The Chromosomal Basis of Inheritance

1
The Chromosomal Basis of Inheritance

• Chapter 15

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The Chromosomal Theory of Inheritance

• Mendelian genes have specific loci (positions)
  along chromosomes.
• It is the chromsomes that undergo segregation and
  independent assortment.

3
Fig. 15-2
P Generation
Yellow-round seeds (YYRR)
Green-wrinkled seeds ( yyrr)
y
Y
r
?
R
R
r
Y
y
Meiosis
Fertilization
r
y
R
Y
Gametes
All F1 plants produce yellow-round seeds (YyRr)
F1 Generation
R
R
y
y
r
r
Y
Y
LAW OF INDEPENDENT ASSORTMENT Alleles of genes on
nonhomologous chromosomes assort independently
during gamete formation.
LAW OF SEGREGATION The two alleles for each
gene separate during gamete formation.
Meiosis
R
r
r
R
Metaphase I
Y
y
y
Y
1
1
R
R
r
r
Anaphase I
Y
Y
y
y
Metaphase II
r
r
R
R
2
2
y
Y
y
Y
y
Y
Y
y
y
Y
y
Y
Gametes
r
R
r
r
r
R
R
R
1/4
1/4
1/4
1/4
yR
yr
Yr
YR
F2 Generation
An F1 ? F1 cross-fertilization
3
3
9
3
3
1
4
Thomas Hunt Morgan

• Drosophila melanogaster
• Wild Type- the phenotype for a character most
  commonly observed in natural populations.
• Mutant Phenotypes- traits that alternatives to
  the wild type.
• A student at UK

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Fig. 15-4b
EXPERIMENT
P
?
Generation
F1
All offspring had red eyes
Generation
RESULTS
F2
Generation
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The Chromosomal Basis of Sex Determination
SRY- Sex determining Region of Y -The presence of
this gene on the Y chromosome codes for the
development of testes. In the absence of this
gene, the gonads develop into ovaries
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Sex Linked Genes

• A gene located on either sex chromosome is
  called
• a sex-linked gene
• In humans, sex-linked usually refers to a gene
  on
• the larger X chromosome
• For a recessive sex-linked trait to be
• expressed
• -A female needs two copies of the allele
• -A male needs only one copy of the allele
• Sex-linked recessive disorders are much
• more common in males than in females

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Fig. 15-7
N normal color vision
XnY
XnY
XNXN
XNXn
?
?
XNY
XNXn
?
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Fig. 15-7
XnY
XnY
XNXN
XNXn
?
?
XNY
XNXn
?
Sperm
Sperm
Sperm
Y
Xn
XN
Y
Xn
Y
Eggs
Eggs
XNXN
Eggs
XNXn
XN
XNY
XN
XNY
XNXn
XNY
XN
XNXn
XNY
XnXN
XnY
XnXn
XnY
XN
Xn
Xn
(a)
(b)
(c)
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Morgans Experiments

• Linked Genes- genes located on the same
  chromosome that tend to be inherited together in
  genetic crosses.

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What type of ratios would you expect to see in
the testcross offspring if the genes were
located on different chromosomes? What if they
were located on the same chromosome and
parental alleles are always inherited together?
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Morgans Experiments

• Linked Genes- genes located on the same
  chromosome that tend to be inherited together in
  genetic crosses.
• Genetic Recombination- the production of
  offspring with combinations of traits that differ
  from those found in either parent.

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Parental Types- offspring that inherit a
phenotype that matches one of the parental
phenotypes.
Gametes from yellow-round heterozygous parent
(YyRr)
Yr
YR
yr
yR
Gametes from green- wrinkled homozygous recessive
parent ( yyrr)
yr
yyRr
YyRr
yyrr
Yyrr
Parental- type offspring
Recombinant offspring
Recombinant Types or Recombinants- offspring that
have new combinations of phenotypes.
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Recombination

• Crossing Over- accounts for the recombination of
  linked genes.
• Genetic Map- an ordered list of the genetic loci
  along a particular chromosome.
• Linkage Map- a genetic map based on recombination
  frequencies. Displays order but not precise
  location.
• Distances are expressed in map units- equivalent
  to 1 recombination frequency. (Centimorgans)

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Exit Slip

• In humans, pseudohypertrophic muscular dystrophy
  is a condition in which the muscles gradually
  waste away, ending in death in the early teens.
  This type of muscular dystrophy is dependent
  upon a sex-linked recessive gene that occurs only
  in boys and has never been reported in girls.
  Hypothesize why it does not appear in girls?
• Why does the sex-linked gene for
  pseudohypertrophic muscular dystrophy not become
  eliminated from the human race since all boys
  showing the trait die before reaching maturity?

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Alteration of Chromosome Number

• Nondisjunction- pairs of homologous chromosomes
  do not separate normally during meiosis
• As a result, one gamete receives two of the same
  type of chromosome and the other gamete doesnt
  receive a copy.
• Aneuploidy- a chromosomal aberration in which one
  or more chromosomes are present in extra copies
  or deficient in number
• Polyploidy- a chromosomal alteration in which the
  organism possesses more than two complete
  chromosome sets.

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Fig. 15-13-3
Meiosis I
Nondisjunction
Meiosis II
Nondisjunction
Gametes
n 1
n 1
n 1
n
n
n 1
n 1
n 1
Number of chromosomes
(b) Nondisjunction of sister chromatids in
meiosis II
(a) Nondisjunction of homologous chromosomes
in meiosis I
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Alteration of Chromosome Structure

• Deletion- removes a chromosomal segment
• Duplication- repeats a segment
• Inversion- reverses a segment within a chromosome
• Translocation- moves a segment from one
  chromosome to another

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Exceptions to the Standard Chromosome Theory

• Genomic Imprinting- a variation in phenotype
  depending on whether an allele is inherited from
  the male or female parent.
• Extranuclear Genes- genes outside of the nucleus.
  Some are located in organelles in the cytoplasm.