GENETICS REVIEWER.pdf

Contents:
A. Definition of Genetics
B. The Beginnings of Genetics
C. The Scope of Genetics
D. Applications of Genetics

Objectives:
At the end of this lecture, you would be able to:
•Define genetics based on your own understanding;
•Create a timeline of the history/ beginnings of genetics;
•Identify introductory terminologies commonly used in the concept of
genetics; and
•Explain the scope and applications of genetics.

Try this!
• Instruction: Create your own
definition of each letter of the word
GENETICS. Be creative and make sure
that the chosen definition is related
to the concept of genetics. The first
letter “G” is already defined to serve
as your guide.
G -ained from mom and dad
E -
N -
E -
T -
I -
C -
S -

Genetics
(n) the study of heredity and the variation of
inherited characteristics.
Source: OxfordLanguages (2023)
Etymology. The word genetics stems from the ancient Greek γενετικός
genetikos meaning "genitive"/"generative", which in turn derives from
γένεσις genesis meaning "origin".

William Bateson (August 1861 – 8 February 1926)
•an English geneticist. He was the
first person to use the term
genetics to describe the study of
heredity and biological
inheritance.

Prehistoric Times: Domesticated Animals and Cultivated Plants
•8000 to 1000 B.C.- horses, camels, oxen, and various breeds of dogs
(derived from the wolf family) were domesticated to serve various
roles.
•7000 to 5000 B.C.- the cultivation of many plants, including maize,
wheat, rice, and the date palm is thought to have been initiated. The
remains of maize dating to this period have been recovered in caves
in the Tehuácan Valley of Mexico.

(A) The caves of San Marcos (cave on the left) and Tecorral (cave on the right). (B) Archaeobotanical sampling in
San Marcos cave conducted in February 2012. (C) Maize specimens SM3 dating 5,280–4,970 cal. y B.P. (Left)
and SM5 dating 5,300–4,980 calibrated y B.P. (Right). (Scale bar, 1.5 cm.) (D) Maize specimens SM9 dating
5,280–4,970 cal. y B.P. (Left) and SM10 5,300–5,040 cal. y B.P. (Right). (Scale bar, 43 mm.)

800 B.C. Pinecone or date palm male inflorescence – metaphorical pollination in
Assyrian art

Prehistoric evidence of cultivated plants and domesticated animals
supports the hypotheses that our ancient ancestors learned that
desirable and undesirable traits are passed to successive generations
and that by influencing their breeding, many desirable varieties of
animals and plants could be obtained.
Human awareness of heredity was thus apparent during prehistoric
times and successful attempts were made to manipulate the genetic
material, even though it was unclear what it might be.

The Greek Influence: Hippocrates and Aristotle
•500 to 400 B.C.- Hippocratic treatise “On the Seed” argues that male
semen is formed in numerous parts of the body and is transported
through blood vessels to the testicles.
•384 to 322 B.C.- Aristotle proposed that male semen was formed
from blood, rather than from each organ, and that its generative
power resided in a “vital heat” that it contained. This vital heat had
the capacity to produce offspring of the same “form” as the parent.

Aristotle describes the animals Alexander has sent him, as seen in a fresco in The Main Hall
of the Assembleé Nationale in Paris, France.

1600-1850: The Dawn of Modern Biology
•1578 to 1657- William Harvey an English anatomist was credited
with the eariest statement of the theory of
epigenesis.
•1700- theory of preformation- states that sex cells contains a
complete miniature adult called “homunculus”.
•1733 to 1794- Casper Wolff, an embryologist, supported the theory
of epigenesis. He believed that several structures such as the
alimentary canal were not initially present in the earliest embryos he
studied, but instead were formed later during development.

Depiction of the “homunculus” , a sperm containing a miniature adult, perfect in proportion,
and fully formed.

• 1800- Carolus Linnaeus popularized the doctrine of the fixity of
species.
• 1808- John Dalton expounded his atomic theory, which stated that all
matter is composed of small, invisible units called atoms.
• 1809- Jean Baptiste Lamarck proposed that organisms acquire or lose
characteristics that then become heritable. His theory was known as
the doctrine of use and disuse.

•1830- Matthias Schleiden and Theodor Schwann proposed the cell
theory, stating that all organisms are composed of basic visible units
called cells which are deived from similar preexisting structures.

Charles Darwin and Evolution
•1859- Charles Darwin published the book on his evolutionary theory
“On the Origin of Species”. In this book, he formulated the theory of
natural selection.
“Natural selection is the process through which populations of living
organisms adapt and change. Individuals in a population are naturally
variable, meaning that they are all different in some ways. This
variation means that some individuals have traits better suited to the
environment than others.”

Charles Darwin and Evolution
•1868- Charles Darwin published his second book “Variation in
Animals and Plants under Domestication”. In this book, he postulated
two major ideas: pangenesis and the inheritance of acquired
characteristics.
“Pangenesis was Charles Darwin's hypothetical mechanism for
heredity, in which he proposed that each part of the body continually
emitted its own type of small organic particles called gemmules that
aggregated in the gonads, contributing heritable information to the
gametes.”

Gregor Johann Mendel
•1856 to 1863 - conducted his
experiments on garden peas
•1866 - published his classic
paper
•1900- rediscovery of his work
by Three botanists - Hugo
DeVries, Carl Correns and
Erich von Tschermak

Major Events in the 20th
Century
•1902: Archibald Garrod discovers that alkaptonuria, a human disease,
has a genetic basis.
•1910: Thomas Hunt Morgan proves that genes are located on the
chromosomes (using Drosophila).
•1918: R. A. Fisher begins the study of quantitative genetics by
partitioning phenotypic variance into a genetic and an environmental
component.
•1926: Hermann J. Muller shows that X-rays induce mutations.

