Introduction to genetics
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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.

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Introduction to genetics

Introduction to Genetics


Introduction to genetics

  • Purebreds and Mutts–A Difference of Heredity

    • Purebred dogs are very similar


Introduction to genetics

  • Mutts, or mixed breed dogs show considerably more genetic variation


Early ideas about heredity

Early Ideas about Heredity

  • Sperm and eggs transmitted information

  • Blending theory

  • Problem:

    • Variation would disappear


Gregor mendel

Petal

Stamen

Carpel

Figure 9.2 A

Figure 9.2 B

Gregor Mendel

  • Experimental genetics

    • Modern genetics

      • Began with Gregor Mendel’s quantitative experiments with pea plants


Gregor mendel1

Gregor Mendel

  • Mendel crossed pea plants that differed in certain characteristics

    • And traced traits from generation to generation

1Removed stamens from purple flower

White

Stamens

Carpel

2 Transferred

pollen from stamens of white flower to carpel of purple flower

Parents(P)

Purple

3 Pollinated carpel matured into pod

4 Planted seeds from pod

Offspring(F1)

Figure 9.2 C


Gregor mendel2

Purple

Flower color

White

Terminal

Axial

Flower position

Green

Yellow

Seed color

Seed shape

Round

Wrinkled

Pod shape

Inflated

Constricted

Green

Yellow

Pod color

Tall

Stem length

Dwarf

Gregor Mendel

  • Mendel hypothesized that there are alternative forms of genes

    • The units that determine heritable traits

Figure 9.2 D


Genes

Genes

  • Units of information about specific traits

  • Passed from parents to offspring

  • Each has a specific location (locus) on a chromosome


Alleles

Alleles

  • Different molecular forms of a gene

  • Arise by mutation

  • Dominant allele masks a recessive allele


Allele combinations

Allele Combinations

  • Homozygous

    • having two identical alleles at a locus

    • AA or aa

  • Heterozygous

    • having two different alleles at a locus

    • Aa


Allele combinations1

Dominantallele

Gene loci

a

B

P

a

b

P

Recessiveallele

Genotype:

PP

aa

Bb

Heterozygous

Homozygousfor thedominant allele

Homozygousfor therecessive allele

Allele Combinations

  • Homologous chromosomes bear the two alleles for each characteristic

    • Reside at the same locus on homologous chromosomes

Figure 9.4


Genotype phenotype

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

Tracking Generations

  • Parental generation P

    mates to produce

  • First-generation offspring F1

    mate to produce

  • Second-generation offspring F2


Mendel s theory of segregation

P plants

Genetic makeup (alleles)

pp

PP

Gametes

All p

All P

F1 plants

(hybrids)

All Pp

1

2

1

2

P

p

Gametes

Sperm

p

P

F2 plants

Phenotypic ratio

3 purple : 1 white

Pp

P

PP

Eggs

Genotypic ratio

1 PP: 2 Pp: 1 pp

Pp

p

pp

Mendel’s Theory of Segregation

  • Allele pairs separate from each other during the production of gametes

Figure 9.3 B


Mendel s theory of segregation1

Mendel’s Theory of Segregation

  • An individual inherits a unit of information (allele) about a trait from each parent


Independent assortment

Hypothesis: Independent assortment

Hypothesis: Dependent assortment

RRYY

P generation

rryy

RRYY

rryy

ry

ry

Gametes

Gametes

RY

RY

RrYy

RrYy

F1 generation

Sperm

Sperm

1

4

1

4

1

4

1

4

ry

ry

RY

RY

1

2

1

2

ry

RY

1

4

RY

1

2

RY

RrYY

RRYY

RRYy

RrYy

F2 generation

Eggs

1

4

ry

1

2

ry

rrYY

rrYy

RrYy

RrYY

Eggs

Yellowround

9

16

1

4

Ry

RrYy

RRyy

RRYy

Rryy

Greenround

3

16

1

4

ry

Yellowwrinkled

Actual resultscontradict hypothesis

3

16

rryy

RrYy

rrYy

Rryy

Greenwrinkled

1

16

Actual resultssupport hypothesis

Independent Assortment

  • Alleles of a pair segregate independently of other allele pairs during gamete formation

Figure 9.5 A


Independent assortment1

Independent Assortment

Metaphase I:

OR

A

A

a

a

A

A

a

a

B

B

b

b

b

b

B

B

Metaphase II:

A

A

a

a

A

A

a

a

B

B

b

b

b

b

B

B

Gametes:

B

B

b

b

b

b

B

B

A

A

a

a

A

A

a

a

1/4 AB

1/4 ab

1/4 Ab

1/4 aB


Independent assortment2

Blind

Blind

Phenotypes

Genotypes

Black coat, normal vision

B_N_

Black coat, blind (PRA)

B_nn

Chocolate coat, normal vision

bbN_

Chocolate coat, blind (PRA)

bbnn

Mating of heterozygotes

(black, normal vision)

BbNn 

BbNn

9 black coat,

normal vision

3 black coat,

blind (PRA)

1 chocolate coat,

blind (PRA)

3 chocolate coat,

normal vision

Phenotypic ratio

of offspring

Figure 9.5 B

Independent Assortment


Monohybrid crosses

Monohybrid Crosses

Experimental intercross between

two F1 heterozygotes

AA X aa

Aa (F1 monohybrids)

Aa X Aa

?


