CHAPTER 9 GENETICS. MENDEL’S LAWS Copyright © 2009 Pearson Education, Inc.

CHAPTER 9

GENETICS

MENDEL’S LAWS

Copyright © 2009 Pearson Education, Inc.

Flower color White

Axial

Purple

Flower position Terminal

YellowSeed color Green

RoundSeed shape Wrinkled

InflatedPod shape Constricted

GreenPod color Yellow

TallStem length Dwarf

Mendel’s law of segregation describes the inheritance of a single character

Example of a monohybrid (HETEROZYGOUS) cross • Parental generation (HOMOZYGOUS): purple flowers

white flowers• F1 generation: all plants with purple flowers • F2 generation: ¾ of plants with purple flowers

¼ of plants with white flowers Mendel needed to explain

• Why one trait seemed to disappear in the F1 generation• Why that trait reappeared in one quarter of the F2

offspring


P generation(true-breedingparents)

Purple flowers White flowers

F1 generation All plants havepurple flowers

F2 generation

Fertilizationamong F1 plants(F1 F1)

of plantshave purple flowers

3–4 of plants

have white flowers

1–4


Four Hypotheses 1. Genes are found in alternative versions called ALLELES; a

GENOTYPE is the listing of alleles an individual carries for a specific gene

2. For each characteristic, an organism inherits two alleles, one from each parent; the alleles can be the same or different

– A homozygous genotype has identical alleles

– A heterozygous genotype has two different alleles



Four Hypotheses 3. If the alleles differ, the DOMINANT allele determines the

organism’s appearance, and the RECESSIVE allele has no noticeable effect

– The PHENOTYPE is the appearance or expression of a trait

– The same phenotype may be determined by more than one GENOTYPE (the alleles carried by the organism)

4. Law of segregation: Allele pairs separate (segregate) from each other during the production of gametes (MEIOSIS) so that a sperm or egg carries only one allele for each gene


P plants

1–2

1–2

Genotypic ratio1 PP : 2 Pp : 1 pp

Phenotypic ratio3 purple : 1 white

F1 plants(hybrids)

Gametes

Genetic makeup (alleles)

All

All Pp

Sperm

Eggs

PP

p

ppPp

Pp

P

pP

pP

P

p

PP pp

All

Gametes

F2 plants

F1 genotypes

1–2

1–2

1–2

1–2

1–4

1–4

1–4

1–4

Formation of eggs

Bb female

F2 genotypes

Formation of sperm

Bb male

B

BB B B

B

b

b

bbbb

COIN TOSS DEMONSTRATION

• 4 students toss coin once…RESULTS?

• 4 students toss coin 3 times…RESULTS?

• All students toss coin once…RESULTS?

• All students toss coin 3 times…RESULTS?

Homologous chromosomes bear the alleles for each character

For a pair of homologous chromosomes, alleles of a gene reside at the same locus

• Homozygous individuals have the same allele on both homologous chromosome

• Heterozygous individuals have a different allele on each homologous chromosome


Gene loci

Homozygousfor thedominant allele

Dominantallele

Homozygousfor therecessive allele

Heterozygous

Recessive allele

Genotype:

P Ba

P

PP

a

aa

b

Bb

HUMAN EXAMPLESof

Dominant and Recessive Genes

Freckles

Widow’s peak

Free earlobe

No freckles

Straight hairline

Attached earlobe

Dominant Traits Recessive Traits

CLASSROOM EXAMPLE

• How many students have free earlobes?

• How many students have attached earlobes?

• Which trait (phenotype) is caused by the DOMINANT gene?

CLASSROOM EXAMPLE (cont.)

• Free earlobe gene = F• Attached earlobe gene = f• What are the possible GENOTYPES for earlobe

shape in the human population?

• What PHENOTYPES are produced by the different genotypes?

CLASSROOM EXAMPLE (cont.)

• A genetics problem:If Mary has attached earlobes and John has free

earlobes (John’s mother has attached earlobes) what are the chances that their children will have attached earlobes? Free earlobes?

Use a PUNNETT SQUARE to prove your answer!

CLASSROOM EXAMPLE

1.) Determine the possible genotypes for Mary and John.

2.) Create a PUNNETT SQUARE.

3.) Determine the PHENOTYPES of the children.

