Crossing Over
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Transcript Crossing Over
Chromosomes and Human
Genetics
Mendel was unaware of chromosomes
The physical structure of genes was unknown
Chromosomal Theory of Inheritance
Genes
are on the chromosomes
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
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Homologous Chromosomes
Chromosomes that pair up during meiosis
Contain
the same genes
May
have different alleles of these genes
One
came from each parent
Each is one long DNA molecule
A
gene is a short region of the molecule
Each
chromosome can have > 1,000 genes
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
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Homologous
Chromosomes
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DISCOVER BIOLOGY 3/e
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Sex Determination
22 pairs of chromosomes are homologous
Called
autosomes
Females have 2 X chromosomes (XX)
Males have X and Y chromosomes (XY)
Y
is very small
Has
few genes (~80) mostly dealing with
maleness
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
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Sex Determination
During reproduction, males determine sex
If
the sperm has an X, it’s a girl
If the sperm has a Y, boy
Y chromosome has the SRY gene
SRY is a master switch that controls
other genes at other loci to develop the
embryo into a boy
Males only have one X chromosome
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
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Linkage and Crossing Over
Linkage: 2 genes on the same
chromosome
Do not sort Independently!
Segregate together on the same chromosome
Exception: Crossing Over
Linked genes should segregate together
Crossing over causes them to separate
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
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Linkage and Crossing Over
Test cross of Linkage
Independent Assortment
predicts ¼ of each
phenotype
Results are much
different
These genes are linked
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
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Linkage and Crossing Over
One would think that linked genes should
always sort together
One expect to see only two phenotypes
Both
recessive or both dominant traits
50% of each as there are 2 chromosomes
Crossing over changes this result
Results
are usually in between total linkage
and independent assortment.
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
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Crossing Over
Meiosis
Before Meiosis
Gametes produced
Homologous chromosomes line up during meiosis
Parts of maternal and paternal chromosomes exchange
The chance that this crossing over occurs increases with
the distance between the two linked loci.
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
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Human Genetic Variation
F1 aren’t identical to sexually reproducing P
They differ with respect to their genes
Variation is important for populations
Variation is produced in 4 ways:
Mutation (creates new genetic variation)
Independent assortment (sorts preexisting
genes)
Crossing over (increases the sorting)
Fertilization (sexual reproduction shuffles the
genes in eggs & sperm & then 2 parents join to
form an offspring)
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
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Meiosis and Independent
Assortment Generate Variation
Each homologous chromosome independently
sorts
23 pairs of chromosomes
223 or 8,388,608 possible gametes
Fertilization: 223 X 223 possible combinations
Over 64 trillion possible offspring
Crossing over increases this!
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
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Human Genetic Disorders
Many diseases are genetically inherited
Studied through Pedigree analyses
Show disease
patterns in
families
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
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Autosomal Recessive Diseases
Represents several thousand disorders
Can be mild or lethal
Usually both parents are Heterozygous
(Aa) and don’t show the phenotype
Two carriers have ¼ chance of producing
affected offspring
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
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Autosomal Dominant Diseases
One copy of this gene causes the disease
More rare than recessive diseases
Affected individuals may not survive to mate
Recur due to new mutations
Some diseases express late in life
More common; individuals can reproduce
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
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Sex-linked
Diseases
Genes on the X
chromosome
Mothers who have a
recessive gene pass it on
50% of sons are affected
(only one X)
Females rarely get these
recessive diseases as
they need to copies
Hemophilia, color blind
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
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Chromosomal Abnormalities
Some disorders are caused by changes in
chromosomes
Less common – usually lethal
Two main types:
Physical changes in chromosome structure
Changes in chromosome number
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
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Changes in Structure
There are four ways the structure can change
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DISCOVER BIOLOGY 3/e
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Changes in Chromosomal
Number
Errors in meiosis result in extra
chromosomes
Most zygotes are not viable
Down’s syndrome – trisomy 21
An extra copy of chromosome 21
Live to adulthood, but with developmental
problems
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
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Sex Chromosomal Number
Extra X and Y chromosomes are possible
XXY
or XYY = males
XXX, XXXX, or X0 (have one X) = females
Viable but usually sterile
Can
have developmental difficulties
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DISCOVER BIOLOGY 3/e
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Concept Quiz
A.
B.
C.
D.
Which of the following is not true of
homologous chromosomes?
They contain the same alleles.
They contain the same genes.
One came from each parent.
Each is duplicated during replication.
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
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Concept Quiz
Genes that are linked on the same
chromosome always segregate together.
True or False?
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
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Concept Quiz
Autosomal dominant disorders
A. Are carried on the X chromosome.
B. Often express late in life.
C. Are spread by carriers (heterozygotes)
mating.
D. Are more common than recessive
disorders.
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
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Cri du Chat
The results of a
chromosomal deletion
Affected individuals have
severe mental retardation
© 2006 W.W. Norton & Company, Inc.
DISCOVER BIOLOGY 3/e
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