Introduction to Genetics
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Transcript of Introduction to Genetics
Introduction to Genetics
• Purebreds and Mutts–A Difference of Heredity– Purebred dogs are very similar
– Mutts, or mixed breed dogs show considerably more genetic variation
Early Ideas about Heredity
• Sperm and eggs transmitted information
• Blending theory
• Problem:– Variation would disappear
Gregor Mendel• Experimental genetics
– Modern genetics • Began with Gregor Mendel’s quantitative
experiments with pea plants
Petal
CarpelStamen
Figure 9.2 BFigure 9.2 A
– Mendel crossed pea plants that differed in certain characteristics • And traced traits from generation to
generation
Figure 9.2 C
1 Removed stamens from purple flower
2 Transferred pollen from stamens of white flower to carpel of purple flower
3 Pollinated carpel matured into pod
4 Planted seeds from pod
Offspring(F1)
Parents(P)
Purple
Carpel
White
Stamens
Gregor Mendel
– Mendel hypothesized that there are alternative forms of genes
• The units that determine heritable traits
Flower color
Flower position
Seed color
Seed shape
Pod color
Pod shape
Stem length
Purple White
Axial Terminal
Round Wrinkled
Inflated Constricted
Tall Dwarf
GreenYellow
Green Yellow
Figure 9.2 D
Gregor Mendel
Genes
• Units of information about specific traits
• Passed from parents to offspring
• Each has a specific location (locus) on a chromosome
Alleles
• Different molecular forms of a gene
• Arise by mutation
• Dominant allele masks a recessive
allele
Allele Combinations• Homozygous
– having two identical alleles at a locus– AA or aa
• Heterozygous – having two different alleles at a locus– Aa
• Homologous chromosomes bear the two alleles for each characteristic
• Reside at the same locus on homologous chromosomes
Figure 9.4
Genotype: PP aa BbHeterozygous
P a b
P a B
Gene loci
Recessiveallele
Dominantallele
Homozygousfor thedominant allele
Homozygousfor therecessive allele
Allele Combinations
Genotype & Phenotype
• Genotype refers genes an individual carries
• Phenotype refers to an individual’s observable traits
• Cannot always determine genotype by observing phenotype
Tracking Generations
• Parental generation Pmates to produce
• First-generation offspring F1
mate to produce
• Second-generation offspring F2
– Allele pairs separate from each other during the production of gametes
Figure 9.3 B
P plants
Gametes
Genetic makeup (alleles)
Gametes
F1 plants(hybrids)
F2 plants
PP pp
All P All p
All Pp
Sperm
12
P
P
P
p
p
PP Pp
Pp pp
EggsGenotypic ratio1 PP : 2 Pp: 1 pp
Phenotypic ratio3 purple : 1 white
12
p
Mendel’s Theory of Segregation
Mendel’s Theory of Segregation
• An individual inherits a unit of information (allele) about a trait from each parent
Slide 4
Aa
fertilization produces heterozygous offspring
AA
AA
aa
aa
meiosis I
A A A A a a a a
meiosis II
gametes
Stepped Art
Figure 11.4Page 180
AA
A
AA
aa
aa
aa
(chromosomes duplicated before
meiosis)
Homozygous dominant parent
Homozygous recessive parent
A
•Alleles of a pair segregate independently of other allele pairs during gamete formation
Figure 9.5 A
Hypothesis: Dependent assortment Hypothesis: Independent assortment
RRYY rryy
Gametes Gametes
RRYY rryy
RrYy RrYy
RY ry ryRY
Sperm Sperm
RY ry
ry
RY
ry
Ry
ry
RYRRYY
RrYY
RRYy
RrYy
RrYY
rrYY
RrYy
rrYy
RRYy
RrYy
RRyy
Rryy
RrYy
rrYy
Rryy
rryy
RY ry ryRY
Actual resultscontradict hypothesis
Actual resultssupport hypothesis
YellowroundGreenround
Yellowwrinkled
Greenwrinkled
Eggs
P generation
F1 generation
F2 generation
Eggs
12
12
12
12
14
14
14
14
14
14
14
14
916
316
3161
16
Independent Assortment
Independent Assortment
Metaphase I:
Metaphase II:
Gametes:
1/4 AB 1/4 ab 1/4 Ab 1/4 aB
A A A A
A A A A
AAAA
B B
B B
BB
B B
BBBB
a a a a
aa aa
aaaa
bb b b
bb b b
b b b b
OR
Independent Assortment
Black coat, normal visionB_N_
Black coat, blind (PRA)B_nn
Chocolate coat, normal visionbbN_
Chocolate coat, blind (PRA)bbnn
Blind Blind
9 black coat, normal vision
3 black coat,blind (PRA)
3 chocolate coat, normal vision
1 chocolate coat, blind (PRA)
BbNn BbNn
PhenotypesGenotypes
Mating of heterozygotes(black, normal vision)
Phenotypic ratioof offspring
Figure 9.5 B
Monohybrid Crosses
Experimental intercross between two F1 heterozygotes
AA X aa Aa (F1 monohybrids)
Aa X Aa ?