CONNECTION: Genetic traits in humans can be tracked through family pedigrees

A pedigree • Shows the inheritance of a trait in a family

through multiple generations• Demonstrates dominant or recessive inheritance• Can also be used to deduce genotypes of family

members


Ff

Female Male

Attached

Free

First generation(grandparents)

Second generation(parents, aunts,and uncles)

Third generation(two sisters)

Ff Ff

Ff

Ff Ff

Ff

ff

ff ff ff

ff

FF

FF

or

or

MORE GENETICS PROBLEMS

• See LAB MANUAL…

The law of independent assortment is revealed by tracking two characters at once

Example of a dihybrid (HETEROZYGOUS) cross

• Parental generation: round yellow seeds wrinkled green seeds • F1 generation: all plants with round yellow seeds • F2 generation: 9/16 of plants with round yellow seeds

3/16 of plants with round green seeds 3/16 of plants with wrinkled yellow

seeds 1/16 of plants with wrinkled green

seeds Mendel needed to explain

• Why nonparental combinations were observed • Why a 9:3:3:1 ratio was observed among the F2 offspring


The law of independent assortment is revealed by tracking two characters at once

Law of independent assortment • Each pair of alleles segregates

independently of the other pairs of alleles during gamete formation

• R = round; r = wrinkledY = yellow; y = green

• For genotype RrYy, four gamete types are possible: RY, Ry, rY, and ry


P generation

1–2

Hypothesis: Dependent assortment Hypothesis: Independent assortment

1–2

1–2

1–2

1–4

1–4

1–4

1–4

1–4

1–4

1–4

1–4

9––16

3––16

3––16

1––16

RRYY

Gametes

Eggs

F1

generation

SpermSperm

F2

generation

Eggs

Gametes

rryy

RrYy

ryRY

ryRY

ry

RY

Hypothesized(not actually seen)

Actual results(support hypothesis)

RRYY rryy

RrYy

ryRY

RRYY

rryy

RrYy

ry

RY

RrYy

RrYy

RrYy

rrYYRrYY

RRYyRrYY

RRYy

rrYy

rrYy

Rryy

Rryy

RRyy

rY

Ry

ry

Yellowround

Greenround

Greenwrinkled

Yellowwrinkled

RY rY Ry

F1 generation R

Metaphase Iof meiosis(alternative

arrangements)

r

Y

y

Rr

Y y

R r

Y y

All round yellow seeds(RrYy)

F1 generation R


arrangements)

r

Y

y

Rr

Y y

R r

Y y


Anaphase Iof meiosis

Metaphase IIof meiosis

R

y

r

Y

r

y

R

Y

R r

Y y

Rr

Y y

F1 generation R


arrangements)

r

Y

y

Rr

Y y

R r

Y y


Anaphase Iof meiosis

Metaphase IIof meiosis

R

y

r

Y

r

y

R

Y

R r

Y y

Rr

Y y

1–4

R

y

Ry

R

y

r

Y

1–4 rY

r

Y

1–4 ry

r

y

1–4RY

R

Y

R

YGametes

Fertilization among the F1 plants

:39 :3 :1F2 generation

r

y

QUESTION…

• Why would genes independently assort even if they are on the same chromosome???

HINT: Think Prophase I (meiosis)…

Gametes

Tetrad Crossing over

Yr yrr y

R YR Y R y

CONNECTION: Many inherited disorders in humans are controlled by a single gene

Inherited human disorders show• Recessive inheritance

– Two recessive alleles are needed to show disease– Heterozygous parents are carriers of the disease-

causing allele– Probability of inheritance increases with inbreeding,

mating between close relatives• Dominant inheritance

– One dominant allele is needed to show disease– Dominant lethal alleles are usually eliminated from

the population


EXAMPLES of HUMAN DISORDERS

The next slide lists a number of Recessive and Dominant AUTOSOMAL disorders…

AUTOSOMAL means that the defective gene is found on one of the autosomes.

AUTOSOMES = Chromosomes that are NOT one of the sex chromosomes

SEX Chromosomes = The X or Y chromosomes that determine the sex of the human

XX = Female XY = Male

Genetics Problem

• Cystic Fibrosis (Autosomal Recessive):• If Mary and John are both heterozygous

(carriers) for the cystic fibrosis gene, what are the chances that their children will have cystic fibrosis???

Parents NormalCarrierCc

Offspring

Sperm

Eggs

ccCystic

Fibrosisc

CcNormal(carrier)

CCNormalC

C c

CcNormal(carrier)

NormalCarrierCc

Genetics Problem

• Huntington’s (Autosomal Dominant):• Only Genotypes = Hh (Huntington’s) or hh

(normal)• If Mary is normal and John will eventually

develop Huntington’s disease, what are the chances that their children will develop Huntington’s disease???

