What is linkage? How you are going to estimate it? What are the differnt methods of detection of linkage?

  

 

Linkage refers to the phenomenon where genes that are located close to each other on the same chromosome tend to be inherited together during meiosis. This violates Mendel's Law of Independent Assortment, which states that genes on different chromosomes are inherited independently of each other.

 

Estimating Linkage

Linkage is typically estimated by calculating the recombination frequency between two genes. Recombination frequency is the proportion of recombinant offspring (those with a combination of traits not found in either parent) produced in a genetic cross.

 

Steps to Estimate Linkage:

1. Perform a Cross:

·        Cross two individuals that are heterozygous for the genes of interest.

2. Count Offspring:

·        Count the number of offspring with parental and recombinant phenotypes.

3. Calculate Recombination Frequency:
4. Determine Linkage:

• Genes are considered linked if the recombination frequency is less than 50%. A recombination frequency close to 50% indicates that the genes are assorting independently, and they are either on different chromosomes or far apart on the same chromosome.

 

Methods of Detection of Linkage

1. Test Cross (or Backcross)

• Definition: A cross between an individual heterozygous for the traits and an individual homozygous recessive for the same traits.
• Purpose: To observe the phenotypes of the offspring and determine the recombination frequency.
• Example: In Drosophila, a fly with genotype AaBb (heterozygous for two linked genes) is crossed with aabb (homozygous recessive).

2. Two-Point Test Cross

• Definition: A test cross involving two genes.
• Purpose: To calculate the recombination frequency between two genes.
• Example: Crossing AaBb (heterozygous) with aabb (homozygous recessive) and analyzing the phenotypes of the offspring.

3. Three-Point Test Cross

• Definition: A test cross involving three genes.
• Purpose: To determine the order of genes on a chromosome and the distances between them.
• Example: Crossing an individual heterozygous for three linked genes (AaBbCc) with a triple homozygous recessive (aabbcc) and analyzing the offspring.

4. Genetic Mapping

• Definition: The process of determining the order and relative distances between genes on a chromosome based on recombination frequencies.
• Purpose: To create a genetic map that shows the positions of genes relative to each other.
• Example: Using recombination data from multiple two-point and three-point crosses to construct a genetic map.

5. Molecular Markers

• Definition: DNA sequences that are associated with particular genes or traits and can be used to track the inheritance of these genes.
• Purpose: To detect linkage by identifying markers that are inherited together with the gene of interest.
• Example: Restriction fragment length polymorphisms (RFLPs), simple sequence repeats (SSRs), and single nucleotide polymorphisms (SNPs).

6. Linkage Analysis in Pedigrees

• Definition: Studying the inheritance patterns of genes in families to detect linkage.
• Purpose: To identify linked genes by analyzing how traits are inherited across generations.
• Example: Using pedigree charts to track the inheritance of a genetic disorder and determine if it is linked to other traits.

Conclusion

Linkage is the phenomenon where genes close to each other on the same chromosome are inherited together. Estimating linkage involves calculating recombination frequencies through genetic crosses. Various methods, such as test crosses, two-point and three-point test crosses, genetic mapping, molecular markers, and linkage analysis in pedigrees, are used to detect linkage. These techniques are crucial for understanding genetic relationships and for applications in plant breeding, genetic research, and medical genetics.