Download 11-5 Linkage and Gene Maps

Biology
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11-5 Linkage and Gene Maps
11-5 Linkage and Gene Maps
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11-5 Linkage and Gene Maps
It’s easy to see how genes
located on the same
chromosome assort
independently, but what
about genes located on the
same chromosome?
Wouldn’t they generally be
inherited together?
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11-5 Linkage and Gene Maps
Gene Linkage
What structures actually assort
independently?
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11-5 Linkage and Gene Maps
Gene Linkage
Gene Linkage
Thomas Hunt Morgan’s research on fruit flies led him to the principle of
linkage.
Morgan discovered that many of the more than 50 Drosophila genes he
had identified appeared to be “linked” together.
They seemed to violate the principle of independent assortment.
Example:
Fruit flies with reddish-orange eyes and miniature wings
Genes for these traits were almost inherited together
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11-5 Linkage and Gene Maps
Gene Linkage
Morgan and his associates grouped the linked genes into four linkage
groups.
Each linkage group assorted independently but all the genes in one group
were inherited together.
Each chromosome is actually a group of linked genes.
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11-5 Linkage and Gene Maps
Gene Linkage
Morgan concluded that Mendel’s principle of independent
assortment still holds true.
Chromosomes assort independently,
not individual genes.
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11-5 Linkage and Gene Maps
The Chromosomal Basis of Sex
• In humans and other mammals there are
two varieties of sex chromosomes, designated
X and Y.
•
The Y chromosome is much smaller than the X chromosome.
• A person who inherits two X chromosomes develops as female.
• A male develops from a zygote containing one X and one Y
chromosome.
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11-5 Linkage and Gene Maps
The Chromosomal Basis of Sex
• In both testes and ovaries, the two sex chromosomes segrgate during
meiosis, and each gamete receives one.
• Each egg contains one X chromosome.
• Half the sperm cells contain a Y chromosome and the other half
contain an X chromosome.
• Therefore, sex determination is a matter of chance ( a fifty-fifty chance).
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11-5 Linkage and Gene Maps
Inheritance of Sex-Linked Genes
• In addition to their role as carriers of genes that determine sex, the sex
chromosomes have many characters unrelated to sex.
• A gene located on either sex chromosome is called a sex-linked
chromosome.
• Fathers pass sex-linked alleles to all of their daughters but to none of
their sons.
• Mothers pass sex-linked alleles to both sons and daughters.
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11-5 Linkage and Gene Maps
Inheritance of Sex-Linked Genes
• If a sex-linked allele is recessive, a female will express the phenotype
only if she is homozygous.
• Any male receiving the recessive allele from his mother will express
the trait.
• More males have sex-linked recessive disorders.
Example:
Color-blindness
Color-blind fathers pass it on to daughters
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11-5 Linkage and Gene Maps
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11-5 Linkage and Gene Maps
Gene Maps
Gene Maps
Crossing-over during meiosis sometimes separates genes that had
been on the same chromosomes onto homologous chromosomes.
Crossover events occasionally separate and exchange linked genes
and produce new combinations of alleles.
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11-5 Linkage and Gene Maps
Gene Maps
• Alfred Sturtevant, a student of Morgan, reasoned that the farther apart
two genes were, the more likely they were to be separated by a
crossover in meiosis.
• Recombination frequencies can be used to determine the distance
between genes.
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11-5 Linkage and Gene Maps
Gene Maps
• Sturtevant created a gene map showing the relative locations of each
known gene on one of the Drosophila chromosomes.
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11-5 Linkage and Gene Maps
Gene Maps
• 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.
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11-5 Linkage and Gene Maps
Gene Maps
Exact location on chromosome
0.0
Chromosome 2
Aristaless (no bristles on antenna)
13.0 Dumpy wing
48.5 Black body
54.5 Purple eye
67.0 Vestigial (small) wing
99.2 Arc (bent wings)
107.0 Speck wing
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11-5 Linkage and Gene Maps
Gene Maps
Exact location on chromosome
Chromosome 2
1.3 Star eye
31.0 Dachs (short legs)
51.0 Reduced bristles
55.0 Light eye
75.5 Curved wing
104.5 Brown eye
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11-5 Linkage and Gene Maps
Advanced Genetics Video
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According to Mendel's principle of independent
assortment, the factors that assort
independently are the
a. genes.
b. chromosomes.
c. chromatids.
d. gametes.
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A chromosome is actually a group of
a. independent genes.
b. linkage groups.
c. crossed-over genes.
d. linked genes.
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Thomas H. Morgan is credited with the discovery
of the principle of
a. segregation.
b. independent assortment.
c. gene linkage.
d. dominance.
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Linkage maps can be produced because the
farther apart two genes are on a chromosome,
a. the less likely they are to assort
independently.
b. the more likely they are to be linked.
c. the more likely they are to be separated by a
crossover.
d. the less likely they are to be separated by a
crossover.
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If two genes are close together on the same
chromosome, they are more likely to
a. behave as though they are linked.
b. behave independently.
c. move to different chromosomes.
d. belong to different linkage groups.
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