Download - Genetics
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Genetics is the science of heredity
• These black Labrador puppies are purebred—their parents and grandparents were black Labs with very similar genetic makeups
– Purebreds often suffer from serious genetic defects
Purebreds and Mutts — A Difference of Heredity
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• The parents of these puppies were a mixture of different breeds
– Their behavior and appearance is more varied as a result of their diverse genetic inheritance
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Modern genetics began with Gregor Mendel’s quantitative experiments with pea plants
9.2 Experimental genetics began in an abbey garden
Figure 9.2A, B
Stamen
Carpel
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Mendel crossed pea plants that differed in certain characteristics and traced the traits from generation to generation
Figure 9.2C
• This illustration shows his technique for cross-fertilization
1 Removed stamensfrom purple flower
White
Stamens
Carpel
PurplePARENTS(P)
OFF-SPRING
(F1)
2 Transferred pollen from stamens of white flower to carpel of purple flower
3 Pollinated carpel matured into pod
4 Planted seeds from pod
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Mendel studied seven pea characteristics
Figure 9.2D
• He hypothesized that there are alternative forms of genes (although he did not use that term), the units that determine heredity
FLOWER COLOR
FLOWER POSITION
SEED COLOR
SEED SHAPE
POD SHAPE
POD COLOR
STEM LENGTH
Purple White
Axial Terminal
Yellow Green
Round Wrinkled
Inflated Constricted
Green Yellow
Tall Dwarf
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• From his experimental data, Mendel deduced that an organism has two genes (alleles) for each inherited characteristic
– One characteristic comes from each parent
9.3 Mendel’s principle of segregation describes the inheritance of a single characteristic
P GENERATION(true-breedingparents)
F1 generation
F2
generation
Purple flowers White flowers
All plants have purple flowers
Fertilization among F1 plants(F1 x F1)
3/4 of plantshave purple flowers
1/4 of plantshave white flowers
Figure 9.3A
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• A sperm or egg carries only one allele of each pair– The pairs of
alleles separate when gametes form
– This process describes Mendel’s law of segregation
– Alleles can be dominant or recessive
GENETIC MAKEUP (ALLELES)
P PLANTS
F1 PLANTS(hybrids)
F2 PLANTS
PP pp
All P All p
All Pp
1/2 P 1/2 p
EggsP
p
P
PPp
Sperm
Pp Pp
pp
Gametes
Gametes
Phenotypic ratio3 purple : 1 white
Genotypic ratio1 PP : 2 Pp : 1 pp
Figure 9.3B
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Alternative forms of a gene (alleles) reside at the same locus on homologous chromosomes
9.4 Homologous chromosomes bear the two alleles for each characteristic
GENE LOCI
Figure 9.4
P a B
DOMINANTallele
RECESSIVEallele
P a b
GENOTYPE: PP aa Bb
HOMOZYGOUSfor thedominant allele
HOMOZYGOUSfor therecessive allele
HETEROZYGOUS
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• The offspring of a testcross often reveal the genotype of an individual when it is unknown
9.6 Geneticists use the testcross to determine unknown genotypes
TESTCROSS:
B_GENOTYPES bb
BB Bbor
Two possibilities for the black dog:
GAMETES
OFFSPRING All black 1 black : 1 chocolate
B
b
B
b
b
Bb Bb bb
Figure 9.6
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• The inheritance of many human traits follows Mendel’s principles and the rules of probability
9.8 Connection: Genetic traits in humans can be tracked through family pedigrees
Figure 9.8A
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Most such disorders are caused by autosomal recessive alleles
– Examples: cystic fibrosis, sickle-cell disease
9.9 Connection: Many inherited disorders in humans are controlled by a single gene
Figure 9.9A
D D
d d
NormalDd
NormalDd
DDNormal
DdNormal(carrier)
