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GENETIC MAPPING IN PLANTS AND ANIMALS
• Genetic mapping is also known as gene mapping
or chromosome mapping
• Its purpose is to determine the linear order of
linked genes along the same chromosome
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Each gene has its
own unique locus
at a particular site
within a
chromosome
simplified genetic linkage map of
Drosophila melanogaster
• Chromosomes are called linkage groups. They
contain a group of genes that are linked
together.
• Genes that are far apart on the same
chromosome may independently assort from
each other. This is due to crossing-over.
• Genetic maps allow us to estimate the relative
distances between linked genes, based on the
likelihood that a crossover will occur between
them.
• Experimentally, the percentage of recombinant
offspring is correlated with the distance between
the two genes.
• If the genes are far apart  many recombinant
offspring
• If the genes are close  very few recombinant
offspring
• One map unit is equivalent to 1% recombination
frequency
The haploid cells contain
the same combination of
alleles as the original
chromosomes
The arrangement of linked
alleles has not been altered
These haploid cells contain a
combination of alleles NOT found
in the original chromosomes
These are
termed
parental or
nonrecombinant
cells
This new combination of
alleles is a result of genetic
recombination
These are termed nonparental or
recombinant cells
• Genetic maps allow us to estimate the relative distances
between linked genes, based on the likelihood that a
crossover will occur between them
• Experimentally, the percentage of recombinant offspring is correlated with
the distance between the two genes
– If the genes are far apart  many recombinant offspring
– If the genes are close  very few recombinant offspring
•
Map distance =
Number of recombinant offspring
Total number of offspring

The units of distance are called map units (mu)
 They are also referred to as centiMorgans (cM)

One map unit is equivalent to 1% recombination frequency
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X 100
Genetic mapping
experiments are
typically accomplished
by carrying out a
testcross
Chromosomes are the
product of a crossover
during meiosis in the
heterozygous parent
A mating between an
individual that is
heterozygous for two or
more genes and one that
is homozygous recessive
for the same genes
Recombinant
offspring are fewer in
number than
nonrecombinant
offspring
•
The data at the bottom of Figure 6.9 can be used to estimate the distance between the two
genes
•
Map distance =
Number of recombinant offspring X 100
Total number of offspring
=
76 + 75
542 + 537 + 76 + 75
X 100
= 12.3 map units

Therefore, the s and e genes are 12.3 map units apart
from each other along the same chromosome
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• The first genetic map was constructed in 1911 by Alfred Sturtevant
– He was an undergraduate who spent time in the laboratory of
Thomas Hunt Morgan
• Sturtevant wrote:
• “In conversation with Morgan … I suddenly realized that the variations in
the length of linkage, already attributed by Morgan to differences in the
spatial orientation of the genes, offered the possibility of determining
sequences [of different genes] in the linear dimension of the chromosome.
I went home and spent most of the night (to the neglect of my
undergraduate homework) in producing the first chromosome map, which
included the sex-linked genes, y, w, v, m, and r, in the order and
approximately the relative spacing that they still appear on the standard
maps.”

Multiple crossovers set a quantitative limit on measurable recombination
frequencies as the physical distance increases
 A testcross is expected to yield a maximum of only 50% recombinant
offspring
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• Genetic maps are useful in many ways
– 1. They allow us to understand the genetic organization
of a particular species
– 2. They can help molecular geneticists to clone genes
– 3. They improve our understanding of the evolutionary
relationships among different species
– 4. They can be used to diagnose, and perhaps,
someday to treat inherited human diseases
– 5. They can help in predicting the likelihood that a
couple will produce children with certain inherited
diseases
– 6. They provide helpful information for improving
agriculturally important strains through selective
breeding programs
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