classical genetics. classical genetics deals with the study of heredity often referred to as...
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Classical Genetics
• Classical genetics deals with the study of heredity• Often referred to as Mendelian genetics• Based on the principles first set forth by Gregor
Mendel in 1866– Austrian monk studying traits of pea plants
• Early analysis of organismal traits was based on morphological characteristics– Prior to Mendel, blending of traits was standard idea
• Later rejected due to subsequent reappearance of traits in offspring
• Mendel studied seven traits: seed shape, seed color, flower color, pod shape, pod color, flower and pod position, and stem length– Traits occurred in two different forms
• His results were largely ignored until 1900
Mendel’s Peas
Petal
Stamen
Carpel
Pea plant traits
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
• Mendel theorized that discrete heritable factors are passed from parent to offspring
• Following traits through multiple generations provided evidence to predict how traits could be passed on
• Mendel cross-fertilized peas of his choosing based on desired characteristics through controlled matings– Called first generation the P1 (parental) generation
• Differed by one trait (hybrid)
– Offspring of the P generation were F1 generation – Offspring of F1generation were F2 generation (dihybrid)
• He made several important discoveries based on observations of experiments
LE 9-2c
Removed stamensfrom purpleflower
White
Carpel
Parents(P)
Purple
Transferred pollenfrom stamens ofwhite flower tocarpel of purpleflower
Stamens
Pollinated carpelmatured into pod
Planted seedsfrom pod
Offspring(F1)
LE 9-3a
P generation(true-breedingparents)
Purple flowers White flowers
All plants havepurple flowers
F1 generation
F2 generation
Fertilizationamong F1 plants(F1 F1)
of plantshave purple flowers
34
of plantshave white flowers
14
Mendel’s Observations & Hypotheses• Some traits disappeared in F1 generation but
reappeared in F2 generation in particular ratios• Some traits mask or dominate expression of the
other trait: dominant form• Some traits are masked by expression of
dominant trait: recessive form• Offspring get alternative forms of discrete
heritable factors (genes) that account for variation in traits passed down: alleles
• Organisms receive one allele from each parent– Supported by law of segregation: one allele on each
chromosome of a (homologous) pair
Homologous Chromsomes
• Alternate forms of the genes (alleles) reside at the same locus on homologous chromosomes
• Supports law of segregation• Either allele may be present, location is
the constant• Identical alleles: homozygous• Differing alleles: heterozygous
LE 9-4
Gene loci
P a B
Dominantallele
P a b
PP BbGenotype:
Homozygousfor thedominant allele
Homozygousfor therecessive allele
Heterozygous
aa
Recessiveallele
Mendel’s Conclusions
• Organism’s appearance doesn’t necessarily reflect its genetic makeup
• Genotype is genetic makeup• Phenotype is expression of traits• In monohybrid cross, 3:1 phenotypic ratio
and 1:2:1 genotypic ratio• Used a Punnett square to demonstrate
possible combinations of crosses
LE 9-3b
P plants
Gametes
Genetic makeup (alleles)
All PpF1 plants(hybrids)
F2 plants
Sperm
Phenotypic ratio3 purple : 1 white
Gametes
PP pp
All P All p
Eggs
12
Genotypic ratio1 PP : 2 Pp : 1 pp
P12 p
pP
P
p
PP Pp
Pp pp
Dihybrid Crosses• Mating a parental generation differing in two traits • Results of mating F1 generations produce F2
generation • Can be used to demonstrate independent
assortment: alleles of two different genes segregate independently of one another
• Demonstrates alleles segregating independently of one another during gametogenesis
• Phenotypic ratio of 9:3:3:1, genotypic ratio of 1:2:2:4:2:1:1:2:1 (forget it!)
LE 9-5a
rryy
RrYy
RRYY rryy
RY ry
RrYy
SpermSperm
Eggs
Eggs
RY
rY
Ry
ry
RY rY Ry ry
ry
ry
RRYY RrYY RRYy RrYy
RrYY rrYY RrYy rrYy
RRYy RrYy RRyy Rryy
RrYy rrYy Rryy rryy
Yellowround
GreenroundYellowwrinkledGreenwrinkled
916
316
316
116
Actual resultssupport hypothesis
14
14
14
14
12
ryRY
RRYY rryyP generation
RY
RY12
12
12
Hypothesis: Dependent assortment Hypothesis: Independent assortment
Gametes
14
14
14
14
Actual resultscontradict hypothesis
F1 generation
F2 generation
Gametes
Independent Assortment
PhenotypesGenotypes
Mating of heterozygotes(black, normal vision)
Phenotypic ratioof offspring
Black coat, normal visionB_N_
Black coat, blind (PRA)B_nn
Blind Blind
Chocolate coat, normal visionbbN_
Chocolate coat, blind (PRA)bbnn
BbNn BbNn
9 black coat,normal vision
3 black coat,blind (PRA)
1 chocolate coat,blind (PRA)
3 chocolate coat,normal vision
• Example: Coat color and vision in Labs• Black or chocolate coat: B or b• Normal vision or PRA: N or n
Discovering Genotypes
• Test crosses can be used to determine genotype
• Homozygous recessive organism is mated with organism of dominant phenotype to determine genotype
• Phenotypic ratio of F1 generation will allow determination of genotype
LE 9-6
Testcross:
Genotypes
Gametes
Offspring All black 1 black : 1 chocolate
Two possibilities for the black dog:
or
B_ bb
Bb
B b
bb
B
BB
Bb Bb bb
Genetics and Probability• Mendel’s laws follow predictable rules of probability• Events following rules of probability occur independently
of one another• Current genetic configuration does not influence future
outcome: sex in subsequent offspring• Multiplication rule: probability of two events occurring
simultaneously is product of the probabilities of the separate events( ½ X ½ = ¼ )
• Addition rule: probability that event can occur in two or more alternative ways is the sum of the separate probabilities of the different ways (¼ + ¼ = ½)
• Can be used to predict probability of combinations of traits occurring in offspring
LE 9-7F1 genotypes
Bb female
Bb male
Formation of sperm
Formation of eggs
F2 genotypes
B b
BB B B b
bb B b b
12
12
12
12
14
14
14
14
Variations on Mendel’s Laws
• Mendel’s laws can be applied to all sexually reproducing organisms, but…
• Some patterns of inheritance don’t follow Mendel’s laws – too complex!
