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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