Page 82 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

7.4 What Mutations Tell Us About Gene Structure 241

Figure 7.22 Complementation testing of Drosophila eye color mutations. (a) A heterozygote has one mutation (m 1 ) on one
chromosome and a different mutation (m 2 ) on its homolog. If the mutations are in different genes, the heterozygote will be wild type; the
mutations complement each other (left). If both mutations affect the same gene, the phenotype will be mutant; the mutations do not
complement each other (right). Complementation testing makes sense only when both mutations are recessive to wild type. (b) This
complementation table reveals five complementation groups (five different genes) for eye color. A plus (+) indicates mutant combinations with
wild-type eye color; these mutations complement and are thus in different genes. Several mutations fail to complement (−) and are thus
alleles of one gene, white. (c) Recombination mapping shows that mutations in different genes are often far apart, while different mutations
in the same gene are very close together.
(a) Complementation testing
Complementation No complementation
Defective gene Functional gene Defective gene Functional gene
m m
Maternal X 1 X 1
chromosome G R G R

Functional gene Defective gene Defective gene Functional gene
m m
2
2
Paternal X X
chromosome G R G R

Conclusion: m and m are in di erent genes. Conclusion: m and m are in the same gene.
1
2
1
2
m /m has wild-type phenotype because m /m has mutant phenotype because the
2
1
1
2
one chromosome supplies gene G function, organism has no gene G function.
while the other supplies gene R function.
(b) A complementation table: X-linked eye color mutations in Drosophila
Mutation white garnet ruby vermilion cherry coral apricot bu
carnation
white – + + + – – – – +
garnet – + + + + + + +
ruby – + + + + + +
vermilion – + + + + +
cherry – – – – +
coral – – – +
apricot – – +
bu
– +
carnation –
(c) Genetic map: X-linked eye color mutations in Drosophila
bu
w coral apricot cherry
w w w 1 w
0.011 m.u.
white ruby vermilion garnet carnation Genes

0 1.5 7.5 33.0 44.4 62.5 Distance (m.u.)

pairs. All known human genes fall somewhere between gene can accommodate different mutations and how these
these extremes. To put these figures in perspective, an aver- different mutations can alter phenotype in different ways.
age human chromosome is approximately 130 million base Does each mutation change the whole gene at a single
pairs in length. stroke in a particular way, or does it change only a specific
part of a gene, while other mutations alter other parts?
In the late 1950s, the American geneticist Seymour
A Gene Is a Set of Nucleotide Pairs Benzer used recombination analysis to show that two dif-
That Can Mutate Independently ferent mutations that did not complement each other and
and Recombine with Each Other were therefore known to be in the same gene can in fact
change different parts of that gene. He reasoned that if a
Although complementation testing makes it possible to gene is composed of separately mutable subunits, then it
distinguish mutations in different genes from mutations in should be possible for recombination to occur within
the same gene, it does not clarify how the structure of a a gene, between these subunits. Therefore, crossovers