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A gene is the nucleic acid sequence that is necessary for the synthesis of a functional peptide or protein in a temporal and tissue-specific manner. Genes are composed of DNA Opens in new window in which is coded the information for making amino acids and proteins, essential components of our bodies.
We are each born with certain characteristics or traits which are inherent. That is why we resemble our relatives more than we resemble others. These inherited traits are part of our constitution and include, for example, eye color and blood groups. These traits are determined by genes.
All life starts off as a single cell, the fertilized egg or ovum, which then undergoes repeated divisions, ultimately to produce all the cells, tissues, and organs of the body.
Each cell contains a nucleus, of which is contained 46 thread-like bodies called chromosomes Opens in new window, 23 of which are derived from one parent and 23 from the other. Thus each gene exists in two copies, known as alleles.
The arrangement of the chromosomes in a standard manner is referred to as a karyotype Opens in new window.
The chromosomes carry the genes, and the same genes are present in every cell of the body – although only certain genes are active in particular tissues or organs.
For example, dystrophine Opens in new window, a gene making muscle protein is active only in muscle tissue and a gene making hemoglobin, the oxygen-carrying chemical in red blood cells, is only active in blood-forming cells. Estimates vary, but there are probably around 50 000 to 100 000 genes in each nucleus.
Genes are categorized based on their location on a specific chromosome (loci). They are described as autosomal (chromosomes 1 to 22) or sex linked (X or Y chromosome).
As mentioned earlier, many genes have one or more alternate forms known as alleles. If both the maternal and paternal alleles for a specific locus are the same, then the person is said to be homozygous for that gene.
Conversely, if the maternal and paternal alleles for a specific locus are different, then the person is said to be heterozygous.
Because more than 30,000 genes are in the human genome, all humans are heterogenic as a result of genetic evolution.
In addition, some genetic traits require only one copy of an allele to be expressed (i.e., dominant trait), whereas others require two copies of the same allele to be expressed (recessive trait).
If a trait is caused by an autosomal dominant Opens in new window, autosomal recessive Opens in new window, X-linked dominant Opens in new window or X-linked recessive Opens in new window gene, a specific pattern of inheritance is found within families, which is identified easily with the construction of a pedigree. Opens in new window
People with one dominant allele and one recessive allele for a given loci that requires two recessive alleles for expression are considered carriers Opens in new window of the recessive allele.
Because they are not homozygous for the recessive allele, they do not express the associated phenotype; however, they can pass the recessive allele to any children.
If the child subsequently receives two copies of the recessive allele (one from each parent), s/he will be homozygous for the recessive trait and will express its effects.
For example, a father has brown eyes and a mother has blue eyes. The expression of brown eyes requires only one copy of the dominant allele (brown) for eye color. The expression of blue eyes requires two copies of the recessive form (blue).
If the brown-eyed parent is homozygous, then all children of this couple will have brown eyes because the father can transmit only the dominant allele.
If, however, he is heterozygous with one dominant allele (brown) and one recessive allele (blue), then each child has a 50% chance to be a brown-eyed heterozygous blue-eyed child like the mother (Lashley, 1998; Lewis, 2001).
An inheritable permanent change in the DNA Opens in new window of a gene is known as a mutation. Some mutations Opens in new window have a benign or no effect; others result in an adverse alteration. Typically, such changes affect growth, development, and wellness.
As with all other human traits, disorders caused by a single copy of an altered gene follow dominant patterns of inheritance, whereas disorders that require two copies of the altered gene follow recessive patterns of inheritance.
More than 3,000 autosomal dominant disorders, 1,500 autosomal recessive disorders, and 300 X-linked disorders have been identified (National Center for Biotechnology Information, 2003).
- Alberts B, Bray D, Lewis J, Raff M, Roberts K, Watson JD. Molecular biology of the cell. 3rd ed. New York: Garland, 1994.
- Darnell J, Lodish H, Baltimore D. Molecular cell biology. 2nd ed. New York: Scientific American Books, WH Freeman, 1990.
- Watson JD, Tooze J, Kurtz DT. Recombinant DNA, a short course. New York: Scientific American Books, WH Freeman, 1983.
- Watson JD, Hopkins NH, Roberts JW, Steitz JA, Weiner AM. Molecular biology of the gene. 4th ed. Menlo Park, CA: Benjamin/Cummings Publishing Co., 1987.
- Meselson M, Stahl FW. The replication of DNA in E. coli Proc Natl Acad Sci USA 1958;44-671-682.