Human Genetics
Article by: Margaret W. Thompson, Departments of Medical Genetics and Pediatrics, University of Toronto and the Hospital for
Sick Children, Toronto, Ontario, Canada. Carter L. Denniston, Department of Medical Genetics, University of Wisconsin, Madison,
Wisconsin. Friedrich Vogel, Institute for Anthropology and Human Genetics, Im Neuenheimer Feld, Germany.
William L. Nyhan, Department of Pediatrics, University of California, San Diego, California.
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A scientific discipline concerned with genetically deter-
mined resemblances and differences among human beings.
Human genetics analyzes all aspects of inheritance occurring in
humans. Although the mechanism by which heredity operates
remained mostly unknown until the twentieth century, the
idea that certain physical and mental characteristics, normal
or abnormal, can "run in families" goes back to ancient times.
Formerly, genetics was thought to be concerned only with the
familial transmission of rare and insignificant characteristics,
but its fundamental biological role is now apparent. Genes,
the units of heredity, have two unique properties: they are
self-replicating, and they carry (in their biochemical structure)
the codes for protein synthesis (Fig. 1). Consequently, genes
play the double role of transmitting genetic information from
generation to generation and of governing all the activities of
living cells.
Expansion of the knowledge of human genetics has come from
various directions. For example, technological advances in the
visualization of human chromosomes have shown that abnor-
malities of chromosome number or structure are surprisingly
common and of many different kinds, and that they account for
birth defects or mental impairment in many individuals, as well
as for numerous early spontaneous abortions. Progress in mo-
lecular biology has clarified the molecular structure of chromo-
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Content
• Chromosome and gene
structure
• Mutation
• Single-gene inheritance
• Quantitative inheritance
• Hereditary diseases
• Chromosomal diseases
• Diseases with mendelian
inheritance
• Multifactorial diseases
• Biochemical genetics
• Inheritance mechanism
• Inborn errors of amino acid
metabolism
• Mitochondrial disease
Key Concepts
• Human cells normally contain 23 pairs of chromosomes (46 total), and are considered
diploid cells.
• Human reproductive cells (eggs and sperm) normally contain 23 total chromo-
somes—22 autosomes and one sex chromosome—and are considered haploid.
• A very large amount of genetic variation exists in the human population.
• Hereditary diseases in humans include chromosomal diseases such as Down
syndrome, diseases with mendelian modes of inheritance such as hemophilia,
and multifactorial diseases such as diabetes mellitus.
• Not all familial traits are hereditary because relatives tend to share common
environments as well as common genes.
• Biochemical genetics involves the study of the molecular mechanisms of inheritance,
inborn errors of amino acid metabolism.
