Question

Deletions can be used to map genes along a chromosome. In order to do this a series of crosses in which one parent is homozygous for a mutant allele is crossed with the other parent that is homozygous for a partial deletion of the region. The progeny are scored to determine whether they have the mutant phenotype ("m") or the wild-type phenotype ("+"). If a mutation falls within the deletion it will have the "m" phenotype, if it falls outside the deletion it will complement and have the ("+") phenotype. Some of the deletions will take out more than one gene and thus will produce the "m" phenotypes with several of the genes in question. Use the following table of progeny phenotypes for 7 different deletions to determine the order of the genes a-g on the chromosome.

	mutations
deletion	a	b	c	d	e	f	g
1	+	+	m	+	+	+	m
2	+	+	+	m	m	+	+
3	+	+	+	m	+	m	+
4	+	+	+	m	m	m	m
5	+	m	m	+	+	+	m
6	m	+	m	m	m	m	m
7	m	+	+	m	+	m	+

The answer is afdegcb. Please EXPLAIN!

Answer 1

On the basis of deletion and mutants produced due to failed complementation, a table can be constructed to identify the genes involved in deletion. The involved genes are either linked or present next to each other.

Deletion	Genes involved
1	c , g
2	d , e
3	d , f
4	d ,e , f, g
5	b , c , g
6	a , c , d, e , f , g
7	a , d , f

To determine the gene order, the table can be analyzed as :

• According to deletion 2 and 3, genes d-e & d-f are present together. This implies that gene d is present between e & f i.e., gene order is e-d-f
• According to deletion 7, genes a , d , f are located together. So, the order will be e-d-f-a
• According to deletion 4, e-d-f is also linked to gene g. Therefore, the order will be g-e-d-f-a.
• According to deletion 5, genes b , c , g lie next to each other. So, the order is either b-c-g-e-d-f-a OR c-b-g-e-d-f-a.
• According to deletion 6, all the genes except b are located together.

Thus, the correct gene order will be "b-c-g-e-d-f-a"

* NOTE - Gene order can be read from any direction.