9.1 Introduction

Learning Objectives

  • Describe what is meant by genetic recombination and relate it to specific stages of meiosis.
  • Identify and use specific notation related to linked genes including the cis and trans configurations.
  • Compare recombination frequencies between completely linked genes vs. those which assort independently.
  • Calculate recombination frequency given experimental data.

As we have already discussed, Mendel reported that the pairs of loci he observed segregated independently of each other; for example, the segregation of seed color alleles was independent of the segregation of alleles for seed shape. This observation was the basis for his Second Law (Independent Assortment), and contributed greatly to our understanding of heredity as single units. However, further research showed that Mendel’s Second Law did not apply to every pair of genes that could be studied. In fact, we now know that alleles of loci located close together on the same chromosome tend to be inherited together. This phenomenon is called linkage, and is a major exception to Mendel’s Second Law of Independent Assortment.

Unlinked genes are on different chromosomes or far apart on the same chromosome, while linked genes are close (enough) together on the same chromosome. The random assortment of the different alleles of genes on different chromosomes depends upon the segregation and independent assortment of the chromosomes during meiosis I. However, genetic recombination of different alleles of genes on the same chromosome can only occur by crossing over. When genes are located physically very near to each other on a particular chromosome, they act as if they are linked and inherited together.

Watch this video, Genetic Linkage, by Steve Baskauf  (2015) on YouTube.

Researchers use linkage to determine the location of genes along chromosomes in a process called genetic mapping. The concept of gene linkage is important to the natural processes of heredity and evolution, as well as our genetic manipulation of crops and livestock.

A horse with Bay Roan Tobiano coat colour which is a mixture of brown and white
Figure 9.1.1 The Coat Colour on This Juvenile Horse is Called “Bay Roan Tobiano.” Bay is the brown base coat colour; Roan is the mixture of white hairs with the base coat, making a ‘foggy’ colour; and Tobiano is the white patches. The genes causing the Roan and Tobiano coat colours, respectfully, are found on the same chromosome and linked. Knowing this, we can predict which coat colour genes are from which parents, and how those genes will be inherited in this horse’s offspring. [Long description]

Media Attribution

Reference

Steve Baskauf, S. (2015, February 16). Genetic linkage (video file). YouTube. https://www.youtube.com/watch?v=iH8b-5BxtuY

Long Description

  • Figure 9.1.1 A juvenile horse with a coat colour called “Bay Roan Tobiano.” Bay is the brown base coat colour; Roan is the mixture of white hairs with the base coat, making a ‘foggy’ colour; and Tobiano is the white patches. The genes causing the roan and tobiano coat colours, respectfully, are found on the same chromosome and linked. Knowing this, we can predict which coat colour genes are from which parents, and how those genes will be inherited in this horse’s offspring. [Back to Figure 9.1.1]

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Introduction to Genetics by Natasha Ramroop Singh, Thompson Rivers University is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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