Mendel s laws
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MENDEL’S LAWS. 9.1 The science of genetics has ancient roots The historical roots of genetics, the science of heredity Date back to ancient attempts at selective breeding. Petal. Stamen. Carpel. Figure 9.2 A. Figure 9.2 B. 9.2 Experimental genetics began in an abbey garden

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Mendel s laws

MENDEL’S LAWS

  • 9.1 The science of genetics has ancient roots

    • The historical roots of genetics, the science of heredity

      • Date back to ancient attempts at selective breeding


Mendel s laws

Petal

Stamen

Carpel

Figure 9.2 A

Figure 9.2 B

  • 9.2 Experimental genetics began in an abbey garden

    • Modern genetics

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


Mendel s laws

  • 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


Mendel s laws

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

  • Mendel hypothesized that there are alternative forms of genes

    • The units that determine heritable traits

Figure 9.2 D


Mendel s laws

P generation

(true-breedingparents)

Purple flowers

White flowers

F1 generation

All plants havepurple flowers

Fertilizationamong F1 plants(F1 F1)

F2 generation

3

4

1

4

of plantshave white flowers

of plantshave purple flowers

  • 9.3 Mendel’s law of segregation describes the inheritance of a single characteristic

    • From his experimental data

      • Mendel deduced that an organism has two genes (alleles) for each inherited characteristic

Figure 9.3 A


Mendel s laws

  • For each characteristic

    • An organism inherits two alleles, one from each parent


Mendel s laws

  • If the two alleles of an inherited pair differ

    • Then one determines the organism’s appearance and is called the dominant allele

  • The other allele

    • Has no noticeable effect on the organism’s appearance and is called the recessive allele


Mendel s laws

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 law of segregation

    • Predicts that allele pairs separate from each other during the production of gametes

Figure 9.3 B


Mendel s laws

Dominantallele

Gene loci

a

B

P

a

b

P

Recessiveallele

Genotype:

PP

aa

Bb

Heterozygous

Homozygousfor thedominant allele

Homozygousfor therecessive allele

  • 9.4 Homologous chromosomes bear the two alleles for each characteristic

    • Alternative forms of a gene (allele)

      • Reside at the same locus on homologous chromosomes

Figure 9.4


Mendel s laws

  • 9.5 The law of independent assortment is revealed by tracking two characteristics at once

    • By looking at two characteristics at once

      • Mendel tried to determine how two characteristics were inherited


Mendel s laws

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

  • Mendel’s law of independent assortment

    • States that alleles of a pair segregate independently of other allele pairs during gamete formation

Figure 9.5 A


Mendel s laws

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

  • An example of independent assortment


Mendel s laws

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

  • 9.6 Geneticists use the testcross to determine unknown genotypes

    • The offspring of a testcross, a mating between an individual of unknown genotype and a homozygous recessive individual

      • Can reveal the unknown’s genotype

Figure 9.6


Mendel s laws

  • 9.7 Mendel’s laws reflect the rules of probability

    • Inheritance follows the rules of probability


Mendel s laws

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

  • The rule of multiplication

    • Calculates the probability of two independent events

  • The rule of addition

    • Calculates the probability of an event that can occur in alternate ways

Figure 9.7


Connection

Dominant Traits

Recessive Traits

Freckles

No freckles

Widow’s peak

Straight hairline

Free earlobe

Attached earlobe

CONNECTION

  • 9.8 Genetic traits in humans can be tracked through family pedigrees

    • The inheritance of many human traits

      • Follows Mendel’s laws

Figure 9.8 A


Mendel s laws

D ?

John

Eddy

Dd

Abigail

Linnell

D ?

Hepzibah

Daggett

Dd

Joshua

Lambert

dd

Jonathan

Lambert

Dd

Elizabeth

Eddy

D ?

Abigail

Lambert

Dd Dd dd Dd Dd Dd dd

Female Male

Deaf

Hearing

  • Family pedigrees

    • Can be used to determine individual genotypes

Figure 9.8 B


Connection1

Table 9.9

CONNECTION

  • 9.9 Many inherited disorders in humans are controlled by a single gene

    • Some autosomal disorders in humans


Mendel s laws

Parents

Normal

Dd

Normal

Dd

Sperm

Dd

Dd

Normal

(carrier)

