Q15-12TYU - Campbell Biology

Question

Assume you are mapping genes A, B, C, and D in Drosophila. You know that these genes are linked on the same chromosome, and you determine the recombination frequencies between each pair of genes to be as follows: A-B, 8%; A-C, 28%; A-D, 25%; B-C, 20%; B-D, 33%. 

1. Describe how you determined the recombination frequency for each pair of genes.

2. Draw a chromosome map based on your data.

Step-by-Step Solution

Answer

(a). An F1 dihybrid offspring for each pair of genes is generated to obtain recombinant frequencies. The F1 hybrid is then test crossed with a homozygous recessive fly. The parental and recombinant offspring are observed and counted to determine the recombination frequency between the two genes.

(b). 

Step 1: Recombination frequency

The number of recombinants produced concerning the number of offspring in a cross is referred to as recombinant frequency. The recombination frequency between gene pairs helps to calculate the distance between the genes on a chromosome.

1% recombinant frequency refers to one map unit. The distance between the genes on the chromosome is referred to as map units.

Step 2: Calculation of recombinant frequency between the pair of genes

Calculate the recombinant frequency for gene A and gene B. First, a homozygous dominant parent (AABB) is crossed with a recessive parent (aabb). The offspring produced would be F1 dihybrid with genotype AbBb. 

The F1 dihybrid is then test crossed with a homozygous recessive fly (aabb). The offspring produced from this cross is observed and segregated as parental and recombinants based on parents' genotype. 

The recombination frequency between the gene pairs is calculated using the formula:

$Recombinant Frequency=\frac{Number of recombinants}{Total number of offspring}\times 100$

Step 3: Chromosome map

According to the given data, the distance between gene A and gene B is 8 map units, between A and C is 28 map units, between A and D is 25 map units, between B and C is 20 map units, and between A and D gene is 33 map units.

The data suggest that gene A and B is located between gene D and C. Therefore, the sequence of genes on the chromosome is DABC.