Question

A cross is made between two strains of Neurospora: one strain requires thiamine in its medium (thi), and the other strain grows in buttonlike colonies (but). The following data is collected: (a) Are these genes linked? (b) Determine the sequence of the points: centromere, thi locus, but locus. (c) Calculate the map distances.

Short answer

The short version of the answer is: 1. Calculate the recombination frequency using the formula: Recombination frequency = ((N3 + N4) / (N1 + N2 + N3 + N4)) * 100 2. Compare the calculated recombination frequency with 50% to determine if the thi and but genes are linked. 3. Find a centromere-proximal and a centromere-distal marker to determine the sequence of the centromere, thi locus, and but locus. 4. Calculate the map distances using the recombination frequencies as map units or centimorgans (cM): Map distance (in centimorgans) = recombination frequency.

Step-by-step solution

Tabulate the given data

To start with the analysis, let's first organize the given data in the form of a table: | Class | Progeny | |-------------|---------| | Parental 1 | N1 | | Parental 2 | N2 | | Recombinant 1 | N3 | | Recombinant 2 | N4 | Note that the actual values for N1, N2, N3, and N4 are not given in the exercise prompt, but the methodology described below can be applied once the values are provided.

Calculate the recombination frequency

The recombination frequency is the proportion of recombinant progeny to the total progeny. We need to calculate it to determine whether the two genes are linked or not. The formula for calculating the recombination frequency is: Recombination frequency = (Number of recombinant progeny / Total progeny) * 100 For our data: Recombination frequency = ((N3 + N4) / (N1 + N2 + N3 + N4)) * 100

Determine if the genes are linked or not

If the recombination frequency is significantly lower than the expected 50% for unlinked genes, it indicates that the genes are linked and are on the same chromosome. Compare the calculated recombination frequency with 50% to determine if the thi and but genes are linked.

Determine the sequence of the centromere and the gene loci

To determine the sequence of the centromere, thi locus, and but locus, we need to find a centromere-proximal and a centromere-distal marker. The centromere-proximal marker corresponds to the allele that is closer to the centromere, whilst the centromere-distal marker corresponds to the allele that is farther from the centromere. Look for the marker that creates frequent recombinants with the thi and but loci. This will give us the location of the centromere in our sequence.

Calculate the map distances

To calculate the map distances, we can use the recombination frequency as an estimate of map units or centimorgans (cM). The larger the recombination frequency between two loci, the farther apart they are on the chromosome. Map distance (in centimorgans) = recombination frequency Use the recombination frequencies between the centromere and the thi and but loci, calculated previously, to compute the map distances.

Key concepts

Neurospora genetics

Neurospora, a model organism in genetics, is a genus of ascomycete fungi. This bread mold has been instrumental in advancing our understanding of genetic principles. One of the key reasons for using Neurospora in genetic studies is its simple life cycle and the ease with which one can observe genetic mutations and map genes.

For instance, genetic markers, such as those needing thiamine (thi) or producing button-like colonies (but), are used to investigate the inheritance patterns in crosses between different strains. By analyzing the progeny of these crosses, scientists can determine how traits pass from one generation to the next, offering a window into more complex genetic mechanisms in other organisms.

Recombination frequency

Recombination frequency is a measure that reflects the likelihood of a crossover event occurring between two gene loci during meiosis. This is crucial because it indicates how often alleles from different genes are inherited together. In essence, the smaller the recombination frequency, the closer together the genes are likely to be on the chromosome, suggesting linkage.

It's computed as the ratio of recombinant progeny (the offspring showing different combinations of traits from the parents) to total progeny, expressed as a percentage. This measure is a fundamental tool used in constructing genetic maps and conducting linkage analysis.

Genetic map distance

The genetic map distance refers to the relative physical distance between two gene loci on a chromosome. Geneticists use the concept of map distance to visually represent the location of genes and their separation on a 'genetic map'.

The greater the recombination frequency, the greater the map distance between two loci. This is due to the increased likelihood of crossover events occurring between distant loci. It is important to highlight that genetic map distances are not always a perfect representation of physical distances on DNA, as some regions are more prone to recombination than others.

Gene loci

The term 'loci' (singular: locus) refers to the specific, fixed positions on a chromosome where genes reside. Each gene locus is the address of a gene, determining where it is in the context of the genome. When conducting genetic linkage analysis, like in the Neurospora example, scientists look at these loci to determine the relationship between different genes.

Understanding the relationship between gene loci can help predict the inheritance patterns of certain traits and is foundational to genetics and heredity studies. Gene loci are the landmarks used to create genetic maps, which help researchers find genes responsible for particular phenotypes or diseases.

Centimorgans

Centimorgans (cM) are the units of measurement for genetic map distance. Named after the geneticist Thomas Hunt Morgan, centimorgans quantitatively describe the distance between gene loci based on the frequency of recombination between them.

One centimorgan is equivalent to a 1% chance that a marker at one genetic locus will be separated from a marker at a second locus due to recombination in a single generation. In the context of the Neurospora example, after calculating recombination frequency, it can be directly converted into centimorgans to ascertain how far apart the thi and but loci are on the chromosome, aiding in the creation of a genetic map.