DD

Normal

D

Offspring

Eggs

Dd

Normal

(carrier)

dd

Deaf

d

  • Recessive Disorders

    • Most human genetic disorders are recessive

Figure 9.9 A


Mendel s laws

  • Dominant Disorders

    • Some human genetic disorders are dominant

Figure 9.9 B


Connection2

CONNECTION

  • 9.10 New technologies can provide insight into one’s genetic legacy

    • New technologies

      • Can provide insight for reproductive decisions


Mendel s laws

  • Identifying Carriers

    • For an increasing number of genetic disorders

      • Tests are available that can distinguish carriers of genetic disorders


Mendel s laws

Chorionic villus sampling (CVS)

Amniocentesis

Needle inserted

through abdomen to

extract amniotic fluid

Ultrasound

monitor

Ultrasound

monitor

Suction tube inserted

through cervix to extract

tissue from chorionic villi

Fetus

Fetus

Placenta

Placenta

Chorionic

villi

Uterus

Cervix

Cervix

Uterus

Amniotic

fluid

Centrifugation

Fetal

cells

Fetal

cells

Biochemical

tests

Several

weeks

Several

hours

Karyotyping

Figure 9.10 A

  • Fetal Testing

    • Amniocentesis and chorionic villus sampling (CVS)

      • Allow doctors to remove fetal cells that can be tested for genetic abnormalities


Mendel s laws

  • Fetal Imaging

    • Ultrasound imaging

      • Uses sound waves to produce a picture of the fetus

Figure 9.10 B


Mendel s laws

  • Newborn Screening

    • Some genetic disorders can be detected at birth

      • By simple tests that are now routinely performed in most hospitals in the United States


Mendel s laws

  • Ethical Considerations

    • New technologies such as fetal imaging and testing

      • Raise new ethical questions


Variations on mendel s laws

VARIATIONS ON MENDEL’S LAWS

  • 9.11 The relationship of genotype to phenotype is rarely simple

    • Mendel’s principles are valid for all sexually reproducing species

      • But genotype often does not dictate phenotype in the simple way his laws describe


Mendel s laws

P generation

Red

RR

White

rr

r

R

Gametes

F1 generation

Pink

Rr

Genotypes:

1

2

1

2

HH

Homozygous

for ability to make

LDL receptors

Hh

Heterozygous

hh

Homozygous

for inability to make

LDL receptors

r

R

Gametes

Sperm

Phenotypes:

1

2

1

2

r

R

LDL

Pink

rR

LDL

receptor

1

2

Red

RR

R

Eggs

F2 generation

Pink

Rr

White

rr

1

2

Cell

r

Mild disease

Severe disease

Normal

  • 9.12 Incomplete dominance results in intermediate phenotypes

    • When an offspring’s phenotype is in between the phenotypes of its parents

      • It exhibits incomplete dominance

Figure 9.12 A

Figure 9.12 B


Mendel s laws

  • 9.13 Many genes have more than two alleles in the population

    • In a population

      • Multiple alleles often exist for a characteristic


Mendel s laws

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

  • The ABO blood type in humans

    • Involves three alleles of a single gene

  • The alleles for A and B blood types are codominant

    • And both are expressed in the phenotype

Figure 9.13


Mendel s laws

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

  • 9.14 A single gene may affect many phenotypic characteristics

    • In pleiotropy

      • A single gene may affect phenotype in many ways

Figure 9.14


Mendel s laws

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

1

8

Fraction of population

Eggs

1

8

6

64

1

8

1

8

1

64

1

8

Skin color

  • 9.15 A single characteristic may be influenced by many genes

    • Polygenic inheritance

      • Creates a continuum of phenotypes

Figure 9.15


Mendel s laws

  • 9.16 The environmental affects many characteristics

    • Many traits are affected, in varying degrees

      • By both genetic and environmental factors

Figure 9.16


Connection3

CONNECTION

  • 9.17 Genetic testing can detect disease-causing alleles

    • Predictive genetic testing

      • May inform people of their risk for developing genetic diseases


The chromosomal basis of inheritance

THE CHROMOSOMAL BASIS OF INHERITANCE

  • 9.18 Chromosome behavior accounts for Mendel’s laws

    • Genes are located on chromosomes

      • Whose behavior during meiosis and fertilization accounts for inheritance patterns


Mendel s laws

All round yellow seeds(RrYy)

F1 generation

R

14

14

14

14

RY

Ry

rY

ry

y

r

Y

r

R

r

R

Metaphase Iof meiosis(alternative arrangements)

y

Y

y

Y

r

R

r

R

Anaphase Iof meiosis

Y

y

y

Y

r

r

R

R

Metaphase IIof meiosis

y

y

Y

Y

y

Y

Y

Y

y

y

Y

y

Gametes

R

r

r

R

r

R

r

R

Fertilization among the F1 plants

: 3

F2 generation

9

: 3

: 1

(See Figure 9.5A)

