Allelic and non-allelic interaction in Drosophila.pptx

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1. Test for allelism or Allelic and non-allelic interaction in Drosophila

2. The Mendelian concept that genes exist in no more than two allelic states had to be modified when genes with three, four, or more alleles were discovered . classic example of a gene with multiple alleles is the one that controls coat color in rabbits Coat Color in rabbits. Different phenotypes are caused by four different alleles of “c gene”

3. This gene has four alleles, three of which are distinguished by a superscript: c (albino), c h ( himalayan ), c ch (chinchilla), c + (wild-type ) In homozygous condition, each allele has a characteristic effect on the coat color. Because most rabbits in wild populations are homozygous for the c allele, this allele is called the wild type. In genetics it is customary to represent wild-type alleles by a superscript plus sign after the letter for the gene. When the context is clear, the letter is sometimes omitted and only the plus sign is used; thus, c may be abbreviated simply as + .

4. The other alleles of the c gene are mutants—altered forms of the wildtype allele that must have arisen sometime during the evolution of the rabbit. The himalayan and chinchilla alleles are denoted by superscripts, but the albino allele is denoted simply by the letter c (for colorless, another word for the albino condition). This notation reflects another custom in genetics nomenclature c c Albino c h c h Himalyan c ch c ch Chinchila c + c + Wild type

5. Allelic Series The functional relationships among the members of a series of multiple alleles can be studied by making heterozygous combinations through crosses between homozygotes. For example, the four alleles of the c gene in rabbits can be combined with each other to make six different kinds of heterozygotes: c h c , c ch c , c + c , c ch c h , c + c ch , and c + c ch These heterozygotes allow the dominance relations among the alleles to be studied ( Figure 4.4). The wild-type allele is completely dominant over all the other alleles in the series; the chinchilla allele is partially dominant over the himalayan and albino alleles , and the himalayan allele is completely dominant over the albino allele. These dominance relations can be summarized as c + > c ch > c h >c .

6. Another example of multiple alleles comes from the study of human blood types. The A, B, AB, and O blood types, Blood groups are identified by testing a blood sample with different sera. One serum detects the A antigen, another the B antigen ;When only the A antigen is present on the cells, the blood is type A; when only the B antigen is present, the blood is type B. When both antigens are present, the blood is type AB, When neither antigen is present , it is type O. Genotype Blood Type A antigen Present B antigen Present Freq. in U.S. White Popn . I A I A or I A i A + - 41 I B I B or IB i B - + 11 I A I B AB + + 4 ii O - - 44

7. TESTING GENE MUTATIONS FOR ALLELISM A mutant allele is created when an existing allele changes to a new genetic state—a process called mutation for example, the c allele mutated to a null allele, a rabbit homozygous for this mutation would have the albino phenotype However, it is not always possible to assign a new mutation to a gene on the basis of its phenotypic effect. In rabbits, for example, several genes determine coat color, and a mutation in any one of them could reduce, alter, or abolish pigmentation in the hairs. Thus, if a new coat color appears in a population of rabbits, it is not immediately clear which gene has been mutated.

8. A simple test can be used to determine the allelic identity of a new mutation, providing that the new mutation is recessive . The procedure involves crosses to combine the new recessive mutation with recessive mutations of known genes . If the hybrid progeny show a mutant phenotype , then the new mutation and the tester mutation are alleles of the same gene . If the hybrid progeny show a wild phenotype , then the new mutation and the tester mutation are not alleles of the same gene . This test is based on the principle that mutations of the same gene impair the same genetic function. If two such mutations are combined , the organism will be abnormal for this function and will show a mutant phenotype , even if the two mutations had an independent origin.

9. As an example, let’s consider the analysis of two recessive mutations affecting eye color in the fruit fly, Drosophila melanogaster Scarlet mutation Cinnabar mutation

10. Two independently isolated recessive mutations, called cinnabar and scarlet , are phenotypically indistinguishable, each causing the eyes to be bright red. In wild-type flies, the eyes are dark red/brown . We wish to know whether the cinnabar and scarlet mutations are alleles of a single color-determining gene or if they are mutations in two different genes? To find the answer, we must cross the homozygous mutant strains with each other to produce hybrid progeny. If the hybrids have bright red eyes, we will conclude that cinnabar and scarlet are alleles of the same gene . If they have dark red eyes, we will conclude that they are mutations in different genes