Question

Two different female Drosophila were isolated, each heterozygousfor the autosomally linked genes black body (b), dachs tarsus (d), and curved wings (c). These genes are in the order \(d-b-c\), with \(b\) closer to \(d\) than to $c .$ Shown in the following table is the genotypic arrangement for each female, along with the various gametes formed by both. Identify which categories are noncrossovers (NCO), single crossovers (SCO), and double crossovers (DCO) in each case. Then, indicate the relative frequency with which each will be produced.

Short answer

Answer: The relative frequencies of NCO, SCO, and DCO for Drosophila are approximately 50%, 50%, and 12.5%, respectively. Note that the actual relative frequencies may vary depending on the distance between genes and other factors involved in crossover events.

Step-by-step solution

Genotypic arrangement for each female

We are given the genotypic arrangement for each female Drosophila and the various gametes formed. Let's represent these arrangements using gene symbols for easier analysis. Female 1: --------- Normal arrangement (N): \(D B C\) Mutant arrangement (M): \(d b c\) Female 2: --------- Normal arrangement (N): \(D B C\) Mutant arrangement (M): \(d b c\)

Determine NCO, SCO, and DCO

To identify the noncrossovers (NCO), single crossovers (SCO), and double crossovers (DCO), we will look for the combinations of genes formed by the gametes for each female. Female 1: --------- NCO: \(DB C\) and \(d b c\) SCO: \(D b C\) and \(d B c\) DCO: \(D B c\) and \(d b C\) Here, NCO represents no exchanges of genes, SCO represents one exchange between \(d\) and \(b\), and DCO represents exchanges between both \(d,b\) and \(b,c\). Female 2: --------- NCO: \(DB C\) and \(d b c\) SCO: \(D b C\) and \(d B c\) DCO: \(D B c\) and \(d b C\) The same combinations are formed for both females as they have the same genotypic arrangements.

Calculate relative frequency

To calculate the relative frequency of NCO, SCO, and DCO, we will compare their occurrence rates to the total number of gametes possible for each female. Given the order of genes as \(d-b-c\), we know that \(b\) is closer to \(d\) than to \(c\). This means that the recombination frequency between \(d\) and \(b\) is smaller than the frequency between \(b\) and \(c\). Combination Rates: ------------------- NCO: \(DB C\) and \(d b c\) (both recombinations not occurring) SCO: \(D b C\) and \(d B c\) (recombination between \(d\) and \(b\)) DCO: \(D B c\) and \(d b C\) (both recombinations occurring) For each combination, the relative frequency can be calculated by dividing the number of occurrences of each gene combination by the total number of possible gametes (assuming all are equally likely). NCO Frequency: \(\frac{1}{4}+\frac{1}{4}=\frac{1}{2}=50\%\) SCO Frequency: \(\frac{1}{4}+\frac{1}{4}=\frac{1}{2}=50\%\) DCO Frequency: \(\frac{1}{16}+\frac{1}{16}=\frac{1}{8}=12.5\%\) (Note: This is an assumption since the actual frequencies depend on the distance between the different genes). In conclusion, the relative frequencies of NCO, SCO, and DCO are approximately 50%, 50%, and 12.5%, respectively. It is important to note that the actual relative frequencies will be affected by various factors, including the distance between genes and the specific processes involved in crossover events.