A 2006 genetic study of a large American family (Ikeda et al., 2006) identified genetic linkage between DNA markers on chromosome 11 and the gene producing the autosomal dominant neuromuscular disorder spinocerebellar ataxia type 5 (SCA5). The following lod score data are taken from the 2006 study:                  

Based on the available information, is DNA marker A linked to the gene producing SCA5? Explain your answer.

Verified step by step guidance

1. Understand the concept of a lod score: A lod (logarithm of the odds) score is used in genetic linkage analysis to determine whether two loci (e.g., a gene and a DNA marker) are likely to be linked. A lod score of 3 or higher is considered evidence of linkage, as it indicates a 1000:1 odds in favor of linkage.
2. Examine the lod score data for DNA marker A: The lod scores for DNA marker A at various recombination fractions (θ values) are provided. These values are 0.35 (θ = 0.01), 0.94 (θ = 0.05), 1.07 (θ = 0.10), 0.99 (θ = 0.20), 0.75 (θ = 0.30), and 0.43 (θ = 0.40).
3. Compare the lod scores for DNA marker A to the threshold for linkage: None of the lod scores for DNA marker A reach or exceed the threshold of 3. This suggests that there is insufficient evidence to conclude that DNA marker A is linked to the gene producing SCA5.
4. Consider the implications of the recombination fraction (θ): The recombination fraction represents the likelihood of recombination between two loci. Smaller θ values indicate closer physical proximity. Even at the smallest θ value (0.01), the lod score for DNA marker A is only 0.35, which is far below the threshold for linkage.
5. Conclude based on the data: Based on the lod scores provided, DNA marker A is not linked to the gene producing SCA5, as the lod scores do not meet the threshold of 3 at any recombination fraction.

Key Concepts

Genetic Linkage

Genetic linkage refers to the tendency of genes located close to each other on a chromosome to be inherited together during meiosis. This phenomenon is measured using lod scores, which indicate the likelihood of linkage between a genetic marker and a trait. A higher lod score suggests a stronger linkage, making it easier to identify genes associated with specific disorders.

Lod Score

A lod score (logarithm of the odds) is a statistical measure used to evaluate the probability of genetic linkage between a marker and a trait. A lod score greater than 3 is typically considered evidence of linkage, while a score less than -2 suggests no linkage. In the context of the study, the lod scores for different theta values help determine the strength of the association between DNA markers and SCA5.

Theta (θ) Value

The theta (θ) value represents the recombination frequency between two genetic loci, indicating how often they are separated during meiosis. A lower θ value suggests a closer physical proximity on the chromosome, which increases the likelihood of linkage. In the provided data, analyzing the θ values alongside the lod scores helps assess whether DNA marker A is linked to the SCA5 gene.