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11–5 Linkage and Gene Maps
Section 11–5
1 FOCUS
Objectives
I
f you thought carefully about Mendel’s principle of independent assortment as you analyzed meiosis, one question might
have been bothering you. It’s easy to see how genes located on
different chromosomes assort independently, but what about
genes located on the same chromosome? Wouldn’t they generally
be inherited together?
Key Concept
• What structures actually assort
independently?
Vocabulary
11.5.1 Identify the structures
that actually assort
independently.
11.5.2 Explain how gene maps are
produced.
gene map
Gene Linkage
The answer to these questions, as Thomas Hunt Morgan first
realized in 1910, is yes. Morgan’s research on fruit flies led him
to the principle of linkage. After identifying more than 50
Drosophila genes, Morgan discovered that many of them
appeared to be “linked” together in ways that, at first glance,
seemed to violate the principle of independent assortment. For
example, a fly with reddish-orange eyes and miniature wings,
like the one shown in Figure 11–18, was used in a series of
crosses. The results showed that the genes for those traits were
almost always inherited together and only rarely became separated from each other.
Morgan and his associates observed so many genes that
were inherited together that before long they could group all of
the fly’s genes into four linkage groups. The linkage groups
assorted independently, but all of the genes in one group were
inherited together. Drosophila has four linkage groups. It also
has four pairs of chromosomes, which led to two remarkable
conclusions. First, each chromosome is actually a group of
linked genes. Second, Mendel’s principle of independent assortment still holds true.
It is the chromosomes, however,
that assort independently, not individual genes.
How did Mendel manage to miss gene linkage? By luck, or
by design, six of the seven genes he studied are on different
chromosomes. The two genes that are found on the same chromosome are so far apart that they also assort independently.
Reading Strategy:
Predicting Before you read,
preview Figure 11–19. Predict
how a diagram like this one can
be used to determine how likely
genes are to assort independently. As you read, note whether
or not your prediction was
correct.
Vocabulary Preview
Have student volunteers describe
what a map is. Elicit the fact that
maps show the locations of places
and things. Ask: What do you think
a gene map is? (It shows the locations of genes on a chromosome.)
Reading Strategy
왔 Figure 11–18 The genes for this
fruit fly’s reddish-orange eyes and
miniature wings are almost always
inherited together. The reason for
this is that the genes are close
together on a single chromosome.
It is the chromosomes that
assort independently, not individual genes.
Gene Maps
If two genes are found on the same chromosome, does
this mean that they are linked forever? Not at all.
Crossing-over during meiosis sometimes separates
genes that had been on the same chromosome
onto homologous chromosomes. Crossover events
occasionally separate and exchange linked
genes and produce new combinations of alleles.
This is important because it helps to generate
genetic diversity.
As students read, encourage them to
write down the main ideas that lead
them to determine whether or not
their prediction was correct.
2 INSTRUCT
Gene Linkage
Build Science Skills
Using Models Students can construct a model of a chromosome with
beads threaded on a pipe cleaner. The
beads represent genes, and the pipe
cleaner represents the chromosome.
Challenge students to demonstrate
why linked genes do not usually assort
independently. Ask: Could there be
exceptions to this? (Yes, if crossingover occurs.) How could crossingover affect the linked genes of a
fruit fly? (Alleles could be exchanged
between a maternal chromatid and a
paternal chromatid.)