Mendel s monohybrid cross results

Mendel’s Monohybrid Cross Results

5,474 round

1,850 wrinkled

6,022 yellow

2,001 green

882 inflated

299 wrinkled

428 green

152 yellow

F2 plants showed dominant-to-recessive ratio

705 purple

224 white

651 long stem

207 at tip

787 tall

277 dwarf


Monohybrid cross illustrated

True-breeding

homozygous recessive

parent plant

F1PHENOTYPES

aa

True-breeding

homozygous dominant

parent plant

Aa

Aa

a

a

Aa

Aa

A

AA

A

Aa

Aa

Aa

Aa

An F1 plant

self-fertilizes

and produces

gametes:

F2PHENOTYPES

Aa

AA

Aa

A

a

A

AA

Aa

a

Aa

aa

Aa

aa

Monohybrid CrossIllustrated


Test cross

Test Cross

  • Individual with dominant phenotype is crossed with individual with recessive phenotype

  • Examining offspring determines the genotype


Test cross1

Testcross:

Genotypes

bb

B_

Two possibilities for the black dog:

BB or Bb

Gametes

B

b

B

b

Bb

b

bb

Bb

1 black : 1 chocolate

All black

Offspring

Test Cross


Introduction to genetics

Homozygous

recessive

Homozygous

recessive

a a

a a

A

A

Aa

Aa

Aa

Aa

a

A

aa

Aa

aa

Aa

Punnett Squares of Test Crosses

Two phenotypes

All dominant phenotype


Dihybrid cross

Dihybrid Cross

Cross between individuals that are homozygous for different versions of two traits


Dihybrid cross f 1 results

Dihybrid Cross: F1 Results

purple

flowers, tall

white

flowers,

dwarf

TRUE-

BREEDING

PARENTS:

AABB

x

aabb

GAMETES:

AB

AB

ab

ab

AaBb

F1 HYBRID

OFFSPRING:

All purple-flowered, tall


Dihybrid cross f 2 results

Dihybrid Cross: F2 Results

X

AaBb

AaBb

1/4 AB

1/4 Ab

1/4 aB

1/4 ab

9/16 purple-flowered, tall

1/4 AB

1/16

AABB

1/16

AABb

1/16

AaBB

1/16

AaBb

3/16 purple-flowered, dwarf

3/16 white-flowered, tall

1/16

AaBb

1/16

AAbb

1/16

Aabb

1/4 Ab

1/16

AABb

1/16 white-flowered, dwarf

1/16

AaBB

1/16

aaBB

1/16

aaBb

1/4 aB

1/16

AaBb

1/16

Aabb

1/16

aaBb

1/16

aabb

1/16

AaBb

1/4 ab


Probability

Probability

The chance that each outcome of a given event will occur is proportional to the number of ways that event can be reached


Probability1

F1 genotypes

Bbmale

Formation of sperm

Bbfemale

Formation of eggs

1

2

1

2

b

B

b

B

B

B

1

2

B

1

4

1

4

F2 genotypes

1

2

B

b

b

b

b

1

4

1

4

Probability

  • The rule of multiplication

    • Probabilty of aa = ?

    • Probabilty of aa and bb?

  • Independent events

Figure 9.7


Inherited disorders

Table 9.9

Inherited Disorders

  • Many are controlled by a single gene


Dominance relations

Dominance Relations

Complete dominance

Incomplete dominance

Codominance


Incomplete dominance

Incomplete Dominance

X

Homozygous

parent

Homozygous

parent

All F1 are

heterozygous

X

F2 shows three phenotypes in 1:2:1 ratio


Codominance abo blood types

Reaction When Blood from Groups Below Is Mixed with

Antibodies from Groups at Left

Blood

Group

(Phenotype)

