NOTES: Ch 14, part 2 –

Extending Mendelian

Genetics

The Spectrum of Dominance

● Complete dominance occurs when the

phenotypes of the heterozygote and

dominant homozygote are identical

● In codominance 2 dominant alleles affect

the phenotype in separate, distinguishable

ways

● Example: the human blood group MN

CODOMINANCE:

CODOMINANCE:● inheritance

characterized by full

expression of both

alleles in the

heterozygote

Example: in chickens,

BB = black feathers,

bb = white, Bb =

“speckled” (both black

and white feathers)

INCOMPLETE DOMINANCE:

● inheritance where one allele is not completely dominant over the other, so the heterozygote has a phenotype that is intermediate between the phenotype of the 2 homozygotes

Ex: RR = red flowers,

rr = white, Rr = pink

The Relationship Between Dominance

and Phenotype

● Dominant and recessive alleles do not

really “interact”

● they lead to synthesis of different

proteins that produce a phenotype

Frequency of Dominant Alleles

● Dominant alleles are not necessarily more

common in populations than recessive

alleles

Multiple Alleles

● Most genes exist in populations in more

than two allelic forms

● The ABO blood group in humans is

determined by multiple alleles

Table 14.2

Polygenic Inheritance

● Many human characters vary in the

population along a continuum and are called

quantitative characters

POLYGENIC INHERITANCE:

● mode of inheritance in which the additive effect of 2 or more genesdetermines a single phenotypic character

Examples: height

hair color (4 gene pairs)

eye color (2 gene pairs)

skin color

AaBbCc AaBbCc

aabbcc Aabbcc AaBbcc AaBbCc AABbCc AABBCc AABBCC

20⁄64

15⁄64

6⁄64

1⁄64

● Quantitative

variation usually

indicates polygenic

inheritance

(An additive effect of

2 or more genes on

a single phenotype)

PLEIOTROPY:● the ability of a single gene to have

multiple phenotypic effects

Examples: sickle cell anemia

Siamese cats & tigers

EPISTASIS:

● interaction between 2 nonallelic genes in

which one modifies the phenotypic expression

of the other

Example: gene for pigment deposition is

epistatic to gene for melanin production in

mice

● An example of epistasis:

BC bC Bc bc1⁄41⁄4

1⁄41⁄4

BC

bC

Bc

bc

1⁄4

1⁄4

1⁄4

1⁄4

BBCc BbCc BBcc Bbcc

Bbcc bbccbbCcBbCc

BbCC bbCC BbCc bbCc

BBCC BbCC BBCc BbCc

9⁄163⁄16

4⁄16

BbCc BbCc

Sperm

Eggs

Nature and Nurture: The Environmental

Impact on Phenotype

● Another departure from simple Mendelian

genetics arises when the phenotype for a

character depends on environment as well

as on genotype

● The norm of reaction is the phenotypic

range of a particular genotype that is

influenced by the environment

Figure 14.13

MULTIFACTORIAL INHERITANCE:

● A trait depends on many factors; a variety of

genotypes as well as environmental

influences (such as certain chemicals,

medicines, or diet)

Examples of disorders that may be a

result of multifactorial inheritance:

diabetes, heart disease, neural tube defects,

autism, Alzheimer disease, ALS, and many

cancer syndromes

~Same genotype

~Phenotype

depends on acidity

of soil

~height controlled by

several genes

~height influenced by

amount of nourishment

Integrating a Mendelian View of Heredity

and Variation

● An organism’s phenotype includes its

physical appearance, internal anatomy,

physiology, and behavior

● the phenotype reflects its overall genotype

and unique environmental history

Inheritance Patterns for

Genetic Diseases in Humans

Pedigree Analysis

● A pedigree is a family tree that describes

the interrelationships of parents and children

across generations

George Arlene

Sandra Tom Sam Wilma Ann Michael

Daniel Alan Tina

Christopher

Carla

● Inheritance patterns of particular traits can

be traced and described using pedigrees

Figure 14.14 A, B

Ww ww ww Ww

wwWwWwwwwwWw

WW

or

Ww

ww

First generation

(grandparents)

Second generation

(parents plus aunts

and uncles)

Third

generation

(two sisters)

Ff Ff ff Ff

ffFfFfffFfFF or Ff

ff FF

or

Ff

Widow’s peak No Widow’s peak Attached earlobe Free earlobe

(a) Dominant trait (widow’s peak) (b) Recessive trait (attached earlobe)

1) Autosomal Recessive:

● recessive alleles that

cause human disorders

are usually defective

versions of normal alleles

● defective alleles code for

either a malfunctioning

protein or no protein at all

● heterozygotes can be

phenotypically normal,

if 1 copy of the normal

allele is all that is

needed to produce

sufficient quantities of

the “good” protein

Examples: cystic

fibrosis, Tay-Sachs,

sickle cell anemia

● “Carriers” are

heterozygous

individuals who

carry the recessive

allele but are

phenotypically

normal

Cystic Fibrosis

● Symptoms of cystic fibrosis include:

-Mucus buildup in the some internal organs

-Abnormal absorption of nutrients in the

small intestine

Sickle-Cell Disease

● Sickle-cell disease:

-Affects one out of 400 African-Americans

-Is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells

● Symptoms include:

-Physical weakness, pain, organ damage, and even paralysis

2) Autosomal Dominant:

● only 1 dominant allele is needed in order to produce the effects of these diseases (heterozygous)

● Lethal homozygous dominant condition results in spontaneous abortion of fetus

● homozygous

recessives are of

normal phenotype

Examples:

achondroplasia,

Huntington’s

Disease

● ACHONDROPLASIA:

a form of dwarfism that

is lethal when

homozygous for the

dominant allele

Figure 14.15

● HUNTINGTON’S DISEASE: a degenerative

disease of the nervous system

-it has no obvious phenotypic effects until

about 35 to 40 years of age

Figure 14.16

Genetic Testing and Counseling

● Genetic counselors can provide information

to prospective parents concerned about a

family history for a specific disease

Counseling Based on Mendelian

Genetics and Probability Rules

● Using family histories

genetic counselors help

couples determine the

odds that their children will

have genetic disorders

Tests for Identifying Carriers

● For a growing number of diseases tests are

available that identify carriers and help

define the odds more accurately

Fetal Testing

● AMNIOCENTESIS: the liquid that bathes

the fetus is removed and tested

● CHORIONIC VILLUS SAMPLING (CVS): a

sample of the placenta is removed and

tested