Question

Assume that investigators crossed a strain of flies carrying the dominant eye mutation Lobe on the second chromosome with a strain homozygous for the second chromosome recessive mutations smooth abdomen and straw body. The F1 Lobe females were then backcrossed with homozygous smooth abdomen, straw body males, and the following phenotypes were observed: smooth abdomen, straw body 820, Lobe 780, smooth abdomen, Lobe 42, straw body 58, smooth abdomen 148, Lobe, straw body 152. Give the gene order and map units between these three loci.

1) smooth abdomen - Lobe - straw body
2) Lobe - smooth abdomen - straw body
3) Lobe - straw body - smooth abdomen

Answer 1

The gene order is Lobe - smooth abdomen - straw body, and the map units between these loci can be calculated using the observed phenotypes.

Step-by-step explanation:

In this experiment, investigators crossed flies with the dominant eye mutation Lobe on the second chromosome with flies homozygous for the recessive mutations smooth abdomen and straw body. The resulting F1 generation was then backcrossed with smooth abdomen, straw body flies. The observed phenotypes were smooth abdomen, straw body; Lobe; and smooth abdomen, Lobe, straw body. By analyzing the phenotypes, we can determine the gene order and map units between these three loci.

The gene order is Lobe - smooth abdomen - straw body, and the map units between these loci can be calculated as follows:

1. Calculate the total map distance by adding together the number of double crossovers and half the number of single crossovers.
2. Calculate the frequency of each type of crossover event.
3. Divide the number of each type of crossover event by the total number of offspring to get the frequency of each event.
4. Calculate the map distance by multiplying the frequency of each crossover event by 100 to get the percentage of offspring with that particular crossover event, and then dividing by 2 to get the map distance in map units.

The gene order is Lobe - smooth abdomen - straw body, based on the analysis of recombinant phenotypes and the phenotypic data provided. The map units are calculated using the recombinant numbers and total offspring.

The question pertains to linkage mapping in Drosophila melanogaster, commonly known as fruit flies. To answer the question of gene order and map units between the three loci, we analyze the given phenotypic data from a backcross involving flies with a dominant eye mutation (Lobe) and recessive mutations for the second chromosome (smooth abdomen and straw body). The recombinant phenotypes, which are less frequent, help determine the gene order. In this case, the phenotypes smooth abdomen, Lobe (42 flies) and straw body (58 flies) represent the recombinants. Their low numbers suggest they are the outcomes of crossing over between the Lobe and the other two genes. Therefore, the Lobe gene must lie between the smooth abdomen and straw body loci. To calculate map units, we add the two recombinant phenotypes (42 + 58) and divide by the total number of offspring (820 + 780 + 42 + 58 + 148 + 152), then multiply by 100. The percentage gives the map distance between the Lobe gene and the genes for smooth abdomen and straw body.