Mendel’s Monohybrid Cross Results
787 tall 277 dwarf
651 long stem
207 at tip
705 purple 224 white
152 yellow428 green
299 wrinkled882 inflated
6,022 yellow 2,001 green
5,474 round 1,850 wrinkled
F2 plants showed dominant-to-recessive ratio
Monohybrid Cross
Illustrated
True-breedinghomozygous recessiveparent plant
True-breedinghomozygous dominantparent plant
An F1 plantself-fertilizesand producesgametes:
F1 PHENOTYPES
F2 PHENOTYPES
aa
Aa
AA
aaAa
Aa
Aa Aa
Aa Aa
Aa Aa
Aa Aa
Aa
Aa
AA
aa
A
A
A
A
a a
a
a
AA
Test Cross
• Individual with dominant phenotype is crossed with individual with recessive phenotype
• Examining offspring determines the genotype
Test Cross
Testcross:
Genotypes
Gametes
Offspring
B_ bb
Two possibilities for the black dog:
BB or Bb
B B b
b Bb b Bb bb
All black 1 black : 1 chocolate
Punnett Squares of Test Crosses
Homozygous recessive
a a
A
a aa
Aa Aa
aa
Homozygous recessive
a a
A
A Aa
Aa Aa
Aa
Two phenotypes All dominant phenotype
Dihybrid Cross
Cross between individuals that are homozygous for different versions
of two traits
Dihybrid Cross: F1 Results
AABB aabbx
AaBb
AB AB ab ab
TRUE-BREEDING PARENTS:
GAMETES:
F1 HYBRID OFFSPRING:
purple flowers, tall
white flowers,dwarf
All purple-flowered, tall
1/16aaBB
1/16aaBb
1/16aaBb
1/16Aabb
1/16Aabb
1/16AAbb
1/16AABB
1/16AABb
1/16AaBB
1/16AaBb
1/16AABb
1/16AaBb
1/16AaBB
1/16AaBb
1/16AaBb
1/4 AB 1/4 Ab 1/4 aB 1/4 ab
1/16aabb
1/4 AB
1/4 Ab
1/4 aB
1/4 ab
AaBb AaBbX
1/16 white-flowered, dwarf
3/16 white-flowered, tall
3/16 purple-flowered, dwarf
9/16 purple-flowered, tall
Dihybrid Cross: F2 Results
Probability
The chance that each outcome of a given event will occur is proportional to the number of ways that event can be reached
• The rule of multiplication
– Probabilty of aa = ?– Probabilty of aa and
bb?
• Independent events
Figure 9.7
F1 genotypes
Bb female
Formation of eggs
F2 genotypes
Bb male
Formation of sperm
B b
BB B B b
b b B b b
12
12
12
12
14
14
14
14
Probability
Inherited Disorders• Many are controlled by a single
gene
Table 9.9
Dominance Relations
Complete dominance Incomplete dominance
Codominance
Incomplete Dominance
XHomozygous parent
Homozygous parent
All F1 are heterozygous
X
F2 shows three phenotypes in 1:2:1 ratio
Codominance: ABO Blood Types
• Gene that controls ABO type codes for a glycolipid on blood cells
• Two alleles (IA and IB) are codominant when paired
• Third allele (i) is recessive to othersBloodGroup(Phenotype) Genotypes
AntibodiesPresent inBlood
Reaction When Blood from Groups Below Is Mixed withAntibodies from Groups at Left
O A B AB
O
A
B
AB
ii
IAIA
orIAi
IBIB
orIBi
IAIB
Anti-AAnti-B
Anti-B
Anti-A
—
Pleiotropy • One gene may have effects on two
or more traits• Sickle-cell disease
Individual homozygousfor sickle-cell allele
Abnormal hemoglobin crystallizes,causing red blood cells to become sickle-shaped
Sickle-cell (abnormal) hemoglobin
Sickle cells
Breakdown ofred blood cells
Clumping of cellsand clogging of
small blood vessels
Accumulation ofsickled cells in spleen
Physicalweakness
Anemia Heartfailure
Pain andfever
Braindamage
Damage toother organs
Spleendamage
Impairedmentalfunction
ParalysisPneumoniaand otherinfections
Rheumatism Kidneyfailure
5,55
5
Polygenic Inheritance
