Gregor Mendel’s Discoveries• Pre-Mendel Blending Theory of

Heredity– Hereditary material from each

parent mixes in the offspring• 2 problems• Individuals of a population should

reach a uniform appearance after many generations

• Once traits are blended, they can’t be separated

• Gregor Mendel Particulate Theory of Heredity

– Traits are inherited as separate factors

• Mendel used quantitative approach

• Studied peas for 3 reasons:– Many varieties– Self pollinating/cross pollinating– Each variety had 2 alternative

forms

• Used true breeding varieties • Used large sample sizes and

accurate observations• Used math to develop

probabilities and perform statistical analyses

• Used terms to define generations as: P, F1, F2

• Developed terms such as:– Alleles (factor)– Dominant/Recessive– Homozygous/ Heterozygous– Phenotype/Genotype– Testcross

• Derived 2 principles:– Law of segregation – two alleles for a character

separate when gametes are formed

Law of Independent assortment – each pair of alleles segregate into gametes independently

Degrees of Dominance

• Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical

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Fig. 14-9

Rr RrSegregation of

alleles into eggs

Sperm

R

R

R RR

R rrr

r

r

r1/2

1/2

1/2

1/2

Segregation ofalleles into sperm

Eggs1/4

1/4

1/41/4

Fig. 14-16

Parents

Normal Normal

Sperm

Eggs

NormalNormal(carrier)

Normal(carrier) Albino

Aa Aa

A

AAA

Aa

a

Aaaa

a

Fig. 14-17

Eggs

Parents

Dwarf Normal

Normal

Normal

Dwarf

Dwarf

Sperm

Dd dd

dD

Dd dd

ddDd

d

d

• Dominant condition• Fatal• Only one Huntington’s

allele needed• Produces abnormal

protein that clumps up in cell nuclei – especially nerve cells in the brain

If an allele for tall plants (T) is dominant to short plants (t), what offspring would youexpect from a TT x Tt cross?

A. ½ tall; ½ shortB. ¾ tall; ¼ shortC. All tall

Concept Quiz

Identify vocab term:

• This allele gets masked in the phenotype

• Genetic Makup

• Alternate forms of a gene

• The allele that gets fully expressed

• Two identical alleles for a trait

• Physical appearance

If two heterozygotes are crossed for dimpled chin. What will be the expected genotypic ratio?

Phenotypic ratio?

• 1:2:1

• 3:1

Extending Mendelian Genetics for a Single Gene

• Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations:– When alleles are on the sex chromosomes– When alleles are not completely dominant or

recessive– When a gene has more than two alleles– When a gene produces multiple phenotypes

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

• In codominance, phenotypes of both alleles are exhibited in the heterozygote

• Affects 1 in 2500 individuals in European populations

Fig. 14-10-1

Red

P Generation

Gametes

WhiteCRCR CWCW

CR CW

Fig. 14-10-2

Red

P Generation

Gametes

WhiteCRCR CWCW

CR CW

F1 GenerationPinkCRCW

CR CWGametes 1/21/2

In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties

Fig. 14-10-3

Red

P Generation

Gametes

WhiteCRCR CWCW

CR CW

F1 GenerationPinkCRCW

CR CWGametes 1/21/2

F2 Generation

Sperm

Eggs

CR

CR

CW

CW

CRCR CRCW

CRCW CWCW

1/21/2

1/2

1/2

Incomplete Dominance

Concept Quiz A red carnation and a white carnationproduce offspring that are all pink. The typeof inheritance pattern occurring is:

A. Complete dominanceB. Incomplete dominanceC. Codominance

Red is dominant to white in flower petal color. If a homozygous dominant is crossed

with a homozygous recessive and this inheritance is incomplete dominance. What

will be the phenotypic ratio of this cross?

• 1:2:1

Fur color in rabbits shows incomplete dominance.FBFB individuals are brown, FBFW individuals arecream, FWFW individuals are white. What is theexpected ratio of a FBFW x FWFW cross?

A. 3 white : 1 brownB. 3 white : 1 creamC. 2 white : 2 cream

Concept Quiz

Multiple Alleles

• Most genes exist in populations in more than two allelic forms

• For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i.

Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Fig. 14-11

IA

IB

i

A

B

none(a) The three alleles for the ABO blood groups and their associated carbohydrates

Allele Carbohydrate

GenotypeRed blood cell

appearancePhenotype

(blood group)

IAIA or IA i A

BIBIB or IB i

IAIB AB

ii O

(b) Blood group genotypes and phenotypes

Who is the universal recipient?Who is the universal donor?

What blood types would the offspring have if one parentIs type O and one parent is AB? Both parents AB?

Pleiotropy The ability of a gene to affect an organism in many ways

Fig. 14-12

BbCc BbCc

Sperm

EggsBC bC Bc bc

BC

bC

Bc

bc

BBCC

1/41/4

1/41/4

1/4

1/4

1/4

1/4

BbCC BBCc BbCc

BbCC bbCC BbCc bbCc

BBCc BbCc

BbCc bbCc

BBcc Bbcc

Bbcc bbcc

9 : 3 : 4

Epistasis

• B = Black• b = Brown• C = Pigment• c = nonpig

A gene at one locusAlters a gene at another locus

Polygenic Inheritance

• Additive effect of 2 or more genes on a single phenotypic character

• Eye color, skin color

Identify each type of inheritance?• Both alleles are expressed in heterozygote• More than two alleles are possible for a trait• The dominant allele masks the recessive

allele• Additive effect of two or more genes• Intermediate phenotype in heterozygotes• A gene at one locus controls a gene at

another locus

Quantitative Genetics

• The environment plays a role – traits such as height, weight, musical ability, susceptibility to cancer,and intelligence

• Quantitative traits show continuous variation; we can see a large range of phenotypes in the population

• The amount of variation in a population is called variance

Genetically the same butPhenotypically different

Nutrition, exercise, and exposure to sun can cause differences in phenotypes

Diversity in Offspring

• Mutation, independent assortment, crossing over, and random fertilization result in unique combinations of alleles

• These processes produce the diversity of individuals found in humans and all other sexually reproducing biological populations

• You are one out of 64 trillion genetically different children that your parents could produce

• Fraternal (non-identical)– dizygotic: two

separate fertilized eggs

– not genetically the same

• Identical

– monozygotic: one single fertilized egg that separates

– genetically the same

Sex Determination andSex Linkage

• Some genes are on the X chromosome and are inherited in a specific manner

• In humans, sex determination involves the X and Y chromosomes

Sex Linkage• The genes on the X or Y

chromosomes are called sex-linked genes

• Genes on X are called “X-linked,” while those on Y are called “Y-linked”

• The X chromosome is much larger and carries far more genetic information

X-Linked Genes

• Since males only have one X chromosome, they are more likely to suffer from X-linked diseases– Hemophilia– red-green color

blindness– muscular dystrophy

• Since females get one X chromosome from each parent, and have two copies, they are less likely to suffer from X-linked diseases

• X inactivation allows some female organisms to shut off their X chromosomes

7.3 Pedigrees

• A pedigree is a chart showing inheritance patterns in a family

• Pedigrees can be used to identify different types of inheritance patterns