DdNormal(carrier)
ddDeaf
Eggs Sperm
PARENTS
OFFSPRING
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• A few are caused by dominant alleles
Figure 9.9B
– Examples: achondroplasia, Huntington’s disease
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Karyotyping and biochemical tests of fetal cells and molecules can help people make reproductive decisions
– Fetal cells can be obtained through amniocentesis
9.10 Connection: Fetal testing can spot many inherited disorders early in pregnancy
Figure 9.10A
Amnioticfluid
Fetus(14-20weeks)
Placenta
Amnioticfluidwithdrawn
Centrifugation
Fetalcells
Fluid
Uterus Cervix Cell culture
Severalweeks later Karyotyping
Biochemicaltests
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• In a population, multiple alleles often exist for a characteristic
– The three alleles for ABO blood type in humans is an example
9.13 Many genes have more than two alleles in the population
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• When an offspring’s phenotype—such as flower color— is
in between the phenotypes of its parents, it exhibits incomplete dominance
9.12 Incomplete dominance results in intermediate phenotypes
P GENERATION
F1 GENERATION
F2 GENERATION
RedRR
Gametes R r
Whiterr
PinkRr
R r
R R
r r
1/21/2
1/2
1/21/2
1/2 SpermEggs
PinkRr
PinkrR
Whiterr
RedRR
Figure 9.12A
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 9.13
– The alleles for A and B blood types are codominant, and both are expressed in the phenotype
BloodGroup(Phenotype)
O
Genotypes
AntibodiesPresent inBlood
Reaction When Blood from Groups Below Is Mixed with Antibodies from Groups at Left
O A B AB
A
B
AB
ii
IA IA
orIA i
IB IB
orIB i
IA IB
Anti-AAnti-B
Anti-B
Anti-A
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
9.14 A single gene may affect many phenotypic characteristics
Individual homozygousfor sickle-cell allele
Sickle-cell (abnormal) hemoglobin
Abnormal hemoglobin crystallizes,causing red blood cells to become sickle-shaped
Sickle cells
Breakdown of red blood cells
Clumping of cells and clogging of
small blood vessels
Accumulation ofsickled cells in spleen
Physical weakness Anemia Heart
failurePain and
feverBrain
damageDamage to
other organsSpleen damage
Kidney failureRheumatism
Pneumonia and other infections
ParalysisImpaired mental
function
• A single gene may affect phenotype in many ways, a phenomenon called pleiotropy
– The allele for sickle-cell disease is an example
Figure 9.14
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 9.16
P GENERATION
F1 GENERATION
F2 GENERATION
aabbcc(very light)
AABBCC(very dark)
AaBbCc AaBbCc
Eggs Sperm
Fra
cti
on
of
po
pu
lati
on
Skin pigmentation
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• The chromosomal basis of Mendel’s principles
Figure 9.17
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 9.21A
X Y
Male
(male)
Parents’diploidcells
(female)
Sperm
Offspring(diploid)
Egg
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Other systems of sex determination exist in other animals and plants
Figure 9.21B-D
– The X-O system
– The Z-W system
– Chromosome number
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
– Their inheritance pattern reflects the fact that males have one X chromosome and females have two
Figure 9.22B-D
– These figures illustrate inheritance patterns for white eye color (r) in the fruit fly, an X-linked recessive trait
Female Male Female Male Female Male
XrYXRXR
XRXr
XRY
XR Xr
Y
XRXr
XR
Xr XRXR
XR
Y
XRY
XrXR XRY
XrY
XRXr
XR
Xr
Xr
YXRXr
XrXr XRY
XrY
XrY
R = red-eye alleler = white-eye allele
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• Most sex-linked human disorders are due to recessive alleles
– Examples: hemophilia, red-green color blindness
– These are mostly seen in males
– A male receives a single X-linked allele from his mother, and will have the disorder, while a female has to receive the allele from both parents to be affected
9.23 Connection: Sex-linked disorders affect mostly males
Figure 9.23A