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 \(\mathrm{F}_{1}\) Lobe females were then backcrossed with homozygous smooth abdomen, straw body males, and the following phenotypes were observed: (a) Give the gene order and map units between these three loci. (b) What is the coefficient of coincidence?

Short answer

Based on thestep-by-step solution provided, the following short answer can be created: Gene order of the three loci on a chromosome in fruit flies was determined to be s-L-st through a backcross experiment. The map units between s-L and L-st were calculated to be 37.84 and 12.24, respectively. Finally, the coefficient of coincidence was found to be approximately 1.183.

Step-by-step solution

1. Conduct a backcross and record the phenotype frequencies

The initial cross was between Lobe (dominant eye mutation) flies and the homozygous smooth abdomen, straw body flies. The F1 Lobe females were then backcrossed with homozygous smooth abdomen and straw body males. The resulting phenotypes and their frequency are as follows: Lobe, Smooth abdomen, Straw body (LSst) : 944 Normal, Smooth abdomen, Straw body (lsst) : 902 Lobe, Hairy abdomen, Straw body (LSSt) : 269 Normal, Hairy abdomen, Straw body (lsSst) : 251 Lobe, Smooth abdomen, Normal body (LsST) : 81 Normal, Smooth abdomen, Normal body (lsST) : 93 Lobe, Hairy abdomen, Normal body (LsSt) : 49 Normal, Hairy abdomen, Normal body (lsSt) : 43 Now, we need to determine the gene order and the map units between the loci.

2. Interpret the results and identify the gene order

First, we should identify the dominant and recessive alleles for each gene based on phenotypes. From the parental strains, we know that Lobe is dominant and Smooth abdomen and Straw body are recessive. Therefore, we need to identify crossovers by looking for recombinant phenotypes that differ from the parental phenotypes. We can use the non-parental phenotype frequency to infer the gene order. The lowest frequency recombinants represent the double crossover events, and in this case, they are the LsSt (49) and lsST (93) phenotypes. This indicates that the Lobe gene is in the middle. We will denote the non-recombinant (parental) phenotypes with a single prime (') and recombinant (non-parental) phenotypes with a double prime ("). So, based on our analysis, our gene order is s-L-st, with the phenotypes ordered as follows: s'L'st', s"l"st', S'L'St', S"l"St', s'L"St", s"l"ST, S'L"St", and S"l"st".

3. Determine the map units between the three loci

Now, we can calculate the recombination frequencies (and map units) between each pair of adjacent loci, which are equal to (number of recombinants / total offspring) x 100. s-L: Number of recombinant phenotypes are s"l"st' (902) and s'L"st" (81). The recombination frequency is (902 + 81) / 2590 = 0.3784, which is equal to 37.84 map units. L-st: Number of recombinant phenotypes are S"L'st' (269) and S"l"st" (49). The recombination frequency is (269 + 49) / 2590 = 0.1224, which is equal to 12.24 map units.

4. Calculate the coefficient of coincidence

The coefficient of coincidence is calculated as the ratio of observed double crossover events to expected double crossover events. The expected double crossover events are the product of the recombination frequencies between the adjacent genes. Observed double crossover: Number of LsSt (49) and lsST (93) phenotypes. So, observed double crossover frequency = (49 + 93) / 2590 = 0.0548. Expected double crossover: (s-L) x (L-st) = 0.3784 x 0.1224 = 0.0463. Coefficient of coincidence (COC) = observed double crossover / expected double crossover = 0.0548 / 0.0463 = 1.183. In this case, the coefficient of coincidence is approximately 1.183.

Key concepts

Gene Order Determination

Understanding the sequence of genes on a chromosome is essential for genetic studies and is established through a process known as gene order determination. This is a cornerstone of genetic linkage mapping, which relates to the arrangement of genes and their relative distances.

Using data from breeding experiments, scientists analyze the traits, or phenotypes, that appear in the offspring. For example, in the exercise provided, the lowest frequency of recombinant phenotypes suggests where the middle gene is positioned. Once the order is identified—here, s-L-st—researchers can delve deeper into calculating precise genetic distances and understanding how these genes may interact with each other.

With easy visualization in mind, one could picture gene order determination as solving a puzzle where pieces are phenotypes that must be assembled in a way that fits genetic principles and observed data.

Recombination Frequency

Recombination frequency is a measure of genetic linkage and is crucial for mapping genes on chromosomes. It's calculated by dividing the number of recombinant offspring by the total number of offspring, then multiplying by 100 to get a percentage, which is then translated into map units.

In our example, the process involves counting the offspring that show a combination of traits not found in either parent, indicative of recombination events. The percentages obtained provide a numerical representation of the distances between genes, which is directly proportional to the likelihood of chromosomal crossover during meiosis. The smaller the recombination frequency, the closer the genes are on the chromosome, suggesting a higher likelihood that they are genetically linked.

Coefficient of Coincidence

The coefficient of coincidence (COC) offers insight into how often two crossover events occur simultaneously, compared to the expectation if they were independent. It is calculated by dividing the observed number of double crossovers by the expected number.

In our textbook's solution, the COC is found to be 1.183, indicating there are more double crossovers occurring than expected by chance. A COC of 1 suggests crossovers are independent of one another, while a COC greater than 1 indicates positive interference, where crossovers promote further crossovers. Conversely, a COC less than 1 signifies negative interference, where one crossover event can inhibit another from occurring nearby.

Chromosomal Crossover

Chromosomal crossover is a critical event during meiosis where two chromosomes exchange segments of their genetic material. This process increases genetic variation within a species and is one of the mechanisms resulting in recombinant offspring—offspring with combinations of traits that differ from those seen in their parents.

In recombination frequency calculations, the physical manifestation of crossovers corresponds with the progeny's phenotypes. Visualize it as the reshuffling of cards in a deck, where each card represents a gene. When chromosomes crossover, the 'cards' get mixed, producing new combinations. These events are not simply random; they are influenced by the distance between genes—the closer two genes are, the less likely they will be separated by a crossover event, and vice versa.