Parents Maryhh

Offspring

Sperm

Eggshh

Normal

h

hhNormal

HhHuntington’s

h

H h

HhHuntington’s

JohnHh

VARIATIONS ON MENDEL’S LAWS


Incomplete dominance results in intermediate phenotypes

Incomplete dominance• Neither allele is dominant over the other• Expression of both alleles is observed as an

intermediate phenotype in the heterozygous individual


Flower Color in Plants…

P generation

1–2

1–2

1–2

1–2

1–2

1–2

F1 generation

F2 generation

RedRR

Gametes

Gametes

Eggs

Sperm

RR rR

Rr rr

R

r

R r

R r

PinkRr

R r

Whiterr

Hypercholesterolemia in Humans…

HHHomozygous

for ability to makeLDL receptors

hhHomozygous

for inability to makeLDL receptors

HhHeterozygous

LDL receptor

LDL

CellNormal Mild disease Severe disease

Genotypes:

Phenotypes:

Sickle Cell Anemia in Humans

• HH = Normal Hemoglobin• Hh = Sickle Cell Trait• hh = Sickle Cell Anemia

The sickle cell gene also affects many phenotypic characters

Pleiotropy• One gene influencing many characteristics• The gene for sickle cell disease

– Affects the type of hemoglobin produced– Affects the shape of red blood cells– Causes anemia– Causes organ damage– Is related to susceptibility to malaria


Clumping of cellsand clogging of

small blood vessels

Pneumoniaand otherinfections

Accumulation ofsickled cells in spleen

Pain andfever

Rheumatism

Heartfailure

Damage toother organs

Braindamage

Spleendamage

Kidneyfailure

Anemia

ParalysisImpairedmental

function

Physicalweakness

Breakdown ofred blood cells

Individual homozygousfor sickle-cell allele

Sickle cells

Sickle-cell (abnormal) hemoglobin

Abnormal hemoglobin crystallizes,causing red blood cells to become sickle-shaped

Genetics Problem: Sickle Cell Anemia

• If Mary has normal hemoglobin and John has sickle cell anemia, what are the chances that their children will have sickle cell anemia? Will have sickle cell trait? Will have normal hemoglobin?

Sickle Cell Anemia Problem (cont.)

• What is Mary’s genotype?• What is John’s genotype?• Create the Punnett Square…

Many genes have more than two alleles in the population: MULTIPLE ALLELES

Multiple alleles• More than two alleles are found in the

population• A diploid individual can carry any two of

these alleles• The ABO blood group has three alleles,

leading to four phenotypes: type A, type B, type AB, and type O blood


CODOMINANCE

Codominance• Neither allele is dominant over the

other• Expression of both alleles is observed

as a distinct phenotype in the heterozygous individual

• Observed for type AB blood


BloodGroup(Phenotype) Genotypes

O

A

ii

IAIA

orIAi

Red Blood Cells

Carbohydrate A

AntibodiesPresent inBlood

Anti-AAnti-B

Reaction When Blood from Groups Below Is Mixedwith Antibodies from Groups at Left

Anti-B

O A B AB

BIBIB

orIBi

Carbohydrate B

AB IAIB —

Anti-A

GENETICS PROBLEMS

• ABO BLOOD TYPE…See Lab Manual

A single character may be influenced by many genes: POLYGENIC INHERITANCE

Polygenic inheritance• Many genes influence one trait• Skin color is affected by at least three genes


P generation

1–8

F1 generation

F2 generation

Fra

ctio

n o

f p

op

ula

tio

n

Skin color

Eggs

Sperm1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

1–8

aabbcc(very light)

AABBCC(very dark)

AaBbCc AaBbCc

1––64

15––64

6––64

1––64

15––64

6––64

20––64

1––64

15––64

6––64

20––64

The environment affects many characters

Phenotypic variations are influenced by the environment

• Skin color is affected by exposure to sunlight • Susceptibility to diseases, such as cancer, has

hereditary and environmental components


SEX CHROMOSOMES AND

SEX-LINKED GENES


Chromosomes determine sex in many species

X-Y system in mammals (HUMANS), fruit flies

• XX = female; XY = male X-O system in grasshoppers and roaches

• XX = female; XO = male

Z-W in system in birds, butterflies, and some fishes• ZW = female, ZZ = male

Chromosome number in ants and bees• Diploid = female; haploid = male


X

Y

GENETICS PROBLEM

• What are the chances that Mary and John’s children will be Boys? Girls?

• Mary = XX• John = XY• Create a PUNNETT SQUARE…

CONNECTION: Sex-linked disorders affect mostly males

Males express X-linked disorders such as the following when recessive alleles are present in one copy

• Hemophilia• Colorblindness• Duchenne muscular dystrophy


QueenVictoria

Albert

Alice Louis

Alexandra CzarNicholas IIof Russia

Alexis

GENETICS PROBLEMS

• SEX LINKED TRAITS…See Lab Manual

CHAPTER 9 GENETICS. MENDEL’S LAWS Copyright © 2009 Pearson Education, Inc.

Documents

pearson education

identical alleles

listing of alleles

purple flowers of plants

purple flowers f2 generation

coin onceresults

heterozygous genotype

trait phenotype