Page 81 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

240 Chapter 7 Anatomy and Function of a Gene: Dissection Through Mutation

Figure 7.21 Drosophila eye color mutations produce a mutated copy of the gene will be able to perform the nor-
variety of phenotypes. Flies carrying different X-linked eye mal function. As a result, no complementation will occur
color mutations. From the left: ruby, white, and apricot; a wild-type and no normal gene product will be made, so a mutant
eye is at the far right. phenotype will appear (Fig. 7.22a, right). Ironically, a col-
(all): © Science Source
lection of mutations that do not complement each other is
known as a complementation group. Geneticists often use
complementation group as a synonym for gene because
the mutations in a complementation group all affect the
same unit of function, and thus, the same gene.
A simple test based on the idea of a gene as a unit of
function can determine whether or not two recessive muta-
tions are alleles of the same gene. You simply examine
the phenotype of a heterozygous individual in which
and 1960s, scientists realized they could also use mutations to one homolog of a particular chromosome carries one of
learn how DNA sequences along a chromosome constitute the mutations and the other homolog carries the other
individual genes. These investigators wanted to collect a large mutation. If the phenotype is wild-type, the mutations can-
series of mutations in a single gene and analyze how these not be in the same gene. This technique is known as a
mutations were arranged with respect to each other. For this complementation test. For example, because a female
approach to be successful, they had to establish that various fruit fly simultaneously heterozygous for garnet and ruby
mutations were, in fact, in the same gene. This was not a (garnet ruby /garnet ruby) has wild-type brick-red eyes, it
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trivial exercise, as illustrated by the following situation. is possible to conclude that the mutations causing garnet
Early Drosophila geneticists identified a large number
of X-linked recessive mutations affecting the normally red and ruby colors complement each other and are therefore in
different genes.
wild-type eye color (Fig. 7.21). The first of these to be Complementation testing has, in fact, shown that gar-
discovered produced the famous white eyes studied by net, ruby, vermilion, and carnation pigmentation are caused
Morgan’s group. Other mutations caused a whole palette of by mutations in separate genes. But chromosomes carrying
hues to appear in the eyes: darkened shades such as garnet mutations yielding white, cherry, coral, apricot, and buff
and ruby; bright colors such as vermilion, cherry, and coral; phenotypes fail to complement each other. These mutations
and lighter pigmentations known as apricot, buff, and car- therefore constitute different alleles of a single gene.
nation. This wide variety of eye colors posed a puzzle: Drosophila geneticists named this gene the white, or w,
Were the mutations that caused them multiple alleles of a gene after the first mutation observed; they designate the
single gene, or did they affect more than one gene?
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wild-type allele as w and the various mutations as w (the
original white-eyed mutation discovered by T. H. Morgan,
Complementation Testing Reveals often simply designated as w), w cherry , w coral , w apricot , and
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Whether Two Mutations Are in a Single w buff . As an example, the eyes of a w / w apricot female are a
Gene or in Different Genes dilute apricot color; because the phenotype of this hetero-
zygote is not wild-type, the two mutations are allelic.
Researchers commonly define a gene as a functional unit Figure 7.22b illustrates how researchers collate data from
that directs the appearance of a molecular product that, in many complementation tests in a complementation table.
turn, contributes to a particular phenotype. They can use this Such a table helps visualize the relationships among a large
definition to determine whether two mutations are in the group of mutants.
same gene or in different genes. In Drosophila, mutations in the w gene map very close
If two homologous chromosomes in an individual each together in the same region of the X chromosome, while
carry a mutation recessive to wild type, that individual will mutations in other sex-linked eye color genes lie elsewhere
have a normal phenotype if the mutations are in different on the chromosome (Fig. 7.22c). This result suggests that
genes. Such a result is called complementation. The genes are not disjointed entities with parts spread out from
normal phenotype occurs because almost all recessive one end of a chromosome to another; each gene, in fact,
mutations disrupt a gene’s function. The dominant wild- occupies only a relatively small, discrete area of a chromo-
type alleles on each of the two homologs can make up for, some. Studies defining genes at the molecular level have
or complement, the defect in the other chromosome by shown that most genes consist of 1000–20,000 contiguous
generating enough of both gene products to yield a normal base pairs (bp). In humans, among the shortest genes are
phenotype (Fig. 7.22a, left). the roughly 500 base pair–long genes that govern the pro-
In contrast, if the recessive mutations on the two duction of histone proteins, while the longest gene so far
homologous chromosomes are in the same gene, no wild- identified is the Duchenne muscular dystrophy (DMD)
type allele of that gene exists in the individual, and neither gene, which has a length of more than 2 million nucleotide