Gene Maps
SECTION RESOURCES
Use Visuals
• Teaching Resources, Lesson Plan 11–5,
Adapted Section Summary 11–5, Section
ve
Summary 11–5, Worksheets 11–5,Sa
Section
e
Review 11–5
• Reading and Study Workbook A, Section 11–5
• Adapted Reading and Study Workbook B,
Section 11–5
• Biotechnology Manual, Lab 2
• iText, Section 11–5
• Transparencies Plus, Section 11–5
Tim
Technology:
r
Print:
Figure 11–19 As students study the
gene map, ask: Would you expect
more crossing-over events to occur
between star eye and speck wing
or between star eye and black
body? Explain. (Star eye and speck
wing; because these genes are located
farther apart, it is more likely that a
crossing-over event will occur between
(continued)
Introduction to Genetics
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Exact location on chromosome
11–5 (continued)
them.) Explain that recombination
rates are calculated by determining
the percentage of recombinants produced in a cross. Recombinant
offspring have a phenotype that is
different from either parent. For
example, in a cross between a
homozygous male with a black body
(bb) and vestigial wings (vv) and a
heterozygous female (BbVv) with a
brown body and normal wings, most
of the F2 offspring will look like either
parent. However some of the offspring, about 20%, will have either a
black body and normal wings or a
brown body and vestigial wings.
3 ASSESS
Evaluate Understanding
Chromosome 2
0.0 Aristaless (no bristles on antenna)
1.3 Star eye
13.0 Dumpy wing
31.0 Dachs (short legs)
0
10
20
30
48.5 Black body
51.0 Reduced bristles
54.5 Purple eye
55.0 Light eye
67.0 Vestigial (small) wing
75.5 Curved wing
99.2 Arc (bent wings)
104.5 Brown eye
107.0 Speck wing
왖
Figure 11–19 This gene map
shows the location of a variety of
genes on chromosome 2 of the fruit
fly. The genes are named after the
problems abnormal alleles cause, not
the normal structure. Interpreting
Graphics Where on the chromosome
is the “purple eye” gene located?
Draw a hypothetical gene map on the
board. Have students tell which genes
would have high frequencies of
crossing-over and which would not.
Reteach
Have students diagram a crossingover event to show how genes that
are located close together have a
lower frequency of recombination
than genes that are located far apart.
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100
110
In 1911, a Columbia University student was working part
time in Morgan’s lab. This student, Alfred Sturtevant, wondered if
rates of crossing-over between genes in meiosis might be a clue to
something important. Sturtevant reasoned that the farther apart
two genes were, the more likely they were to be separated by a
crossover in meiosis. That meant that he could use recombination
frequencies to determine the distances between genes. Sturtevant
gathered up several notebooks of lab data and took them back to
his room. The next morning, he presented Morgan with a gene
map showing the relative locations of each known gene on one of the
Drosophila chromosomes, as shown in Figure 11–19. If two
genes are close together, the recombination frequency between
them should be low, since crossovers are rare. If they are far
apart, recombination rates between them should be high.
Sturtevant’s method has been used to construct genetic maps,
including maps of the human genome, ever since.
11–5 Section Assessment
Paragraphs should explain that if
the genes are usually inherited
together, they are located near
each other on the same chromosome. If they were far apart,
crossing-over events would make
them appear to be located on different linkage groups.
If your class subscribes to the iText,
use it to review the Key Concepts in
Section 11–5.
Answer to . . .
Figure 11–19 The “purple eye” gene
is located at 54.5.
280
Chapter 11
1.
Key Concept How does
the principle of independent
assortment apply to chromosomes?
2. What are gene maps, and how
are they produced?
3. How does crossing-over make
gene mapping possible?
4. Critical Thinking Inferring
If two genes are on the same
chromosome but usually assort
independently, what does that
tell you about how close together
they are?
Cause-Effect Paragraph
In your own words, explain
why the alleles for reddishorange eyes and miniature
wings in Drosophila are
usually inherited together.
Include the idea of gene
linkage. Hint: To organize
your ideas, draw a causeeffect diagram that shows
what happens to the two
alleles during meiosis.
11–5 Section Assessment
1. It is the chromosomes that assort independently, not individual genes.
2. A gene map shows the relative locations of
genes on a chromosome. The frequency of
crossing-over between genes is used to produce a map of distances between genes.
3. The farther apart two genes are, the more
likely they are to be separated during a
crossover in meiosis. Therefore, the frequency
of crossing-over is equal to the distance
between two genes.
4. The two genes are located very far apart
from each other.