Lecture 8:
Genetics: Heredity
Overview
Mendel’s laws and Modern Genetic terminology
Patterns of inheritance
Genetic disease
the chromosomal basis of inheritance
Prepared by Mayssa Ghannoum
Overview
 Mendel
discovered the basic principles of heredity
by breeding garden peas in planned experiments.
 A heritable feature that varies among individuals,
such as flower color, is called character.
 Each variant for a character, such as purple or
white color for flowers, is termed a trait.
Overview


Hereditary: the passing on of characteristics
(traits) from parents to offspring.
Genetics is the study of heredity.
Mendel Hereditary Laws

Mendel used garden peas, because they reproduce
sexually having two distinct, male and female, sex
cells called gametes
→ and because their traits are easy to isolate and
notice
→ and are available in many varieties, such as
purple or white color for flowers.
Mendel Hereditary Laws
 Mendel crossed the peas, and
fertilization happened (the
uniting of male and female
gametes).
 Then he took the resulted plants
seeds and grew them, and he
produced offspring carrying
different traits than those of the
parent plants.
 Cross: combining gametes from
parents with different traits.
P Generation
(true-breeding
parents)
F1 Generation
(hybrids)
F2 Generation
When F1 hybrid pea plants are
allowed to self-pollinate.
 Then
he discovered different laws
and rules that explain factors
affecting heredity.
Mendel Law and Rules
1. Rule of Unit Factors
Each organism has two alleles for each trait
→ Genes: located on chromosomes, they control how an
organism develops.
→ Alleles: different forms of the same gene..
Mendel Law and Rules
2. Rule of Dominance
→ The trait that is observed in the offspring is the dominant
trait (abbreviated in an uppercase letters).
→ The trait that disappears in the offspring is the recessive
trait (abbreviated in an lowercase letters).
3. Law of Segregation
→ The two alleles for a trait must separate when gametes are
formed.
→ A parent randomly passes only one allele for each trait to
each offspring.
4. Law of Independent Assortment:
→ The genes for different traits are inherited independently of
each other.
Phenotype & Genotype:
→ Phenotype: the way an organism looks (such as red
hair or brown hair)
→ Genotype: the gene combination of an organism such
as AA or Aa or aa.
Dihybrid & Monohybrid:
→ Dihybrid Cross: crossing parents who differ in two
traits (AAEE with aaee)
→ Monohybrid Cross: crossing parents who differ in
only one trait (AA with aa).
Heterozygous & Homozygous:
→ Heterozygous: if the two alleles for a trait are
different (Aa).
→ Homozygous: if the two alleles for a trait are the same
(AA or aa).
Patterns of Inheritance
 The
inheritance of characters determined by single
gene deviates from simple Mendelian patterns when
alleles are not completely dominant or recessive.
 We will discuss in the section below these situations
about Degrees of Dominance.
Degrees of Dominance

Alleles can show different degrees of dominance and
recessiveness in relation to each other.
a. Complete dominance
In Mendel’s classic pea crosses, the F1 offspring always looked
like one of the two paternal varieties because one allele in a pair
showed complete dominance over the other.
In such situations, the phenotypes of the heterozygote and the
dominant homozygote are the same.
Degrees of Dominance
b. Incomplete dominance
For some genes, neither allele is
completely dominant, and the F1
hybrids have a phenotype somewhere
between those of the two paternal
varieties.
This phenomenon is called incomplete
dominance.
Degrees of Dominance
c. Co-dominance
A condition in which the alleles of a gene pair in
a heterozygote are fully expressed thereby resulting
in offspring with a phenotype that is
neither dominant nor recessive.
A typical example showing codominance is the ABO blood group
system.
For instance, a person having A allele and B allele will have
a blood type AB because both the A and B alleles are codominant
with each other.
Multiple Alleles

Only two alleles exist for the pea character that Mendel studied.

But most genes exist in more than two allelic forms.

The ABO blood groups in human are determined by three alleles
of a single gene: IA, IB and i alleles.

A person’s blood group may be one of four types:
A, B, AB, O.

These letters refer to two carbohydrates A and B that might be
present on the surface of red blood cells.
Multiple Alleles

A person’s blood cells may have
carbohydrate A (type A blood),
carbohydrate B (type B blood) ,
both (type AB blood), or neither
(type O blood).

