Question

A backcross was set up between two homozygous laboratory mouse strains \(A\) and \(B,\) with the \(F_{1}\) backcrossed to \(B\). The \(F_{2}\) were typed using SNPs \(x\) and \(y,\) which varied between strains \(A\) and \(B\left(x^{A}, x^{B}, y^{A}, y^{B}\right) .\) Out of 100 mice, 38 were \(x^{A} y^{A}, 40\) were \(x^{B} y^{B}\) 11 were \(x^{A} y^{B},\) and 11 were \(x^{B} y^{A} .\) What is the genetic distance between SNPs \(x\) and \(y ?\)

Short answer

Answer: The genetic distance between SNPs x and y is 22 centiMorgans.

Step-by-step solution

Identify Double Cross-Over Offspring and Total Offspring

In the problem, we are given the number of mice with each genotype: - 38 \(x^{A} y^{A}\) mice - 40 \(x^{B} y^{B}\) mice - 11 \(x^{A} y^{B}\) mice - 11 \(x^{B} y^{A}\) mice Double cross-over offspring are the ones with \(x^{A} y^{B}\) and \(x^{B} y^{A}\) genotypes, which amounts to a total of 22 mice (11 + 11). The total number of offspring in the experiment is 100.

Calculate Recombination Frequency

We will use the recombination frequency formula to calculate the genetic distance between SNPs x and y: $$Recombination\ frequency = \frac{Number\ of\ double\ cross - over\ offspring}{Total\ number\ of\ offspring}$$ Substitute the numbers we found in step 1: $$Recombination\ frequency = \frac{22}{100}$$ Simplify: $$Recombination\ frequency = 0.22$$

Calculate Genetic Distance

Genetic distance, which is measured in centiMorgans (cM), is calculated by multiplying the recombination frequency by 100: $$Genetic\ distance\ (cM) = Recombination\ frequency \times 100$$ Substitute the recombination frequency that we calculated in step 2: $$Genetic\ distance\ (cM) = 0.22 \times 100$$ Simplify: $$Genetic\ distance\ (cM) = 22$$ The genetic distance between SNPs x and y is 22 centiMorgans.

Key concepts

Recombination Frequency

Recombination frequency is a crucial concept in genetics. It describes how often recombination happens between two genetic markers during meiosis. In simpler terms, it shows us the probability that alleles from two loci on a chromosome will be inherited together or separately in offspring.

Recombination occurs during meiosis, where chromosomes exchange segments. This process generates genetic variation in populations. Traditional examples include fruit flies and other organisms where it's been meticulously studied.

In genetic studies like the backcross described, identifying the recombinant offspring (those with new combinations of alleles) is essential. This example shows 22 recombinants out of 100 total offspring. To calculate the recombination frequency, we use:

• Number of recombinants: 22
• Total offspring: 100

The formula is:\[Recombination\ frequency = \frac{22}{100} = 0.22\]This result tells us that there is a 22% chance per meiosis that recombination occurs between these two loci.Understanding recombination frequency helps predict genetic linkage and informs us about the arrangement of genes on chromosomes.

Genetic Distance

Genetic distance estimates how far apart two loci are on a chromosome by using the recombination frequency. This concept is especially significant when creating genetic maps. A genetic map visualizes the order and distance between genes or markers, such as SNPs.

Genetic distance is represented in centiMorgans (cM), a unit named after the geneticist Thomas Hunt Morgan. One centiMorgan represents a 1% recombination frequency. Thus, it's calculated by multiplying the recombination frequency by 100:

• Recombination frequency: 0.22
• Conversion factor: 100

The calculation is simple:\[Genetic\ distance\ (cM) = 0.22 \times 100 = 22\ cM\]This means there is a 22 centiMorgan distance between SNPs x and y.

The utility of genetic distance lies in its ability to reveal whether or not two markers are likely to be inherited together, which is useful for understanding hereditary patterns and identifying genes associated with diseases.

SNP Analysis

Single Nucleotide Polymorphisms (SNPs) are among the most common types of genetic variation present in the genome. They involve a change in just one nucleotide – an example is a substitution of an adenine (A) with a guanine (G). These minute changes are powerful tools in genetic mapping.

SNP analysis is increasingly used in identifying regions of interest on a chromosome that might be associated with traits or diseases. In the mouse backcross example, different SNPs were used to differentiate between offspring genotypes. Such differentiation is crucial for identifying genetic linkages and associations.

Key benefits of SNPs include:

• High abundance: They occur frequently throughout the genome.
• Stability: SNPs are relatively easy to track over generations due to their low mutation rates.

An essential application of SNP analysis is in studies of genetic diversity and personalized medicine, as it can help tailor treatments based on a person's unique genetic makeup.

SNPs also hold potential in phylogenetic studies, where understanding evolutionary relationships is essential. Overall, SNP analysis is a versatile and robust tool in the geneticist's toolkit.