• A range of small differences in a given trait among individuals
• Effected by the number of genes and env. Factors
P generation
F1 generation
F2 generation
Sperm
Eggs
aabbcc(very light)
AABBCC(very dark)
AaBbCc AaBbCc
18
18
18
18
18
18
18
18
18
18
18
18
18
18
18
18
1 64
6 64
15 64
2064
1564
6 64
1 64
1 64
2064
15 64
6 64
Skin color
Fra
ctio
n of
pop
ulat
ion
Temperature Effects on Phenotype
• Rabbit is homozygous heat-sensitive version of an enzyme
• Melanin is produced in cooler areas of body
Environmental Effects on Plant Phenotype
• Hydrangea macrophylla
• Flower color ranges from pink to blue
– Certain genes are linked
• Inheritedtogether because they are close together onthe same chromosome
Experiment
Explanation: linked genes
PpLI PpLILong pollen
Observed PredictionPhenotypes offspring (9:3:3:1)
Purple longPurple roundRed longRed round
Parentaldiploid cellPpLI
Most gametes
Mostoffspring Eggs
3 purple long : 1 red roundNot accounted for: purple round and red long
Meiosis
Fertilization
Sperm
284212155
215717124
P I
P L
P L
P L
P LP LP I
P L P I
P I
P L
P I
P I
P I
P I
P L
Purple flower
Figure 9.19
Linkages
– Crossing over can separate linked alleles
• Producing gametes with recombinant chromosomes
A B
a b
Tetrad Crossing over
A B
A b
a b
a B
GametesFigure 9.20 A
Crossing Over
Crossover Frequency
A B C D
Full Linkage
x
AB ab
50% AB 50% ab
All AaBb
meiosis, gamete formation
Parents:
F1 offspring:
Equal ratios of two types of gametes:
AB
ab
AB
ab
ab
AB
Figure 12.8aPage 201
Incomplete Linkage
Parents:
F1 offspring:
Unequal ratios of four types of gametes:
All AaCc
x
meiosis, gamete formation
AC acA
C A
C
AC
a
c
ac
Ac
aC
a
c
parental genotypes
recombinant genotypes
Figure 12.8bPage 201
Discovering Linkage
homozygous dominant female
recessive male
Gametes:
XX X Y
All F1 have red eyes
x
1/4
1/4
1/4
1/4
1/2
1/2 1/2
1/2
F2
generation:
XX X Y
xGametes:
– Recombination frequencies • Used to map the relative positions of
genes on chromosomes.
Figure 9.21 B
Mutant phenotypes
Shortaristae
Blackbody(g)
Cinnabareyes(c)
Vestigialwings(l)
Browneyes
Long aristae(appendageson head)
Gray body(G)
Redeyes(C)
Normalwings(L)
Redeyes
Wild-type phenotypes
Chromosomeg c l
9% 9.5%
17%
Recombinationfrequencies
Figure 9.21 C
Discovering Linkage
Sex Determination
X
X Y
X
XX
XY
XX
XY
X X
Y
X
x
x
eggs sperm
female(XX)
male(XY)
Human sex determination interaction.
Effect of YChromosome
10 weeks
Y present
Y absent
7 weeks
birth approaching
appearance of structuresthat will give rise toexternal genitalia
appearance of “uncommitted” duct system
of embryo at 7 weeks
Y present
Yabsent
testis
ovary
testes ovaries
The X Chromosome
• Carries more than 2,300 genes
• Most genes deal with nonsexual traits
• Genes on X chromosome can be expressed in both males and females
22+
XX
22+X
76+
ZW
76+
ZZ
32 16
Figure 9.22 D
Figure 9.22 C
Figure 9.22 B
Sex Determination
X-Linked Recessive Inheritance
• Mutant gene on X chromosome
• Males affected more often
Examples of X-Linked Traits
• Cannot be passed from father to son
• Color blindness
• Hemophilia– Blood-clotting disorder
– 1/7,000 males has allele for hemophilia A
Examples of X-linked Traits
Queenvictoria
Albert
Alice Louis
Alexandra CzarNicholas IIof Russia
AlexisFigure 9.24 A Figure 9.24 B