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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Chapter 14Mendel and the Gene Idea
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 14.1 Gregor Mendel and his garden peas
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 14.2 Research Method Crossing Pea Plants
1
5
4
3
2
Removed stamensfrom purple flower
Transferred sperm-bearing pollen fromstamens of white flower to egg-bearing carpel of purple flower
Parentalgeneration(P)
Pollinated carpelmatured into pod
Carpel(female)
Stamens(male)
Planted seedsfrom pod
Examinedoffspring:all purpleflowers
Firstgenerationoffspring(F1)
APPLICATION By crossing (mating) two true-breedingvarieties of an organism, scientists can study patterns ofinheritance. In this example, Mendel crossed pea plantsthat varied in flower color.
TECHNIQUETECHNIQUE
When pollen from a white flower fertilizeseggs of a purple flower, the first-generation hybrids all have purpleflowers. The result is the same for the reciprocal cross, the transferof pollen from purple flowers to white flowers.
TECHNIQUERESULTS
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 14.3 When F1 pea plants with purple flowers are allowed to self-pollinate, what flower color appears in the F2 generation
P Generation
(true-breeding parents) Purple
flowersWhiteflowers
F1 Generation (hybrids)
All plants hadpurple flowers
F2 Generation
EXPERIMENT True-breeding purple-flowered pea plants andwhite-flowered pea plants were crossed (symbolized by ). Theresulting F1 hybrids were allowed to self-pollinate or were cross-pollinated with other F1 hybrids. Flower color was then observedin the F2 generation.
RESULTS Both purple-flowered plants and white-flowered plants appeared in the F2 generation. In Mendel’sexperiment, 705 plants had purple flowers, and 224 had whiteflowers, a ratio of about 3 purple : 1 white.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Table 14.1 The Results of Mendel’s F1 Crosses for Seven Characters in Pea Plants
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 14.4 Alleles, alternative versions of a gene
Allele for purple flowers
Locus for flower-color gene
Homologouspair ofchromosomes
Allele for white flowers
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 14.5 Mendel’s law of segregation (layer 1)
P Generation
F1 Generation
P p
Appearance:Genetic makeup:
Purple flowerPP
White flowerspp
Purple flowersPp
Appearance:Genetic makeup:
Gametes:
Gametes:
Each true-breeding plant of the parental generation has identicalalleles, PP or pp.
Gametes (circles) each contain only one allele for the flower-color gene. In this case, every gamete produced by one parent has the same allele.
Union of the parental gametes produces F1 hybrids having a Pp combination. Because the purple-flower allele is dominant, allthese hybrids have purple flowers.
When the hybrid plants producegametes, the two alleles segregate, half the gametes receiving the P allele and the other half the p allele.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 14.5 Mendel’s law of segregation (layer 2)
P Generation
F1 Generation
F2 Generation
P p
P p
P p
P
p
PpPP
ppPp
Appearance:Genetic makeup:
Purple flowerPP
White flowerspp
Purple flowersPp
Appearance:Genetic makeup:
Gametes:
Gametes:
F1 sperm
F1 eggs
1/21/2
Each true-breeding plant of the parental generation has identicalalleles, PP or pp.
Gametes (circles) each contain only one allele for the flower-color gene. In this case, every gamete produced by one parent has the same allele.
Union of the parental gametes produces F1 hybrids having a Pp combination. Because the purple-flower allele is dominant, allthese hybrids have purple flowers.
When the hybrid plants producegametes, the two alleles segregate, half the gametes receiving the P allele and the other half the p allele.
This box, a Punnett square, shows all possible combinations of alleles in offspring that result from an F1 F1 (Pp Pp) cross. Each square represents an equally probable product of fertilization. For example, the bottomleft box shows the genetic combinationresulting from a p egg fertilized bya P sperm.
Random combination of the gametesresults in the 3:1 ratio that Mendelobserved in the F2 generation. 3 : 1
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 14.6 Phenotype versus genotype
3
1 1
2
1
Phenotype
Purple
Purple
Purple
White
Genotype
PP(homozygous)
Pp(heterozygous)
Pp(heterozygous)
pp(homozygous)
Ratio 3:1 Ratio 1:2:1
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 14.7 The Testcross
Dominant phenotype,unknown genotype:
PP or Pp?
Recessive phenotype,known genotype:
pp
If PP,then all offspring
purple:
If Pp,then 1⁄2 offspring purpleand 1⁄2 offspring white:
p p
P
P
Pp Pp
PpPp
pp pp
PpPpP
p
p p
APPLICATION An organism that exhibits a dominant trait,such as purple flowers in pea plants, can be either homozygous forthe dominant allele or heterozygous. To determine the organism’sgenotype, geneticists can perform a testcross.
TECHNIQUE In a testcross, the individual with theunknown genotype is crossed with a homozygous individualexpressing the recessive trait (white flowers in this example). By observing the phenotypes of the offspring resulting from this cross, we can deduce the genotype of the purple-flowered parent.
