Mendel and the Gene Idea. The Father of Genetics Mendel introMendel Bio Mendel introMendel Bio.
Beyond Mendel
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Transcript of Beyond Mendel
Chromosomal theory of inheritance
• Mendelian genes have specific loci (positions) along chromosomes and it is the chromosomes that undergo segregation and independent assortment.
Figure 15.2P Generation
F1 Generation
Yellow-roundseeds (YYRR)
Green-wrinkledseeds (yyrr)
Meiosis
Fertilization
Gametes
Y
YR R
YR
y
yr
y r
All F1 plants produceyellow-round seeds (YyRr).
Meiosis
Metaphase I
Anaphase I
Metaphase II
R R
R R
R R
R R
R R R R
r r
r r
r r
r r
r r r r
Y Y
Y Y
Y Y
Y Y
Y Y Y Y
y y
y y
y y
y y
yy y y
Gametes
LAW OF SEGREGATIONThe two alleles for eachgene separate duringgamete formation.
LAW OF INDEPENDENTASSORTMENT Alleles of geneson nonhomologous chromosomesassort independently duringgamete formation.
1
2 2
1
1/41/4
1/41/4YR yr Yr yR
F2 Generation
3 3Fertilization recombinesthe R and r alleles at random.
Fertilization results in the 9:3:3:1 phenotypic ratioin the F2 generation.
An F1 F1 cross-fertilization
9 : 3 : 3 : 1
r
Figure 15.2a
P Generation Yellow-roundseeds (YYRR)
Green-wrinkledseeds (yyrr)
Meiosis
Fertilization
Gametes
Y
YR R
YR
y
yr
y r
r
Figure 15.2b
F1 Generation
All F1 plants produceyellow-round seeds (YyRr).
Meiosis
Metaphase I
Anaphase I
Metaphase II
R R
R R
R R
R R
R R R R
r r
r r
r r
r r
r r r r
Y Y
Y Y
Y Y
Y Y
Y Y Y Y
y y
y y
y y
y y
yy y y
Gametes
LAW OF SEGREGATIONThe two alleles for eachgene separate duringgamete formation.
LAW OF INDEPENDENTASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation.
1
2 2
1
1/41/4
1/41/4YR yr Yr yR
Figure 15.2c
F2 Generation
3Fertilization recombines the R and r alleles at random.
Fertilization results in the 9:3:3:1 phenotypic ratio in the F2 generation.
An F1 F1 cross-fertilization
9 : 3 : 3 : 1
LAW OF SEGREGATION LAW OF INDEPENDENTASSORTMENT
3
All offspringhad red eyes.
PGeneration
F1
Generation
F2
Generation
F2
Generation
F1
Generation
PGeneration
Eggs
Eggs
Sperm
Sperm
XX
XY
w
w
ww w
w
ww w
w
w
w
w
w
ww w
RESULTS
EXPERIMENT
CONCLUSION
F2
Generation
PGeneration
Eggs
Eggs
Sperm
Sperm
Xw
CONCLUSION
XX Y
w
ww w
w
ww w
w
w
w
w
w
ww w
F1
Generation
Parents
orSperm
or
Egg
Zygotes (offspring)
44 XY
44 XX
22 X
22 Y
22 X
44 XX
44 XY
22 XX
22 X
76 ZW
76 ZZ
32 (Diploid)
16 (Haploid)
(a) The X-Y system
(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.
Eggs Eggs Eggs
Sperm Sperm Sperm
(a) (b) (c)
XNXN XnY XNXn XNY XNXn XnY
Xn Y XN Y YXn
Xn Xn
XN
XN
XN XNXNXn XNY
XNY
XNY XNY
XnY XnYXNXn XNXn
XNXnXNXN
XnXn
Transmission of sex-linked recessive traits:a) Father with trait passes trait to all daughters - carriersb) Female carrier passes trait to half her sons and daughtersc) 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 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.
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.