Mendel and the Gene Idea
Mendel and
the Gene Idea
KC#1 Mendel used the scientific approach to identify two
laws of inheritance.
KC#2 The laws of probability govern Mendelian inheritance.
KC#3 Inheritance patterns are often more complex than
predicted by simple Mendelian genetics.
Conrado T. Sotelo
High School Faculty MGCNLCA
SY 2014-2015
Key Concept 3
Inheritance patterns are often
more complex than predicted
by simple Mendelian genetics
KC#4 Many human traits follow Mendelian patterns of
inheritance.
Extending
Mendelian
Genetics for a
Single Gene
Extending Mendelian Genetics for a Single Gene
Complete Dominance of One Allele
The inheritance of characters determined by a single
gene deviates from simple Mendelian patterns when:
Heterozygous phenotype same
as that of homozygous dominant
1. alleles are not completely dominant or
recessive;
2. when a particular gene has more than two
alleles; or
3. when a single gene produces multiple
phenotypes.
1. Incomplete Dominance of Either Allele
Heterozygous phenotype intermediate
between the two homozygous phenotypes
2. Codominance
2. Codominance
The two alleles each affect
the phenotype in separate,
distinguishable ways
Both phenotypes expressed in
heterozygotes
3. Multiple Alleles
Only two alleles
exist for each of
the seven pea
characters that
Mendel studied,
but most genes
exist in more than
two allelic forms.
3. Multiple Alleles
In the whole population, some
genes have more than two alleles
Example: ABO blood group
alleles
Blood Group in Humans
Blood Group in Humans
The ABO blood groups in humans
are determined by three alleles of
a single gene: IA, IB, and i.
Each person has two alleles of the
three for the blood group gene,
which determines his or her blood
group (phenotype): A, B, AB, or O.
Mother
(Type A)
Father
(Type B)
IA IA
IB i
IA IA
IB IB
IA i
IB i
Pleiotropy
So far, we have treated Mendelian
inheritance as though each gene
affects only one phenotypic character.
Most genes, however, have
multiple phenotypic effects, a
property called pleiotropy.
Examples of Pleiotropy
Humans: Pleiotropic alleles are
responsible for the multiple
symptoms associated with certain
hereditary diseases, such as cystic
fibrosis and sickle-cell disease.
Examples of Pleiotropy
Effects of the alleles of a single gene
Pea plants: The gene that determines
flower color also affects the color of
the coating on the outer surface of
the seed, which can be gray or white.
1.Dominance relationships (complete,
incomplete and codominance)
2. Multiple alleles
3. Pleiotropy
Epistasis
Extending
Mendelian
Genetics for Two
or More Genes
Epistasis
The phenotypic expression of a
gene at one locus alters that of a
gene at a second locus.
One allele hiding the effects of
another allele.
Epistasis
BBEE - black
BbEE - black
BbEe - black
bbEE - brown
bbEe - brown
Polygenic Inheritance
Mende’s Flower Color: “either-or” basis
BBee - yellow
Bbee - yellow
bbee - yellow
Polygenic Inheritance
Human skin color and height: either-or
classification is impossible because the characters
vary in the population in gradations along a
continuum
Quantitative characters
Polygenic Inheritance
Polygenic Inheritance
Quantitative variation usually indicates
polygenic inheritance,
the additive effect of two or more
genes on a single phenotypic character.
Skin pigmentation in humans is
controlled by at least three
separately inherited genes
(probably more).
Mendel and the Gene Idea
KC#1 Mendel used the scientific approach to identify two
laws of inheritance.
KC#2 The laws of probability govern Mendelian inheritance.
KC#3 Inheritance patterns are often more complex than
predicted by simple Mendelian genetics.
KC#4 Many human traits follow Mendelian patterns of
inheritance.
Key Concept 4
Many human traits follow
Mendelian patterns of
inheritance.
Peas are convenient subjects
for genetic research, but
humans are not.
It wouldn’t be ethical to ask
pairs of humans to breed so
that the phenotypes of their
offspring could be analyzed!
Pedigree Analysis
Unable to manipulate the
matings of people, geneticists
instead analyze results that have
already occurred by collecting
information about a family’s
history for a particular trait.
A tree diagram that describes
the traits of parents and
children across the generations
Recessive Disorders
Most human genetic
disorders are recessive.
Recessive Disorders
Recessive Disorders
They range in severity from relatively
mild, such as albinism (lack of
pigmentation), to invariably fatal,
such as Tay-Sachs disease.
Most people who have recessive
disorders are born to normal parents
who are both heterozygotes.
Autosomal
Disorders:
Recessive Disorders
Albinism (Achromasia)
Lack of pigment
in the skin, hair,
and eyes
Incidence: 1/22,000
Cystic Fibrosis
Excess mucus in the lungs, digestive tract,
liver; increased susceptibility to infections;
death in early childhood unless treated
1
Incidence: 1/2,500 Caucasians
Comments: Prone to skin cancer
Galactosemia
Accumulation of galactose in tissues;
mental retardation; eye and liver
damage
1
Incidence: 1/100,000
Phenylketonuria
Accumulation of phenylalanine (amino
acid) in blood; lack of normal skin
pigment; mental retardation
1
Incidence: 1/10,000 in US and Europe
Sickle Cell Disease
Tay-Sachs Disease
Sickled red blood cells; damage to
many tissues
Lipid accumulation in brain cells; mental
deficiency; blindness; death in childhood
Incidence: 1/400 African Americans
Incidence: 1/3,500 Jews from Central
Europe
Autosomal
Disorders:
Dominant Disorders
1
Although many harmful
alleles are recessive, a
number of human disorders
are caused by dominant
alleles
Achondroplasia
Dwarfism
Incidence:
1/25,000
Huntington Disease
Alzheimer’s Disease
Mental deterioration; usually strikes late in life
Incidence: not known
Comments: Familial (inherited) Alzheimer's is a
rare form of the disease
Hypercholesterolemia
Mental deterioration
and uncontrollable
movements; trikes in
middle age
Incidence: 1/25,000
Excess cholesterol in the blood; heart disease
Incidence: 1/500 are heterozygous
Multifactorial Diseases
Multifactorial Diseases (Polygenic + Environment)
Multifactorial basis - a genetic
component plus a significant
environmental influence
Heart disease, diabetes, cancer,
alcoholism, certain mental
illnesses such as schizophrenia
and bipolar disorder
New technologies can provide insight into one’s
genetic legacy.
Carrier screening, fetal testing, fetal imaging, and
newborn screening can provide information for
reproductive decisions but may create ethical
dilemmas.