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Genetics and Inheritance

18 GENETICS AND INHERITANCE

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Introduction

G enetics is the science of the way traits are passed from parent to offspring. For all forms of life,

continuity of the species depends upon the genetic code being passed from parent to offspring.
Evolution by natural selection is dependent on traits being heritable. Genetics is very important in
human physiology because all attributes of the human body are affected by a person’s genetic code. It
can be as simple as eye color, height, or hair color. Or it can be as complex as how well your liver
processes toxins, whether you will be prone to heart disease or breast cancer, and whether you will you
be color blind. Defects in the genetic code can be tragic. For example: Down Syndrome, Turner
Syndrome, and Klinefelter's Syndrome are diseases caused by chromosomal abnormalities. Cystic
fibrosis is caused by a single change in the genetic sequence. Genetic inheritance begins at the time of
conception. You inherited 23 chromosomes from your mother and 23 from your father. Together they
form 22 pairs of autosomal chromosomes and a pair of sex chromosomes (either XX if you are female,
or XY if you are male). Homologous chromosomes have the same genes in the same positions, but may
have different alleles (varieties) of those genes. There can be many alleles of a gene within a
population, but an individual within that population only has two copies, and can be homozygous (both
copies the same) or heterozygous (the two copies are different) for any given gene.

Genetics is important to medicine. As more is understood about how genetics affects certain
defects and diseases, cures and treatments can be more readily developed for these disorders. The
sequence of the human genome (approximately 30 billion base pairs but fewer than 30,000 genes) was
completed in 2003, but we are far from understanding the functions and regulations of all the genes. In
some ways medicine is moving from diagnosis based on symptoms towards diagnosis based on
genetics, and we are moving into what many are calling the age of personalized medicine.

DNA

Deoxyribonucleic acid (DNA) is the macromolecule that stores the information necessary to build
structual and functional cellular components. It also provides the basis for inheritance when DNA is
passed from parent to offspring. The union of these concepts about DNA allows us to devise a working
definition of a gene. A gene is a segment of DNA that codes for the synthesis of a protein and acts as a
unit of inheritance that can be transmitted from generation to generation. The external appearance
(phenotype) of an organism is determined to a large extent by the genes it inherits (genotype). Thus,
one can begin to see how variation at the DNA level can cause variation at the level of the entire
organism. These concepts form the basis of genetics and evolutionary theory.

Gene

A gene is made up of short sections of DNA which are contained on a chromosome within the
nucleus of a cell. Genes control the development and function of all organs and all working systems in
the body. A gene has a certain influence on how the cell works; the same gene in many different cells
determines a certain physical or biochemical feature of the whole body (e.g. eye color or reproductive

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