Multiple Cross Overs and Interference: Videos & Practice Problems

The product law in genetics is used to calculate the likelihood of multiple independent events occurring together. To find the frequency of double crossovers, you multiply the individual crossover probabilities. For example, if the crossover frequency between genes A and B is 20% (0.2) and between genes B and C is 30% (0.3), the frequency of a double crossover is calculated as follows:

$0.2\times 0.3\times 100=6\%$

This means the expected frequency of a double crossover is 6%.

Genetic interference occurs when a crossover in one region of a chromosome affects the likelihood of a crossover in an adjacent region. It is calculated using the coefficient of coincidence (COC), which is the ratio of observed double recombinants to expected double recombinants. The formula for interference (I) is:

$I=1-\frac{\mathrm{Observed}}{\mathrm{Expected}}$

For example, if you observe 4 double recombinants but expect 6, the interference is:

$1-\frac{4}{6}=0.33\times 100=33\%$

This means there were 33% fewer double crossovers than expected.

Multiple crossovers are more difficult to detect in genetic mapping because they occur less frequently and can mask each other. A single crossover changes the genotype of a gamete, but multiple crossovers can reverse these changes, making it hard to identify the events without knowing the gene order. Accurate detection often requires extensive data and careful analysis. Additionally, double crossovers can be mistaken for no crossover events if not properly accounted for, complicating the calculation of recombination frequencies and gene mapping.

The coefficient of coincidence (COC) measures the actual occurrence of double crossovers compared to the expected occurrence. It is calculated as:

$\frac{\mathrm{Observed}}{\mathrm{Expected}}$

Genetic interference (I) is then determined using the formula:

$I=1-\frac{\mathrm{Observed}}{\mathrm{Expected}}$

If the observed number of double crossovers is less than expected, interference is positive, indicating that a crossover in one region reduces the likelihood of another nearby. Conversely, if the observed number is higher, interference is negative.

Calculating recombination frequency is crucial in genetic mapping as it helps determine the distance between genes on a chromosome. Recombination frequency is the proportion of recombinant offspring produced in a cross, reflecting the likelihood of crossover events between genes. A higher recombination frequency indicates genes are further apart, while a lower frequency suggests they are closer. This information is used to create genetic maps, which are essential for understanding gene linkage, inheritance patterns, and for identifying the location of genes associated with specific traits or diseases.