2.2 Two Loci on Different Chromosomes

The separation of gametes through the process of meiosis has already been introduced. But what does that mean when you are taking multiple, different genes (or loci) into account?

Remember the main stages of meiosis. The homologous pairs align during Metaphase I, and complete one round of cell division. Then, during Metaphase II, the replicated chromosomes in those two cells align individually and the sister chromatid separate. So when complete, you have two daughter cells. Let’s say one chromosome has gene A on it, and another chromosome has gene B on it, and the individual is heterozygous at each gene (a.k.a. has the genotype A/a ; B/b). There are a variety of ways that the homologous pairs can align themselves during metaphase I. The orientation of that alignment will affect the alleles each gamete receives at the end of telophase II (Figure 2.2.1).

Because the alignment at Metaphase I is always random, you will see a random, equal distribution of alleles in all the gametes produced. This means that one allele doesn’t affect the distribution of another allele, or in other words, each allele assorts independently (Independent Assortment).

 

Independent assortment as seen on two different chromosomes. coloured blue and green
Figure 2.2.1 Independent Assortment as Seen on Two Different Chromosomes. Gene A is found on the short chromosome and Gene B is found on the long chromosome, and both genes are heterozygotes for the dominant (A and B) and recessive (a and b) alleles. The orientation that the chromosomes align themselves during metaphase I affect the alleles found in the 4 gametes produced after telophase II. These are just two of many orientations the chromosomes can arrange themselves in at metaphase I. The full stages of meiosis were removed for simplicity; refer to Chapter 1 to understand the divisions that lead to the 4 gametes seen in telophase II.

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Reference

Canham, L. (2017). Figure 3. Independent assortment as seen on two different chromosomes [digital image]. In Locke, J., Harrington, M., Canham, L. and Min Ku Kang (Eds.), Open Genetics Lectures, Fall 2017 (Chapter 17, p. 2). Dataverse/ BCcampus. http://solr.bccampus.ca:8001/bcc/file/7a7b00f9-fb56-4c49-81a9-cfa3ad80e6d8/1/OpenGeneticsLectures_Fall2017.pdf

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Open 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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