Question

In a series of two-point map crosses involving five genes located on chromosome II in Drosophila, the following recombinant \((\sin -\) gle- crossover) frequencies were observed:$$\begin{array}{lr} p r-a d p & 29 \\ p r-v g & 13 \\ p r-c & 21 \\ p r-b & 6 \\ a d p-b & 35 \\ a d p-c & 8 \\ a d p-v g & 16 \\ v g-b & 19 \\ v g-c & 8 \\ c-b & 27 \end{array}$$ (a) If the adp gene is present near the end of chromosome II (locus 83 ), construct a map of these genes. (b) In another set of experiments, a sixth gene ( \(d\) ) was tested against \(b\) and \(p r,\) and the results were \(d-b=17 \%\) and \(d-p r=23 \% .\) Predict the results of two-point maps between \(d\) and \(c, d\) and \(v g,\) and \(d\) and \(a d p\)

Short answer

Question: Construct a genetic map of five genes on chromosome II in Drosophila using the given recombinant frequencies and predict the two-point maps between a sixth gene and the other genes. Answer: The genetic map constructed from the given recombinant frequencies is as follows: pr adp vg c b |----------------|-------------|------------|--------------| 0 29 45 53 80 (map units) Predicted two-point maps between the sixth gene (d) and the other genes: - d-pr: 23 map units - d-adp: 6 map units - d-vg: 22 map units - d-c: 30 map units - d-b: 17 map units

Step-by-step solution

List the genes involved in the given data set

The genes involved in this problem are as follows: 1. pr 2. adp 3. vg 4. c 5. b

Identifying relative positions of other genes with respect to adp gene

We know that the adp gene is at locus 83, and we have recombinant percentages for each adjacent gene pair. We will now determine the order of genes based on these values: pr-adp = 29 pr-vg = 13 pr-c = 21 pr-b = 6 adp-b = 35 adp-c = 8 adp-vg = 16 vg-b = 19 vg-c = 8 c-b = 27 There are different possible orders of genes. One possible genetic order is pr-adp-vg-c-b.

Calculate the distances between gene pairs

Using the recombinant frequencies, we can calculate the map distances between the genes: pr-adp = 29 map units adp-vg = 16 map units vg-c = 8 map units c-b = 27 map units Now, we can map these genes with respect to the adp gene at locus 83.

Construct the genetic map

Based on the calculated map units, we can construct a genetic map: pr adp vg c b |----------------|-------------|------------|--------------| 0 29 45 53 80 (map units)

Predict the map distances for the sixth gene (d) from given data

We are given that d-b = 17% and d-pr = 23%. So, we can predict the possible map relationships for d: distance between d and pr: 23 map units distance between d and b: 17 map units

Predict the two-point maps between d and c, d and vg, and d and adp

From the genetic map of pr-adp-vg-c-b, we can now predict the likely two-point maps between gene d and the existing genes: 1. Predicting d-c distance: If d is between pr and adp, using the genetic map, distance between d and c = (23 + 16) + 8 = 47 map units If d is between adp and vg, using the genetic map, distance between d and c = abs(29-23) + 16 + 8 = 30 map units It is more probable that d is closer to the adp gene, so the predicted distance is d-c = 30 map units 2. Predicting d-vg distance: If d is between pr and adp, using the genetic map, distance between d and vg = abs(29-23) + 16 = 22 map units Otherwise, the distance between d and vg is between d and b (beyond the five genes in the given map). Predicting the latter distance is not possible with the given data, so the predicted distance is d-vg = 22 map units 3. Predicting d-adp distance: From steps 5 and 6, it's likely that d is between pr-adp, so the distance between d and adp is d-adp = abs(29-23) = 6 map units

Key concepts

Genetic Map Distances

Genetic map distances are a way of quantifying the separation between genes on a chromosome. They are measured in map units or centiMorgans (cM), which reflect the frequency of recombination between two genetic markers. A distance of 1 map unit corresponds to a 1% recombinant frequency, meaning there is a 1% chance that a crossover will occur between the two genes during meiosis.

When constructing genetic maps, the distances between genes are estimated by analyzing the offspring of organisms through test crosses. For instance, in Drosophila, which is frequently used for genetic studies due to its well-characterized genome and quick breeding cycle, the offspring can display various phenotypes based on the combination of alleles they receive. From these phenotypic ratios, scientists can deduce the recombinant frequencies and hence calculate the genetic map distances.

Such mapping is crucial in understanding the linkage of traits and can help predict the outcome of genetic crosses. This information is instrumental for both basic genetic research and for practical applications like breeding programs and genetic disorder investigations.

Drosophila Gene Mapping

Drosophila melanogaster, the common fruit fly, is a classic model organism in genetics. Gene mapping in Drosophila involves determining the linear order of genes along a chromosome and their relative distances from one another, using recombination frequencies derived from cross-breeding experiments.

In the exercise, chromosomal positions are deduced by analyzing recombination data from two-point test crosses that involve pairs of genes. The locus of the adp gene is an anchoring point, providing a reference for mapping additional genes. Since genes that are closer together on a chromosome tend to be inherited together more often, their recombinant frequency is lower. Conversely, genes further apart have a higher likelihood of recombination. Given Drosophila's rapid life cycle and the ease of observing its phenotypes, it serves as an excellent organism for conducting genetic mapping studies.

Recombinant Frequencies

Recombinant frequencies are central to genetic mapping; they represent the proportion of offspring with new combinations of genes not found in either parent, as a result of genetic recombination during meiosis.

This occurs when homologous chromosomes exchange equivalent pieces of DNA during crossover events. In the context of our Drosophila exercise, recombinant frequencies are critical inputs for constructing genetic maps. They can be directly converted into map units, thus enabling the construction of a linear model of gene placement along a chromosome.

For instance, if the recombinant frequency between two genes is 8%, this means that in the offspring, 8 out of 100 will show new combinations of those genes' alleles not present in the parents, suggesting those two genes are 8 map units apart. A lower recombinant frequency indicates closer genetic linkage while a higher frequency implies genes are further apart.

Chromosome Mapping

Chromosome mapping, or linkage mapping, is the process of establishing the relative positions of genes on a chromosome. It’s a crucial aspect of genetics that helps in understanding how traits are passed on through generations.

By using recombination frequencies, researchers create a visual representation of a chromosome with markers denoting the position of genes. In our Drosophila example, steps are taken to calculate the distances between the gene pairs using the given recombinant percentages. Subsequently, a diagrammatic model of the chromosome is created – a genetic map that reflects these distances in map units relative to a known locus, in this case, adp at locus 83.

Having such a map is valuable for exploring genetic linkage and can highlight synteny—conserved blocks of genes between different species. This information is beneficial for tracing the inheritance of traits and can also be applied to locating genes that might be responsible for diseases.