Genetic Linkage
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
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Printed: November 13, 2015
AUTHORS
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
www.ck12.org
C HAPTER
Chapter 1. Genetic Linkage
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Genetic Linkage
• Explain linked genes and sex-linked genes.
• Summarize the relationship between linkage and crossing-over.
• Explain linkage and linkage maps.
What does it mean to be linked?
For a pair of hands, the above image may suggest a certain type of linkage. For genes, it might suggest that they are
very hard to separate.
Linkage
Genes that are located on the same chromosome are called linked genes. Alleles for these genes tend to segregate
together during meiosis, unless they are separated by crossing-over. Crossing-over occurs when two homologous
chromosomes exchange genetic material during meiosis I. The closer together two genes are on a chromosome,
the less likely their alleles will be separated by crossing-over. At the following link, you can watch an animation
showing how genes on the same chromosome may be separated by crossing-over: http://www.biostudio.com/d_%
20Meiotic%20Recombination%20Between%20Linked%20Genes.htm .
Linkage explains why certain characteristics are frequently inherited together. For example, genes for hair color and
eye color are linked, so certain hair and eye colors tend to be inherited together, such as blonde hair with blue eyes
and brown hair with brown eyes. What other human traits seem to occur together? Do you think they might be
controlled by linked genes?
Sex-Linked Genes
Genes located on the sex chromosomes are called sex-linked genes. Most sex-linked genes are on the X chromosome, because the Y chromosome has relatively few genes. Strictly speaking, genes on the X chromosome are
X-linked genes, but the term sex-linked is often used to refer to them.
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Sex-linked traits are discussed at http://www.youtube.com/watch?v=-ROhfKyxgCo (14:19).
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/287
Mapping Linkage
Linkage can be assessed by determining how often crossing-over occurs between two genes on the same chromosome. Genes on different (nonhomologous) chromosomes are not linked. They assort independently during meiosis,
so they have a 50 percent chance of ending up in different gametes. If genes show up in different gametes less than
50 percent of the time (that is, they tend to be inherited together), they are assumed to be on the same (homologous)
chromosome. They may be separated by crossing-over, but this is likely to occur less than 50 percent of the time.
The lower the frequency of crossing-over, the closer together on the same chromosome the genes are presumed to
be. Frequencies of crossing-over can be used to construct a linkage map like the one in Figure 1.1. A linkage map
shows the locations of genes on a chromosome.
FIGURE 1.1
Linkage Map for the Human X Chromosome. This linkage map shows
the locations of several genes on the X chromosome. Some of the genes
code for normal proteins. Others code for abnormal proteins that lead to
genetic disorders. Which pair of genes would you expect to have a lower
frequency of crossing-over: the genes that code for hemophilia A and
G6PD deficiency, or the genes that code for protan and Xm?
Summary
• Linked genes are located on the same chromosome.
• Sex-linked genes are located on a sex chromosome, and X-linked genes are located on the X chromosome.
• The frequency of crossing-over between genes is used to construct linkage maps that show the locations of
genes on chromosomes.
Explore More
Explore More I
Use these resources to answer the questions that follow.
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Chapter 1. Genetic Linkage
• Recombination and Estimating the Distance Between Genes at http://www.ndsu.edu/pubweb/~mcclean/
plsc431/linkage/linkage2.htm .
1.
2.
3.
4.
What is recombination?
What determines the amount of recombination between two genes?
What are recombinant gametes?
What is a centimorgan?
Explore More II
• T. H. Morgan at http://www.dnalc.org/resources/nobel/morgan.html .
Review
1.
2.
3.
4.
What are linked genes?
Explain how you would construct a linkage map for a human chromosome. What data would you need?
People with red hair usually have very light skin. What might be a genetic explanation for this observation?
How often does crossing-over occur between non-linked genes? Explain your answer.
References
1. Sam McCabe. Linkage map for the human X chromosome . CC BY-NC 3.0
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