Human Genetics: Concepts and Applications (Lewis), 9th Edition

Chapter 1: Overview of Genetics

Chapter Outline


Chapter 1 is a synopsis of the basic concepts of genetics and introduces many aspects of daily life in which genetic information is important. Foundational concepts and terms are introduced. A new era of genomics has begun. Researchers study the human genome at the gene, genome and expression levels. The traditional focus on single-gene traits has broadened with the understanding that most genes do not act alone, but interact with other genes and the environment in complex ways. In health care, genetic information is being used to design diagnostic and predictive genetic screens based on single nucleotide polymorphism (SNPs). Genetic information is also used for identification and commercial purposes. The technique known as DNA profiling compares patterns in DNA sequences between individuals and is very useful in forensic analysis and understanding of historical events such as human history and ancestry. Genetically modified (GM) transgenic plants and animals contain genes from other species added to their genomes. The technique is being used to rapidly introduce valuable traits into organisms for agricultural purposes. Genomic techniques are being used to enhance our understanding of ecosystems.


1.1 Introducing Genes

  1. Genetics is a branch of biology concerned with heredity and variation. With the completion of the Human Genome Project, it has grown from a largely academic science to a clinical science with practical and societal implications.
  2. Genes are the unit of inheritance and are composed of DNA.
  3. An organism's genome is its complete set of genetic information.
  4. Genomics is a field of study that uses genome information to answer questions in health, research, and the environment.
  5. Bioethics is a field of study concerned with issues of privacy, confidentiality, and discrimination that arise from the availability of new genetic knowledge.
1.2 Levels of Genetics

The Instructions: DNA, Genes, Chromosomes, and Genomes

  1. The effects of genes are noticeable at the molecular, cellular, individual, and population levels.
  2. Genes are DNA sequences that instruct cells to produce particular proteins, which in turn determine traits.
  3. The field of proteomics studies the production of proteins within specific cell types.
  4. Human genes known to cause genetic disorders have been catalogued in the Online Mendelian Inheritance in Man (MIM) database.
  5. Genes can exist in more than one form and the variants, termed alleles, arise by mutation.
  6. A polymorphism is a particular sequence of DNA that varies in one percent or more of the population.
  7. Single nucleotide polymorphisms (SNPs) are single base pair sites that differ among individuals.
  8. Researchers study the human genome at the gene, genome and expression levels.
  9. Chromosomes consist of genes and associated proteins.
  10. The human genome consists of 22 pairs of autosomes and one pair of sex chromosomes.
  11. A karyotype is the photographic catalog of the human chromosomes.
The Body: Cells, Tissues, and Organs

  1. The human body is composed of 50 to 100 trillion cells. Of these, all except mature red blood cells contain the entire genome. Differential expression creates the distinctive cell types which combine to form the organs and organ systems of the body.
  2. Most organs also contain stem cells which can reproduce themselves and differentiate for repair and replacement of specialized cells.
Relationships: From Individuals to Families

  1. The allelic makeup of an individual is termed the genotype, while phenotype refers to the observable or measureable expression of the alleles.
  2. Dominant alleles are expressed when one copy is present. Recessive alleles generally require two copies for expression.
  3. Pedigree diagrams enable recessive and dominant traits to be followed through individuals across multiple generations of a family.
The Bigger Picture: From Populations to Evolution

  1. Population genetics concerns the study of allele frequencies in members of the same species in a specified geographic area. Population genetics has applications in health care, forensics and evolution.
  2. Comparative genomics is leading to a more detailed and subtle understanding of evolutionary relationships among species.
1.3 Genes and Their Environment

  1. The traditional focus of genetics on traits determined by single genes is giving way to the realization that most genes do not function alone.
  2. Most traits are multifactorial traits and are determined by multiple genes and environmental effects.
  3. Symptoms of an illness may have an environmental, single-gene or multifactorial basis.
  4. Genetic determinism is the point of view that the environment has little effect on gene expression.
1.4 Applications of Genetics

Establishing Identity

  1. DNA profiling, previously known as DNA fingerprinting, is becoming increasingly common. It was used for identification following the World Trade Center attack on Sept. 11, 2001, the tsunami of 2004, and hurricane Katrina in 2005.
  2. DNA profiling can exclude an individual from being biologically related to someone else, or from having committed a crime.
  3. DNA profiling can be used to characterize living and dead populations of humans and other organisms revealing history and ancestry.
Health Care

  1. Genetics is changing health care as genetic information is being incorporated into diagnosis and treatment.
  2. The field of pharmacogenomics is concerned with patient response to medication based on genotype.
  3. Genetic diseases differ from infectious diseases in that the recurrence risks are predictable, pre-symptomatic diagnosis using genetic testing is possible, characteristic frequencies are observed in different populations and gene therapy may be used to treat or possibly prevent certain genetic disorders.
  4. Research into gene expression patterns is yielding new insight into diseases. A diseaseosome depicts genetic relationships among diseases.

  1. Agriculture, both traditional and biotechnological, applies genetic principles.
  2. Traditional agriculture involves controlled breeding to select new combinations of inherited traits.
  3. Using organisms for the production of products or services is termed Biotechnology.
  4. Genetically modified (GM) foods have genes introduced from the genomes of other organisms.

  1. Genomic techniques are being used to enhance our understanding of ecosystems.
  2. DNA collected in the field from scarce or hard to cultivate organisms can be studied in the laboratory.
A Global Perspective

  1. Genetics and genomics are rapidly spawning new technologies.
  2. For the near term, genetic testing will not be widely available.
  3. Genome information does have the potential to spur development of many new kinds of drugs.
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