• Complete vs. Incomplete Dominance– Complete dominance: dominant allele exerts its
affect regardless of number of copies– Incomplete dominance: heterozygote shows
intermediate characteristics of two homozygous conditions• Not the same as blending• Hypercholesterolemia in humans, flower color in
snapdragons
LE 9-12aP generation
F1 generation
F2 generation
Gametes
Gametes
Eggs
Whiterr
PinkRr
R
R
r
r
Sperm
12
12
12
12
R12
r12
RedRR
PinkrR
PinkRr
Whiterr
RedRR
R r
LE 9-12b
HHHomozygous
for ability to makeLDL receptors
Genotypes:
HhHeterozygous
Phenotypes:
LDL
LDLreceptor
Cell
Normal Mild disease Severe disease
hhHomozygous
for inability to makeLDL receptors
Genes and Multiple Alleles
• Many genes have more than 2 alleles: multiple alleles
• Example: ABO blood types in humans: A, B, AB, O
• Codominance of A and B alleles in heterozygotes phenotype
• Six possible genotypes in ABO system
LE 9-13
BloodGroup(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
iiAnti-AAnti-B
Anti-B
Anti-A
IAIA
orIAi
IBIB
orIBi
IAIB
Gene Linkages
• Inheritance patterns inconsistent with Mendelian laws first noted in 1908
• Sweet peas failed to show predicted ratios in the F2 generation
• Genes located close together on the same chromosome are linked
• Don’t follow Mendel’s laws of independent assortment
• Usually inherited together
LE 9-19Experiment
Purple flower
Purple long
Purple round
Red long
Red round
284
21
21
55
Explanation: linked genes
Parentaldiploid cellPpLl
Mostgametes
P L
p l
Meiosis
P L p l
Fertilization
Sperm
P L p l
P L P L
P L p l
P L
p l
p l p l
p lP L
3 purple long : 1 red roundNot accounted for: purple round and red long
Mostoffspring Eggs
215
71
71
24
Long pollenPpLl PpLl
PhenotypesObservedoffspring
Prediction(9:3:3:1)
Crossing Over• New combinations of alleles produced
from crossing over• Occurs during meiosis between
homologous chromosomes• Results in new combinations of alleles in
gametes
Crossing Over
• T.H. Morgan used fruit flies for genetic studies in early 1900s
• Easy to grow, short generation time, very inexpensive• Studied mutant and “wild-type” phenotypes• Was able to determine genes were on chromosomes:
chromosome basis of inheritance• Didn’t know about crossing over, but something “breaks
linkages” according to Morgan• Crossover data used to help map genes: determine their
relative positions on chromosomes• Nucleotide distances used to determine gene maps now
Fruit FliesExperiment
Gray body,long wings(wild type)
GgLl
Female
Black body,vestigial wings
ggll
Male
Offspring
Gray long Black vestigial Gray vestigial Black long
965 944 206 185
Parentalphenotypes
Recombinantphenotypes
Recombination frequency =391 recombinants
2,300 total offspring= 0.17 or 17%
Explanation
GgLl(female)
ggll(male)
g l
g l
g lg LG lg lG L
G L
g l
Eggs Sperm
Offspring
G L g l G l g L
g lg l g l g l
Drosophila crosses
• Drosophila melanogaster can be used to study Mendelian patterns of inheritance
• Many mutant strains available to study • Mutations found on various chromosomes• Maintained as inbred lines for study• Linked and non-linked mutations available• Linkages occur on various chromosomes
Sex Linkage• Genes unrelated to sex determination, but
located on sex chromosomes• X-linked in humans• Inheritance follows peculiar patterns
– Eye color in fruit flies: three possible patterns of inheritance
LE 9-23b
Female Male
XRXR Xr Y
Sperm
XRXr XRY
Xr Y
XREggs
R = red-eye alleler = white-eye allele
LE 9-23c
Female Male
XRXr XRY
Sperm
XRXR XRY
XR Y
XR
Eggs
Xr XrXR Xr Y
LE 9-23d
Female Male
XRXr Xr Y
Sperm
XRXr XRY
Xr Y
XR
Eggs
XrXr Xr Xr Y
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