You obtain 7 homozygous mutant lines of geckos that all lack spots (normal gecko

Question

You obtain 7 homozygous mutant lines of geckos that all lack spots (normal geckos have spots). After three years of work breeding them to one another in pairwise combinations you obtain the results shown in the table below.

According to these results, how many genes control gecko spotting?

Line 1 2 3 4 5 6 7 1 No Spots 2 Spots No Spots 3 Spots Spots No Spots 4 Spots Spots No Spots No Spots 5 Spots Spots Spots Spots No Spots 6 Spots No Spots Spots Spots Spots No Spots 7 Spots Spots Spots Spots No Spots Spots No Spots

Explanation / Answer

Table given is for pairwise combinations obtained after breeding mutant lines of gecko in a complementation test. To calculate number of genes controlling gecko spotting, we first need to identify the complementation groups and their number. This number is achieved by looking at each pair of mutants given and then looking for complementation between them. Three points to identify complementation groups are :

(i) A mutant never complements itself.

(ii) Spots occuring (+) means that the mutants complement each other. When mutants complement it means that they belong to different complementation group.

(iii) No spots occur (-) means that the mutants do not complement each other. When mutants do not complement it means that they belong to the same complementation group.

Proceeding down the rows, we see that 1 complements with 2.This means that they belong to different complementation group. 2 does not complement with 6. This means that 2 and 6 belong to the same group. 3 does not complement with 4 and they belong to the same group. 5 and 7 does not complement each other so they belong to the same group. 1 belongs to a different group.

So now we have 4 groups:

Group 1- 1

Group 2- 2 and 6

Group 3- 3 and 4

Group 4- 5 and 7

Since we have four complementation groups it means that 4 genes are controlling gecko spotting.

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