Linkage and Crossing Over

Easily Learn about Crossing Over, Linkage and Genetic Mapping

Linked Genes

1. Why is Mendel’s Second Law not always valid for two or more phenotypical traits of an individual?

Mendel’s Second Law, or the law of the independent assortment, is valid for genes located in different chromosomes. These genes segregate independently during meiosis.

However, Mendel’s Second Law is not valid for phenotypical features conditioned by genes located in the same chromosome (genes under linkage), since these genes, known as linked genes, do not separate during meiosis (except for the phenomenon of crossing over).

Linkage and Crossing Over - Biology Questions and Answers

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2. Why is drosophila a convenient animal for studying linked genes?

The fruit fly, or drosophila, is suitable for studying genetics because it presents many distinct traits but only has four chromosomes (one sex chromosome and three autosomes). 

Linkage Definition

3. What is linkage?

Two genes are said to be under linkage, or linked, when they are located on the same chromosome.

For example, research on the human genome discovered that the gene for factor III of clotting gene and the gene for factor V of clotting are located on the same chromosome (the human chromosome 1). However, the factor VII gene is not linked to those genes, since it is located on chromosome 13.

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Crossing Over Definition

4. What is crossing over? How is meiosis related to this phenomenon?

Linked alleles, for example, A-b and a-B, form the gametes A-b and a-B, which maintain the linkage of the alleles. This type of linkage is called complete linkage. However, in the first division of meiosis (meiosis I), the crossing over phenomenon may occur. Chromosomes from a pair of homologous chromosomes may exchange ends and certain once-linked alleles, for example, A-b and a-B, recombine to form different gametes, in this case, A-B and a-b.

Crossing over may happen when the arms of the chromatids of each homologous chromosomes are paired during meiosis. Matching portions of the ends of two non-sister chromatids (one from one homologous chromosome of the pair) break off and the pieces are exchanged, each of them becoming part of the arm of the other chromatid. For example, if the allele A is situated to one side of the arm relating to the point of breaking and the allele b is located on the other side, they will be separated and gametes A-B and a-b will be formed, instead of A-b and a-B.

(The percentage of recombinant gametes compared to normal gametes depends on the crossing over rate, which in turn depends on how far apart the given alleles are in the chromosome.)

  • Linkage and Crossing Over Review - Image Diversity: crossing over

5. In genetic recombination by crossing over, what is the difference between parental gametes and recombinant gametes?

Parental gametes are the gametes that maintain the original linkage of genes (alleles) in the chromosome. Recombinant gametes are those in which the original linkage is undone due to exchange of chromosomal pieces via crossing over during meiosis.

Recombination Frequency and Genetic Mapping

6. What is recombination frequency?

Recombination frequency, or the crossing over rate, is the percentage of recombinant gametes produced via crossing over (in relation to the number of parental gametes produced). It always refers to two genes located in the same chromosome.

7. Why does the recombination frequency of genes vary depending on the distance between them in the chromosome?

The larger the distance between the loci of two genes in a chromosome, the higher the recombination frequency between these genes. This is true because when alleles are closer together within the chromosome, it is more probable that they will be maintained united when chromosomal ends are exchanged by crossing over. On the other hand, if they are farther apart, it will be easier for them to separate by crossing over.

8. What is a centimorgan?

A centimorgan, or recombination unit, by convention is the distance between two linked genes that corresponds to 1% of the recombination frequency of these genes.

9. How can the concept of recombination frequency be used in genetic mapping?

Genetic mapping is the determination of the location of genes on a chromosome.

By determining the recombination frequency between several different linked genes, it is possible to estimate the distance between them on the chromosome. For example, if a gene A has a recombination frequency of 20% with gene B, gene B has a recombination frequency of 5% with  gene C, and gene C has a recombination frequency of 15% with gene A, it is possible to determine that gene A is located at a distance of 20 centimorgans from gene B and that between them lies gene C, located at a distance of 15 centimorgans from gene A.

Crossing Over and Evolutionary Diversity

10. Is crossing over important for the diversity of biological evolution?

Sexual reproduction and the recombination of linked genes (crossing over) are, along with mutations, the main instruments of biological variability. Sexual reproduction allows many combinations between genes located in different chromosomes. Crossing over, however, is the only means to provide the recombination of alleles located on the same chromosome. Crossing over probably emerged and has been maintained by evolution because of its importance for biological diversity.

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