Question

A cross is made between an Hfr strain that is StrS a+ b+ d+ in genotype and an F– strain that is StrR a– b– d– in genotype. Interrupted-mating studies show that b+ enters the recipient strain last, and that the Str locus is very far away from b+, so it never enters the recipient strain. The b+ recombinants are then tested for the presence of the a+ and d+ alleles. The following data were obtained:

a+ b+ d+	326
a– b+ d+	2
a+ b+ d–	14
a– b+ d–	58
total	400

a) What is the gene order?

1. a d b Str
2. b a d Str
3. b d a Str
4. d a b Str

b) What is the map distance between markers d and b?

1. 4 map units
2. 15 map units
3. 19 map units
4. 22 map units

Answer 1

A) Given in the question

A cross is made between an Hfr strain that is StrS a+ b+ d+ in genotype and an F– strain that is StrR a– b– d– in genotype. Interrupted-mating studies show that b+ enters the recipient strain last, and that the Str locus is very far away from b+, so it never enters the recipient strain. The b+recombinants are then tested for the presence of the a+ and d+ alleles. The following data were obtained:

a+ b+ d+	326
a– b+ d+	2
a+ b+ d–	14
a– b+ d–	58
total	400

Gene order

As you can see in the table lowest number of recombinant found was 2 of a-b+d+

Hence a must be in the middle of the gene the because if two allele is transferred the middle allele present between them come along with them.

b and str are far away as given in the question hence answer will be b a d str option b

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