  • The chromosomal basis of Mendel’s laws

Figure 9.18


Mendel s laws

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

  • 9.19 Genes on the same chromosome tend to be inherited together

    • Certain genes are linked

      • They tend to be inheritedtogether because they reside close together onthe same chromosome

Figure 9.19


Mendel s laws

A

B

a

b

A

B

a

b

A

b

a

B

Crossing over

Tetrad

Gametes

  • 9.20 Crossing over produces new combinations of alleles

    • Crossing over can separate linked alleles

      • Producing gametes with recombinant chromosomes

Figure 9.20 A


Mendel s laws

  • Thomas Hunt Morgan

    • Performed some of the early studies of crossing over using the fruit fly Drosophila melanogaster

Figure 9.20 B


Mendel s laws

Experiment

Black body,

vestigial

wings

Gray body,

long wings

(wild type)

GgLI

ggll

Male

Female

Offspring

Gray long

Black long

Black vestigial

Gray vestigial

965

944

206

185

Parental

phenotypes

Recombinant

phenotypes

391 recombinants

Recombination frequency =

= 0.17 or 17%

2,300 total offspring

Explanation

g

l

G

L

ggll

(male)

GgLI

(female)

g

g

l

l

g

g

g

G

L

l

G

l

L

l

Eggs

Sperm

g

g

L

G

L

G

l

l

g

g

g

g

l

l

l

l

Offspring

  • Morgan’s experiments

    • Demonstrated the roleof crossing over in inheritance

Figure 9.20 C


Mendel s laws

  • 9.21 Geneticists use crossover data to map genes

    • Morgan and his students

      • Used crossover data to map genes in Drosophila

Figure 9.21 A


Mendel s laws

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

  • Recombination frequencies

    • Can be used to map the relative positions of genes on chromosomes.

Figure 9.21 B

Figure 9.21 C


Sex chromosomes and sex linked genes

(male)

(female)

44

+

XX

44

+

XY

Parents’

diploid

cells

22

+

X

22

+

X

22

+

Y

Egg

Sperm

44

+

XX

Offspring

(diploid)

44

+

XY

SEX CHROMOSOMES AND SEX-LINKED GENES

  • 9.22 Chromosomes determine sex in many species

    • In mammals, a male has one X chromosome and one Y chromosome

      • And a female has two X chromosomes

Figure 9.22 A


Mendel s laws

  • The Y chromosome

    • Has genes for the development of testes

  • The absence of a Y chromosome

    • Allows ovaries to develop


Mendel s laws

22

+

X

22

+

XX

76

+

ZW

76

+

ZZ

32

16

  • Other systems of sex determination exist in other animals and plants

Figure 9.22 B

Figure 9.22 C

Figure 9.22 D


Mendel s laws

  • 9.23 Sex-linked genes exhibit a unique pattern of inheritance

    • All genes on the sex chromosomes

      • Are said to be sex-linked

    • In many organisms

      • The X chromosome carries many genes unrelated to sex


Mendel s laws

  • In Drosophila

    • White eye color is a sex-linked trait

Figure 9.23 A


Mendel s laws

Female

Male

Female

Male

Female

Male

Xr Y

XR XR

XR Xr

Xr Y

XR Xr

XR Y

Sperm

Sperm

Sperm

Xr

Y

XR

Y

Xr

Y

XR Y

XR Xr

Eggs

XR

XR

XR XR

XR Y

XR

XR Xr

XR Y

Eggs

Eggs

R = red-eye allele

r = white-eye allele

Xr Xr

Xr Y

Xr Y

Xr XR

Xr

Xr

Figure 9.23 B

Figure 9.23 C

Figure 9.23 D

  • The inheritance pattern of sex-linked genes

    • Is reflected in females and males


Connection4

Queen

victoria

Albert

Louis

Alice

Alexandra

Czar

Nicholas II

of Russia

Alexis

CONNECTION

  • 9.24 Sex-linked disorders affect mostly males

    • Most sex-linked human disorders

      • Are due to recessive alleles

      • Are mostly seen in males

Figure 9.24 A

Figure 9.24 B


Mendel s laws

  • A male receiving a single X-linked allele from his mother

    • Will have the disorder

  • A female

    • Has to receive the allele from both parents to be affected


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