After Bonaparte After Williston Figure 5. The root of this proposed phylogeny is central, near the top. The two great clades of dinosaurs, Omithischia 1; left and Saurischia 2; right , are curved downward to fit the page. The Saurischia included the Sauropoda 3 and Therapoda 4 , of which birds Aves are the only survivors 5.
Ornithischia included stegosaurs 6 and ceratopsians 7. All ornithiseman lineages are now extinct. From Sereno , 5ci",ce Most or all of these transposable element-induced effects have been observed within experimental populations of organisms such as maize corn, Zea mays and Drosophila melnllogaster.
The transposition rate of various retroelements ranges from about to per copy in inbred lines of Drosophila, resulting in an appreciable rate of mutation Nuzhdin and Mackay All the kinds of changes engendered by transposable elements can be found by comparing genes and genomes of organisms of the same or different species.
For instance, L] retrotransposon insertions are associated with many disease-causing mutations in both mice and humans Kazazian , and a difference in flower color behveen hiVO species of Petllnia has been caused by the insertion and subsequent incomplete excision of a transposon that disabled a gene that controls anthocyanin pigment production Quattrocdtio et al.
Examples of mutations Geneticists have learned an enormous amOLmt about the nature and causes of mutations by studyiJlg model organisms such as Drosophila and E. Human mutations are usually rather rare variants that can be compared with normal forms of the gene; in some instances, newly arisen mutations have been found that are lacking in both of a patient's parents.
Single base pair substitutions are responsible for conditions such as sickle-cell anem.. Because many different alterations of a protein can diminish its function, the same phenotypi.
For exall1ple, cystic fibrosis, a fatal condition afflicting olle in live births in northern Europe, is caused by mutations in the gene encoding a sodium channel protejn. The most common such mutation is a 3 bp deletion that deletes a single am.
Mutations in any of the many different genes that contribute to the normal development of some characteristics can also result in similar phenotypes.
For example, retinitis pigmentosa, a degeneration of the retina, can be caused by mutations in genes on 8 of the 23 chromosomes in the haploid human genome Avise It is believed to have arisen by unequal crossing over betvveen two normal gene copies, due to out-oF-register pairing between tvvo of the repeated sequences Alu, shown as blue boxes in the introns.
The numbered boxes are exons. After Hobbs et aI. Figure 8. J Hemophilia can be caused by mutations in tvvo different genes that encode blood-clotting proteins. Huntington disease, a fatal neurological disorder that strikes in mjdlife, is caused by an excessive number of repeats of the sequence CAG: the normal gene has 10 to 30 repeats, the mutant gene more than Unequal crossing over between the hvo tandemly arranged genes for a-hemoglobin see Figure 8.
The deletjon of one of the Joci causes a-thalassemia, a severe anemia. Another case of deletion, which results in high cholesterol levels, is tJle lack of exon 5 in a low-density lipoprotein gene.
This deletion has been attributed to unequal crossing over, facilitated by a short, highly repeated sequence called AlII that is located in the introns of this gene and in many oU,er sites in the genome Figure 8.
TI1ese examples might make it seem as if mutations are nothing but bad news. While this is close to the truth-far more mutations are harmful than helpful-these mutations represent a biased sample. Phosphoglycerate kinase is encoded by two genes.
The other, on an autosome, lacks introns, and clearly arose from the X-linked gene by reverse transcription. It is expressed only in the testes, a novel pattern of tissue expression that suggests that the gene plays a new functional role see Li When biologists seek those genes that have been involved in the evolution of a specific characteristic, they often use rare deleterious mutations of the kind described here as indicators of CANDIDATE GENES, those that may be among the genes they seek.
For example, a rare mutation in the human FOXP2 gene jorkhead box 2, which encodes a transcription factor causes severe speech and language disorders. Two research groups, led by Jianzhi Zhang Zhang et al. This is a l1"'ltiCh higher rate of protein evolution than would be expected, considering that only one other such substitution has ocew"red between these species and the mouse, which diverged almost 90 Mya Figure 8.
Both research groups propose that these substitutions occurred in the human Lineage less than , years ago and that they are among the important steps in the evolution of human language and speech. Each box shows the number of nonsynonymous yellmv boxes and synonymous white boxes substitutions in the FOXP2 gene. The two nonsynonymolts substitutions in the human lineage represent an unusually high rate of evolution of the FOXP2 protein, and may represent mutations that have been important in the evolution of language and speech.
After Zhang et aJ. Tn classical genetics, a mutation was detected by its phenotypic effects, such as white versus red eyes in Drosophila. Thus phenotypically detected rates of mutation Lmderestirnate the rate at which all mutations occur at a locus. A relatively direct method is to count the number of mutations arising in a laboratory stock which is usually initially homozygous , scoring mutations either by their phenotypic effects or by molecular methods. An indirect method Box A is based on the nLUnber of base pair differences between homologous genes in different species, relative to the number of generations that have elapsed since they diverged from thelr common ancestor.
This method depends on the neulTal theory of molecular evolution, which is described in Chapter Mutation rates vary among genes and even among regions within genes, but on average, as measured by phenotypic effects, a locus mutates at a rate of about to mutations per gamete per generation Table 8. TIle average mutation rate per base pair, based mostly on the indirect method of comparing DNA sequences of different species, has been estimated at about ]0- 11 to per replication in prokaryotes see Table 8.
The mutation rate in the human genome has been estimated at about 4. Back mutation is mutation of a "mutant" allele back to the allele usually the wild type from which it arose. Back mutations are ordinarily detected by their phenotypic effects. At the molecular level, most phenotypically detected back mutations are not restorations of the original sequence, but instead result from a second amino TABLE 8.
One possible fate is that a mutation will become fixed-that is, at- tain a frequency of l. D-entil"ely by chance. In each genera- tion, therefore, the probability is II that a mutation that occurred at some time in the past will become fixed.
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