RESULTS
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 14.8 Do the alleles for seed color and seed shape sort into gametes dependently (together) or independently?
YYRRP Generation
Gametes YR yr
yyrr
YyRrHypothesis ofdependentassortment
Hypothesis ofindependentassortment
F2 Generation(predictedoffspring)
1 ⁄2 YR
YR
yr
1 ⁄2
1 ⁄2
1 ⁄2 yr
YYRR YyRr
yyrrYyRr
3 ⁄4 1 ⁄4
Sperm
Eggs
Phenotypic ratio 3:1
YR1 ⁄4
Yr1 ⁄4
yR1 ⁄4
yr1 ⁄4
9 ⁄163 ⁄16
3 ⁄161 ⁄16
YYRR YYRr YyRR YyRr
YyrrYyRrYYrrYYrr
YyRR YyRr yyRR yyRr
yyrryyRrYyrrYyRr
Phenotypic ratio 9:3:3:1
315 108 101 32 Phenotypic ratio approximately 9:3:3:1
F1 Generation
Eggs
YR Yr yR yr1 ⁄4 1 ⁄4 1 ⁄4 1 ⁄4
Sperm
RESULTS
CONCLUSION The results support the hypothesis of independent assortment. The alleles for seed color and seed shape sort into gametes independently of each other.
EXPERIMENT Two true-breeding pea plants—one with yellow-round seeds and the other with green-wrinkled seeds—were crossed, producing dihybrid F1 plants. Self-pollination of the F1 dihybrids, which are heterozygous for both characters, produced the F2 generation. The two hypotheses predict different phenotypic ratios. Note that yellow color (Y) and round shape (R) are dominant.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 14.9 Segregation of alleles and fertilization as chance events
Rr
Segregation ofalleles into eggs
Rr
Segregation ofalleles into sperm
R r
rR
R
R
R1⁄2
1⁄2 1⁄2
1⁄41⁄4
1⁄41⁄4
1⁄2 r
rR r
r
Sperm
Eggs
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 14.10 Incomplete dominance in snapdragon color
P Generation
F1 Generation
F2 Generation
RedCRCR
Gametes CR CW
WhiteCWCW
PinkCRCW
Sperm
CR
CR
CR
Cw
CR
CRGametes1⁄2 1⁄2
1⁄2
1⁄2
1⁄2
Eggs1⁄2
CR CR CR CW
CW CWCR CW
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Table 14.2 Determination of ABO Blood Group by Multiple Alleles
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 14.11 An example of epistasis
BC bC Bc bc1⁄41⁄41⁄41⁄4
BC
bC
Bc
bc
1⁄4
1⁄4
1⁄4
1⁄4
BBCc BbCc BBcc Bbcc
Bbcc bbccbbCcBbCc
BbCC bbCC BbCc bbCc
BBCC BbCC BBCc BbCc
9⁄163⁄16
4⁄16
BbCc BbCc
Sperm
Eggs
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 14.12 A simplified model for polygenic inheritance of skin color
AaBbCc AaBbCc
aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCc AABBCC
20⁄64
15⁄64
6⁄64
1⁄64
Fra
cti o
n o
f p
rog
en
y
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 14.13 The effect of environment on phenotype
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 14.14 Pedigree analysis
Ww ww ww Ww
wwWwWwwwwwWw
WWor
Ww
ww
First generation(grandparents)
Second generation(parents plus aunts
and uncles)
Thirdgeneration
(two sisters)
Ff Ff ff Ff
ffFfFfffFfFF or Ff
ff FForFf
Widow’s peak No Widow’s peak Attached earlobe Free earlobe
(a) Dominant trait (widow’s peak) (b) Recessive trait (attached earlobe)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 14.15 Achondroplasia
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 14.16 Large families as excellent case studies of human genetics
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 14.17 Testing a fetus for genetic disorders
(a) Amniocentesis
Amnioticfluidwithdrawn
Fetus
Placenta Uterus Cervix
Centrifugation
A sample ofamniotic fluid canbe taken starting atthe 14th to 16thweek of pregnancy.
(b) Chorionic villus sampling (CVS)
Fluid
Fetalcells
Biochemical tests can bePerformed immediately onthe amniotic fluid or lateron the cultured cells.
Fetal cells must be culturedfor several weeks to obtainsufficient numbers forkaryotyping.
Severalweeks
Biochemicaltests
Severalhours
Fetalcells
Placenta Chorionic viIIi
A sample of chorionic villustissue can be taken as earlyas the 8th to 10th week ofpregnancy.
Suction tubeInserted throughcervix
Fetus
Karyotyping and biochemicaltests can be performed onthe fetal cells immediately,providing results within a dayor so.
Karyotyping
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Unnumbered Figure p.273
George Arlene
Sandra Tom Sam Wilma Ann Michael
Daniel Alan Tina
Christopher
Carla
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