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