Download Slide 1

Beyond Mendel
Rediscovery of Mendel’s Work
Carl Correns
Erich von Tschermak
Hugo De Vries
Chromosomal theory of inheritance
Walter Sutton
Theodor Boveri
Chromosomal theory of inheritance
• Mendelian genes have specific loci
(positions) along chromosomes and it is
the chromosomes that undergo
segregation and independent assortment.
Chromosomal theory of inheritance
Figure 15.2
P Generation
Yellow-round
seeds (YYRR)
Y
Y
Green-wrinkled
seeds (yyrr)
ry

R R
r
y
Meiosis
Fertilization
y
R Y
Gametes
r
All F1 plants produce
yellow-round seeds (YyRr).
F1 Generation
R
y
r
Y
R
r
Y
y
Meiosis
LAW OF SEGREGATION
The two alleles for each
gene separate during
gamete formation.
r
R
r
R
Y
y
LAW OF INDEPENDENT
ASSORTMENT Alleles of genes
on nonhomologous chromosomes
assort independently during
gamete formation.
Metaphase I
Y
y
1
1
R
r
r
R
Y
y
Anaphase I
Y
y
R
r
Y
y
r
R
Y
y
2
2
Gametes
R
R
1/
4
YR
F2 Generation
3
y
Y
Y
Fertilization recombines
the R and r alleles at
random.
Metaphase II
r
1/
4
Y
Y
y
r
r
r
1/
yr
4
y
y
R
R
1/
Yr
4
yR
An F1  F1 cross-fertilization
3
9
:3
:3
:1
Fertilization results in the
9:3:3:1 phenotypic ratio
in the F2 generation.
Figure 15.2a
P Generation
Yellow-round
seeds (YYRR)
Y
Y
Green-wrinkled
seeds (yyrr)

R R
r
y
y
r
Meiosis
Fertilization
Gametes
R Y
y
r
Figure 15.2b
All F1 plants produce
yellow-round seeds (YyRr).
F1 Generation
R
y
r
R
y
r
Y
Y
LAW OF INDEPENDENT
ASSORTMENT Alleles of
genes on nonhomologous
chromosomes assort
independently during gamete
formation.
Meiosis
LAW OF SEGREGATION
The two alleles for each
gene separate during
gamete formation.
r
R
Y
y
r
R
Metaphase I
y
Y
1
1
R
r
r
R
Y
y
Anaphase I
Y
y
r
R
2
y
Y
Y
R
R
1/
4
YR
r
1/
4
yr
y
Y
Y
Y
y
r
R
2
y
Y
Gametes
r
Metaphase
II
r
r
1/
4
Yr
y
y
R
R
1/
4
yR
Figure 15.2c
LAW OF INDEPENDENT
ASSORTMENT
LAW OF SEGREGATION
F2 Generation
3 Fertilization
recombines the
R and r alleles
at random.
An F1  F1 cross-fertilization
9
:3
:3
:1
3 Fertilization results
in the 9:3:3:1
phenotypic ratio in
the F2 generation.
Thomas
Hunt
Morgan
The common fruit fly – Drosophila melanogaster
Red eye – the “wild type”
White eye – a mutant
Drosophila melanogaster
EXPERIMENT
P
Generation
F1
Generation
All offspring
had red eyes.
RESULTS
F2
Generation
CONCLUSION
P
Generation
X
X
w
X
Y
w
w
Eggs
F1
Generation
Sperm
w
w
w
w
w
w
Eggs
F2
Generation
w
Sperm
w
w
w
w
w
w
w
EXPERIMENT
P
Generation
F1
Generation
RESULTS
F2
Generation
All offspring
had red eyes.
CONCLUSION
P
Generation
X
X
w
X
Y
w
w
Eggs
F1
Generation
Sperm
w
w
w
w
w
Eggs
F2
Generation
w
w
Sperm
w
w
w
w
w
w
w
Human x and y chromosomes
X
Y
44 
XY
44 
XX
Parents
22 
22 
X or Y
22 
X
Sperm
Egg
44 
XX
or
44 
XY
(a) The X-Y system Zygotes (offspring)
22 
XX
22 
X
76 
ZW
76 
ZZ
32
(Diploid)
16
(Haploid)
(b) The X-0 system
(c) The Z-W system
(d) The haplo-diploid system
Sex determination in Humans
• In humans, the anatomical signs of sex first appear when the
embryo is about two months old.
• In individuals with the SRY gene (sex-determining region of
the Y chromosome), the generic embryonic gonads are
modified into testes.
– Activity of the SRY gene triggers a cascade of
biochemical, physiological, and anatomical features
because it regulates many other genes.
– In addition, other genes on the Y chromosome are
necessary for the production of functional sperm.
• In individuals lacking the SRY gene, the generic embryonic
gonads develop into ovaries.
XNXN
Sperm Xn
XNXn
XnY
Sperm XN
Y
XNY
XNXn
Sperm Xn
Y
XnY
Y
Eggs XN
XNXn XNY
Eggs XN
XNXN XNY
Eggs XN
XNXn XNY
XN
XNXn XNY
Xn
XNXn XnY
Xn
XnXn XnY
(a)
(b)
(c)
Transmission of sex-linked recessive traits:
a) Father with trait passes trait to all daughters - carriers
b) Female carrier passes trait to half her sons and daughters
c) Female carrier mates with male with trait – half of offspring will have trait, half of
daughters will be carriers, half of males will be free of trait
Duchenne Muscular Dystrophy
Duchenne muscular dystrophy
• Duchenne muscular dystrophy affects one in
3,500 males born in the United States.
– Affected individuals rarely live past their early
20s.
– This disorder is due to the absence of an X-linked
gene for a key muscle protein, called dystrophin.
– The disease is characterized by a progressive
weakening of the muscles and a loss of
coordination.
Dystrophin muscle complex
X Inactivation
• Although female mammals inherit two X chromosomes, only
one X chromosome is active.
• Therefore, males and females have the same effective dose
(one copy ) of genes on the X chromosome.
– During female development, one X chromosome per cell
condenses into a compact object, a Barr body.
– This inactivates most of its genes.
• The condensed Barr body chromosome is reactivated in
ovarian cells that produce ova.
• Mary Lyon, a British geneticist, has demonstrated that the
selection of which X chromosome to form the Barr body
occurs randomly and independently in embryonic cells at the
time of X inactivation.
• As a consequence, females consist of a mosaic of cells, some
with an active paternal X, others with an active maternal X.
Mary Lyon
X Inactivation Mosaic
X inactivation and coat color in tortoiseshell cats