Gene Mutations
Gene Mutations
Gene mutations are chemicophysical changes in genes resulting in an alteration of the sequence of bases in the DNA. These mutations can be studied directly by determining the DNA sequence and indirectly through their effects on organismal phenotype, if such effects are present. A mutation may result in a codon substitution as, for example, in the condition in humans known as sickle cell anemia. Homozygotes with sickle cell trait often die before the age of 30 because the ability of their red blood cells to carry oxygen is greatly impaired, a result of the substitution of only a single amino acid in the amino acid sequence of their hemoglobin. Other mutations may involve the deletion of one or more bases or the insertion of additional bases into the DNA chain. The translation of mRNA will thus be shifted, leading to codons that specify incorrect amino acids.
Once a gene is mutated, it faithfully reproduces its new self just as it did before it was mutated. Many mutations are harmful, many are neither helpful nor harmful, and sometimes mutations are advantageous. Helpful mutations are of great significance to evolution because they furnish new possibilities on which natural selection works to build adaptations. Natural selection determines which new alleles merit survival; the environment imposes a screening process that passes the beneficial and eliminates the harmful.
When an allele of a gene is mutated to the new allele, it tends to be recessive and its effects are normally masked by its partner allele. Only in the homozygous condition can such mutant alleles be expressed. Thus a population carries a reservoir of mutant recessive alleles, some of which are homozygous lethals but which are rarely present in the homozygous condition. Inbreeding encourages the formation of homozygotes and increases the probability of recessive mutants being expressed in the phenotype.
Most mutations are destined for a brief existence. There are cases, however, in which mutations may be harmful or neutral under one set of environmental conditions and helpful under a different set. Should the environment change, there could be a new adaptation beneficial to the species. The earth’s changing environment has provided numerous opportunities for new gene combinations and mutations, as evidenced by the great diversity of animal life today.
Gene mutations are chemicophysical changes in genes resulting in an alteration of the sequence of bases in the DNA. These mutations can be studied directly by determining the DNA sequence and indirectly through their effects on organismal phenotype, if such effects are present. A mutation may result in a codon substitution as, for example, in the condition in humans known as sickle cell anemia. Homozygotes with sickle cell trait often die before the age of 30 because the ability of their red blood cells to carry oxygen is greatly impaired, a result of the substitution of only a single amino acid in the amino acid sequence of their hemoglobin. Other mutations may involve the deletion of one or more bases or the insertion of additional bases into the DNA chain. The translation of mRNA will thus be shifted, leading to codons that specify incorrect amino acids.
Once a gene is mutated, it faithfully reproduces its new self just as it did before it was mutated. Many mutations are harmful, many are neither helpful nor harmful, and sometimes mutations are advantageous. Helpful mutations are of great significance to evolution because they furnish new possibilities on which natural selection works to build adaptations. Natural selection determines which new alleles merit survival; the environment imposes a screening process that passes the beneficial and eliminates the harmful.
When an allele of a gene is mutated to the new allele, it tends to be recessive and its effects are normally masked by its partner allele. Only in the homozygous condition can such mutant alleles be expressed. Thus a population carries a reservoir of mutant recessive alleles, some of which are homozygous lethals but which are rarely present in the homozygous condition. Inbreeding encourages the formation of homozygotes and increases the probability of recessive mutants being expressed in the phenotype.
Most mutations are destined for a brief existence. There are cases, however, in which mutations may be harmful or neutral under one set of environmental conditions and helpful under a different set. Should the environment change, there could be a new adaptation beneficial to the species. The earth’s changing environment has provided numerous opportunities for new gene combinations and mutations, as evidenced by the great diversity of animal life today.
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