Antibodies

Present in

Blood

Genotypes

O A B AB

Anti-A

Anti-B

ii

O

IAIA

or

IAi

Anti-B

A

IBIB

or

IBi

Anti-A

B

AB

IAIB

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 others


Pleiotropy

Individual homozygous

for sickle-cell allele

Sickle-cell (abnormal) hemoglobin

Abnormal hemoglobin crystallizes,

causing red blood cells to become sickle-shaped

Sickle cells

5,555

Clumping of cells

and clogging of

small blood vessels

Breakdown of

red blood cells

Accumulation of

sickled cells in spleen

Brain

damage

Pain and

fever

Spleen

damage

Damage to

other organs

Physical

weakness

Heart

failure

Anemia

Impaired

mental

function

Pneumonia

and other

infections

Kidney

failure

Paralysis

Rheumatism

Pleiotropy

  • One gene may have effects on two or more traits

  • Sickle-cell disease


Polygenic inheritance

Polygenic Inheritance

  • A range of small differences in a given trait among individuals

  • Effected by the number of genes and env. Factors


Introduction to genetics

P generation

aabbcc

(very light)

AABBCC

(very dark)

F1 generation

AaBbCc

AaBbCc

20

64

15

64

15

64

1

64

6

64

1

64

6

64

Sperm

1

8

1

8

1

8

1

8

1

8

1

8

1

8

1

8

20

64

1

8

F2 generation

1

8

15

64

1

8

Fraction of population

1

8

Eggs

1

8

6

64

1

8

1

8

1

64

1

8

Skin color


Temperature effects on phenotype

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

Environmental Effects on Plant Phenotype

  • Hydrangea macrophylla

  • Flower color ranges from pink to blue


Linkages

Experiment

Purple flower

PpLI PpLI

Long pollen

  • ObservedPrediction

  • Phenotypesoffspring(9:3:3:1)

Purple long

Purple round

Red long

Red round

215

71

71

24

284

21

21

55

Explanation: linked genes

P L

Parental

diploid cell

PpLI

P I

Meiosis

Most

gametes

P L

P I

Fertilization

Sperm

P I

P L

P L

P L

P L

Most

offspring

P L

P I

Eggs

P I

P I

P I

P I

P L

3 purple long : 1 red round

Not accounted for: purple round and red long

Linkages

  • Certain genes are linked

    • Inheritedtogether because they are close together onthe same chromosome

Figure 9.19


Crossing over

A

B

a

b

A

B

a

b

A

b

a

B

Crossing over

Tetrad

Gametes

Crossing Over

  • Crossing over can separate linked alleles

    • Producing gametes with recombinant chromosomes

Figure 9.20 A


Crossover frequency

Crossover Frequency

A

B

C

D


Full linkage

A

A

a

B

B

b

A

a

B

b

a

b

Full Linkage

x

Parents:

AB

ab

F1 offspring:

All AaBb

meiosis, gamete formation

Equal ratios of two types of gametes:

Figure 12.8aPage 201

50% AB

50% ab


Incomplete linkage

A

a

a

c

c

C

A

C

Incomplete Linkage

AC

ac

x

Parents:

F1 offspring:

All AaCc

meiosis, gamete formation

a

a

A

A

Unequal ratios of four types of gametes:

C

c

C

c

parental

genotypes

recombinant

genotypes

Figure 12.8bPage 201


Discovering linkage

homozygous dominant

female

recessive male

Discovering Linkage

x

Gametes:

X

X

X

Y

All F1

have red eyes

x

Gametes:

X

X

X

Y

1/2

1/2

1/4

1/2

1/2

1/4

1/4

F2

generation:

1/4


Discovering linkage1

Mutant phenotypes

Black

body

(g)

Cinnabar

eyes

(c)

Vestigial

wings

(l)

Brown

eyes

Short

aristae

Chromosome

g

l

c

17%

9%

9.5%

Recombination

frequencies

Normal

wings

(L)

Red

eyes

Long aristae

(appendages

on head)

Gray

body

(G)

Red

eyes

(C)

Wild-type phenotypes

Discovering Linkage

  • Recombination frequencies

    • Used to map the relative positions of genes on chromosomes.

Figure 9.21 B

Figure 9.21 C


Sex determination

X

X

x

X

Y

x

Y

X

X

X

XX

XX

XY

XY

female

(XX)

male

(XY)

Sex Determination

eggs

sperm

Human sex determination interaction.


Effect of y chromosome

appearance of structures

that will give rise to

external genitalia

appearance of

“uncommitted” duct system

of embryo at 7 weeks

Effect of YChromosome

7 weeks

Y

present

Y

absent

Y

present

Y

absent

testes

ovaries

10 weeks

ovary

testis

birth approaching


The x chromosome

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


Sex determination1

22

+

X

22

+

XX

76

+

ZW

76

+

ZZ

32

16

Sex Determination

Figure 9.22 B

Figure 9.22 C

Figure 9.22 D


X linked recessive inheritance

X-Linked Recessive Inheritance

  • Mutant gene on X chromosome

  • Males affected more often


Examples of x linked traits

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 traits1

Queen

victoria

Albert

Louis

Alice

Alexandra

Czar

Nicholas II

of Russia

Alexis

Examples of X-linked Traits

Figure 9.24 A

Figure 9.24 B


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