Matching compatible blood groups is
critical for safe blood transfusions.
Epistasis
 In
epistasis, a gene at one locus alters the phenotypic
expression of a gene at a second locus.
 Example:
In mice and many mammals, black coat color
is dominant to brown. Let’s designate B and b as the two
alleles for this character.
For a mouse to have brown fur, its genotype must be bb.
 But
there is a second gene that determines whether or
not pigment will be deposited in the hair.
Epistasis
 The
dominant allele, symbolized by C (for color),
results in the deposition of either black or brown
pigment, depending on the genotype at the first locus.
If the mouse is homozygous recessive for the second
locus (cc), then the coat is albino, regardless of the
genotype at the black/brown locus.
 In
this case the gene for pigment deposition (C/c) is
said to be epistatic to the gene that codes for black or
brown pigment (B/b).
Epistasis
Illustration of the genotypes and phenotypes predicted for offspring of matings
between two black mice of genotype BbCc.
The C/c gene, which is epistatic to the B/b gene coding for hair pigment,
controls whether or not pigment of any color will be deposited in the hair.
Polygenic inheritance

Polygenic inheritance occurs when one phenotypic
character is controlled by two or more genes.

These characters are called quantitative characters.

Examples of human polygenic inheritance are height,
skin color, eye color and weight.

There is an evidence, that skin pigmentation in humans
is controlled by at least three separately inherited
genes.
Pedigree Analysis
Pedigree: A diagram of a family tree
showing the occurrence of heritable traits in parents and
offspring over multiple generations.
 An importance application of a pedigree is to help us
calculate
the probability that a child will have a particular genotype
and phenotype.


The figure below shows a pedigree of a family
where we focus on a recessive trait:
attached earlobes.
F: is for the dominant allele ( free earlobe)
f: is for the recessive allele (attached earlobe).
Genetic Disorders
 A genetic
disorder is a genetic problem caused by
one or more abnormalities in the genome, especially
a condition that is present from birth (congenital).
Most genetic disorders are quite rare and affect one
person in every several thousands or millions.
 Genetic



disorders can be classified as:
Recessively inherited Disorders
Dominantly inherited Disorders
Multifactorial Disorders
Recessively inherited Disorders
Disorders known to be inherited as simple
recessive trait.
These disorders range in severity from relatively
mild, such as albinism ( lack of pigmentation,
which results in susceptibility to skin cancers and
vision problems), to life threatening, such as
cystic fibrosis.
 These disorders shows up only in the
homozygous individuals who inherit one
recessive allele from each parent.
Heterozygotes, phenotypically normal, may
transmit the recessive allele to their offspring and
thus are called carriers.

Cystic Fibrosis
The most common lethal genetic disease in the US.
 The normal allele for the defected gene codes for a membrane
protein that functions in the transport of chloride ions between
certain cells and the extracellular fluid.
 These chloride transport channels are defective or absent in the
plasma membranes of children who inherit two recessive
alleles for cystic fibrosis.
 The result is an abnormal high concentration of extracellular
chloride, which causes the mucus that coats cells to become
thicker and stickier than normal.
 The mucus build up in the pancreas, lungs, digestive tract and
other organs, leading to multiple effects, including poor
absorption of nutrients from intestines, chronic bronchitis..

Sickle-Cell Disease
 It’s
caused by the substitution of a single amino acid in
the hemoglobin protein of red blood cells.
 When
the oxygen content of an affected individual’s
blood is low, the sickle-cell hemoglobin molecules
aggregate into long rods that deform the red cells into a
sickle shape.
 Sickled
cells may clump and clog small blood vessels
often leading to other symptoms throughout the body,
including physical weakness, pain, organ damage, and
even paralysis.
Dominantly Inherited Disorders
 Disorders
that occur due to dominant alleles, such as
achondroplasia, a form of dwarfism that occurs in
one of every 25,000 people.

