during replication. They can also occur when pieces of different chromosomes get mixed up during cell division, or when whole chromosomes are not divided equally among daughter cells during cell division. Mutations often have negative effects. A mutation in the DNA can produce a pheno-type that is not normal. When natural selection acts against these abnormalities, the mutations are called deleterious mutations. Only very rarely does mutation produce a variant of a phenotype that is better than normal. If natural selection favors this phenotype, the mutation is a beneficial mutation and the trait that results from natural selection is an adaptation. Adaptations can spread throughout a population over a few generations.

Genetic Variation and Biological Evolution

Genes are the raw material for biological evolution. Genes are the only things that are inherited in sexually reproducing organisms. Combinations of different alleles for the same gene and different combinations of alleles at different genes make up genetic variation. Genetic variation comes from genetic mutation and from processes related to sexual reproduction, including recombination and independent assortment. Without genetic variation, biological evolution can not take place.

Evolution is a change in the frequency of a gene in a population over time. Natural selection, the most important of the five forces that cause biological evolution, selects on phenotypes of individuals. However the genes, not the genotype or the phenotype, are passed on to the next generation. The other forces that cause evolution do effect the genes. Nonrandom mating pairs up different combinations of genes in new individuals, or keeps existing combinations of genes together. Gene flow from other populations can introduce new genetic variation into a population. Mutation can change the genes directly during cell division and create new genes, both deleterious and beneficial, during reproduction. Random genetic drift can also change the gene frequency in a population. Random genetic drift is a sub-sampling of a population, for example, if there is a big die off from disease. When only a few individuals are left, only a few alleles are present for each gene, and the combinations that exist are just a few of the possible combinations. Random genetic drift can dramatically change the genetic variation and the gene frequencies of a population, causing much evolution.

Genetics and biological evolution are typically even more complicated. Most traits, such as how tall humans are, do not have categorical differences but vary continuously. For example, humans are not 1.5 to 1.8 meters (5-6 feet) tall, but instead are 5 feet 1 inch or 5 feet 2 inches, and height can be measured in even smaller increments. Quantitative traits such as height typically result from many genes; they are polygenic traits. Alleles at several loci interact to produce the overall height of an individual. The environment can interact with the genotype and affect the phenotype of an individual. For example, if a child does not have the proper nutrition growing up, he or she will be shorter as an adult than if well nourished.

Molecular genetics has changed how genetics is performed and what we can understand about the origin and diversity of animals. In April, 2000, Celera Genomics announced it had sequenced the entire genome of one human being. Being able to know the entire genetic sequence of not only one organism, but of several different organisms, will revolutionize genetics. Being able to compare whole genomic sequences of different organisms will provide a new understanding of how evolution created and maintains the diversity of organisms on Earth.

see also Biological Evolution; Genes; Geneticist; Mendel, Gregor; Morphology.

Laura A. Higgins

Bibliography

Campbell, Neil A., Jane B. Reece, and Lawrence G. Mitchell. Biology, 5th ed. Menlo Park, CA: Addison Wesley Longman, Inc., 1999.

Griffiths, Anthony J. F., Jeffrey H. Miller, David T. Suzuki, Richard C. Lewontin, and William M. Gelbart. An Introduction to Genetic Analysis, 6th ed. New York: W. H. Freeman and Company, 1996.

Lewis, Ricki. Human Genetics, 2nd ed. Chicago: Wm. C. Brown Publishers, 1997.