Heterozygous individuals have the dwarf
phenotype.
 Dominant alleles
that cause a lethal disease are
much less common than recessive alleles that do so.
 All
lethal alleles arise by mutations ( changes to
DNA) in cells that produce sperm or eggs.
Huntington’s Disease
 A degenerative
disease of the nervous system, is
caused by a lethal dominant allele that has no
obvious phenotypic effect until the individual is
about 35 to 45 years old.
 Once
the deterioration of the nervous system begins,
it is irreversible and inevitably fatal.
 Any
child born to a parent who has the allele for
Huntington’s disease has a 50% chance of inheriting
the allele and the disorder.
Multifactorial Disorder
 The
hereditary diseases we have discussed so far are
described as simple Mendelian disorders because they
result from abnormality of one or both alleles at a single
genetic locus.
 Many
more people are susceptible to diseases that have
a multifactorial basis- a genetic component plus a
significant environmental influence.
 Heart
disease, diabetes, cancer, alcoholism, certain
mental illnesses such as schizophrenia and many other
diseases are multifactorial.
Multifactorial Disorder
 In
many cases of multifactorial diseases, the hereditary
component is polygenic.
 For
example, many genes affect cardiovascular health,
making some of us more prone to than others to heart
attacks and strokes.
 Exercise,
a healthful diet, abstinence from smoking, and
an ability to handle stressful situations all reduce our
risk of heart disease and some types of cancer.
The chromosomal basis of inheritance
 Description
of the chromosomal basis for the
transmission of genes from parents to offspring, along
with some important exceptions to the standard mode of
inheritance.
 Chromosomes and genes are both present in pairs in
diploid cells; homologous chromosomes separate and
alleles segregate during the process of meiosis
 Fertilization restores the paired condition for both
chromosomes and genes.
 Mendelian genes have specific loci along chromosome,
and it is the chromosomes that undergo segregation and
independent assortment.
Inheritance of Sex Linked Genes
 In
humans and other mammals, there are two
varieties of sex chromosomes: a larger X
chromosome and a smaller Y chromosome.
 In
addition to their role as carriers of genes that
determine sex , the sex chromosomes especially X
chromosomes have genes for many characters
unrelated to sex.
 A gene
located on either sex chromosome is called a
sex linked gene.
Inheritance of Sex Linked Genes
 Females are XX, and
males are XY.
 Each ovum contains an X
chromosome, while a sperm
may contain either an X or a
Y chromosome.
Sex Linked Disorders in humans

Some disorders caused by recessive alleles on
the X chromosome in humans:
- Color blindness
-
Hemophilia
It’s a sex linked recessive disorder defined by
the absence of one or more of the proteins
required for blood clotting.
Linked genes
 Each
chromosome has hundreds or thousands of
genes.
 Genes
located on the same chromosome that tend
to be inherited together are called linked genes.
Genetic Recombination
 The
production of offspring with combination of
traits that differ from those found in either parent.
 After
genetic recombination there are two types of
offspring produced:
1. Parental type (like parents).
2. Recombinant type or Recombinants (not like
parents).
Abnormal chromosome number

Non disjunction: pairs of homologous chromosomes do not
separate during meiosis1 or sister chromatids fail to separate
during meiosis2.
→ As a result, one gamete receives two of the same type of
chromosome, and another gamete receives no copy.

Aneuploidy results from the fertilization of gametes in
which nondisjunction occurred.
→ Offspring or zygote with this condition have an abnormal
number of a particular chromosome.
→ Zygote (a fertilized egg) is produced from the union of an
ovum with a sperm; gametes.
Fig. 15.13, nondisjunction
Meiosis I
Nondisjunction
Meiosis II
Nondisjunction
Gametes
n+1
n+1
n–1
n–1
n+1
n–1
n
Number of chromosomes
(a) Nondisjunction of homologous
chromosomes in meiosis I
(b) Nondisjunction of sister
chromatids in meiosis II
n
Types of Aneuploidy
A monosomic zygote
2n-1
A trisomic zygote
2n+1
One missing
chromosome in
zygote
One extra
chromosome in
zygote
Human Disorders due to Chromosomal
Alterations
 Down Syndrome (trisomy 21)

it is an aneuploid condition that
results from three copies of
chromosome 21

Each body cell has total 47
chromosomes.

Affected persons have facial
features, short stature, heart
defects, susceptible to
respiratory infections and mental
retardation.
Figure 15.16 Down syndrome
Alterations of Chromosomal Structures
1. Deletion: when a chromosomal fragment is lost.
2. Duplication: when a fragment of chromosome is
repeated.
3. Inversion: chromosomal fragment attaching to the
original chromosome but in reverse orientation.
4. Translocation: breakage of a fragment and
joining to a non homologous chromosome.
(a)
(b)
(c)
(d)
A B C D E
F G H
A B C D E
F G H
A B C D E
F G H
A B C D E
F G H
Deletion
Duplication
A B C E
F G H
A B C B C D E
Inversion
A D C B E
F G H
F G H
M N O C D E
F G H
Reciprocal
translocation
M N O P Q
R
A B P Q
R
Fig. 15.15