More 20th
Century Events
•1944: Oswald Avery, Colin MacLeod and Maclyn McCarty show that
DNA can transform bacteria, demonstrating that DNA is the
hereditary material.
•1953: James Watson and Francis Crick determine the structure of the
DNA molecule, which leads directly to knowledge of how it
replicates.
•1966: Marshall Nirenberg solves the genetic code, showing that 3
DNA bases code for one amino acid.

20th
Century Events Continued
•1972: Stanley Cohen and Herbert Boyer combine DNA from two
different species in vitro, then transform it into bacterial cells: first
DNA cloning.
•2001: Sequence of the entire human genome is announced which
consisof 3.055 billion–base pair sequence.

Molecular Reality (current view)
•Almost all inheritance is based on DNA:
• the sequence of ACGT nucleotides encodes all instructions needed to
build and maintain an organism.
•A chromosome is a single DNA molecule together with other
molecules (proteins and RNA) needed to support and read the DNA.
•A gene is a specific region of a chromosome that codes for a single
polypeptide.
•A polypeptide is a linear chain of amino acids

Molecular Reality (current view)
•Proteins are composed of one or more polypeptides, plus in some
cases other small helper molecules (co-factors). Proteins do most of
the work of the cell.

Scope and Applications of Genetics
1. As basis of biologcal sciences
- provide foundation for biological studies.
- Laws of inheritance helps us to understand the principles of
embryology, population, taxonomy, evolution, and ecology.
2. Food Production
- rules of genetics helps us to discover new varieties of plants and
livestocks.

3. Disease Control
- gene therapy help to cure many genetics-based diseases.
4. Conservation of Wildlife
- conservation of wild life can be achieved by conserving the
germplasm of endangered species.

5. Genetic Engineering/ Biotecnology
- development of transgenic crops
- gene therapy
- improvement in food production
- control of genetic diseases

References:
•Estrada et.al. (2016). The earliest maize from San Marcos Tehuacán is
a partial domesticate with genomic evidence of inbreeding. Retrieved
from: https://www.pnas.org/doi/10.1073/pnas.1609701113
•King, Keith (2014). History of Genetics. Retrieved from:
https://www.slideserve.com/lane/history-of-genetics-by-keith-king
•Klug, W. & Cummings (2018). Concepts of Genetics. Pearson
Education,Inc.
•Shahzad, T (2021). History of Genetics, Scope and applications.
Retrieved from:
https://www.slideshare.net/TahirShahzad17/history-of-genetics-gene
tics-scope-and-applications-of-genetics
•Winchester, A.M. (2022). Genetics. Retrieved from:
https://www.britannica.com/science/genetics

References:
•Ziffer, I. (2019). Pinecone or date palm male inflorescence –
metaphorical pollination in Assyrian art. Retrieved from:
https://brill.com/view/journals/ijps/66/1-2/article-p19_19.xml?langu
age=en

Contents
A. The Cell
B. The Chromosome Structure
C. Cell Division
D. Life Cycles

Objectives:
At the end of this lesson, you will be able to:
•Understand the structures and purposes of basic components of
prokaryotic and eukaryotic cells;
•Describe the structure and functions of chromosomes ;
•State that a normal human body cell contains 46 chromosomes (23
pairs);
•Describe the processes of the cell cycle, including events that take
place during interphase, mitosis, and cytokinesis,

Objectives:
•State that DNA replication takes place during interphase;
•Outline how checkpoints are used to control the cell cycle;
•Define mitosis and meiosis;
•Identify what occurs at each phase of cell division;
•Compare and contrast mitosis and meiosis; and
•Describe the general pattern of Eukaryotic Life Cycles.

Cell Structure
& Function
http://koning.ecsu.ctstateu.edu/cell/cell.html

Definition of Cell
A cell is the smallest unit that is
capable of performing life
functions.

Examples of Cells
Amoeba Proteus
Plant Stem
Red Blood Cell
Nerve Cell
Bacteria

Two Types of Cells
•Prokaryotic
•Eukaryotic

Prokaryotic
•Do not have structures
surrounded by membranes
•Few internal structures
•Ex: blue-green algae, bacteria,
mycoplasma.
http://library.thinkquest.org/C004535/prokaryotic_cells.html

Eukaryotic
•Contain organelles surrounded by membranes
•Most living organisms
Plant Animal
http://library.thinkquest.org/C004535/eukaryotic_cells.html

“Typical” Animal Cell
http://web.jjay.cuny.edu/~acarpi/NSC/images/cell.gif

http://waynesword.palomar.edu/images/plant3.gif
“Typical” Plant Cell

Cell Membrane
•Outer membrane of cell
that controls movement in
and out of the cell
•Double layer
http://library.thinkquest.org/12413/structures.html

Cell Wall
•Most commonly found in plant
cells & bacteria
•Supports & protects cells

Nucleus
•Directs cell activities
•Separated from cytoplasm by
nuclear membrane
•Contains genetic material - DNA

Nuclear Membrane/Envelope
•Surrounds nucleus
•Made of two layers: inner and
outer nuclear membranes
•Openings allow material to enter
and leave nucleus

Nucleolus
A spherical structure found in
the cell's nucleus whose primary
function is to produce and
assemble the cell's ribosomes.

Chromosomes
•In nucleus
•Made of DNA
•Contain instructions for traits &
characteristics

Cytoplasm
•Gel-like mixture
•Surrounded by cell membrane
•Contains hereditary material

Ribosomes

Mitochondria
•Produces energy through
chemical reactions – breaking
down fats & carbohydrates
•Controls level of water and
other materials in cell
•Recycles and decomposes
proteins, fats, and
carbohydrates

Golgi Bodies
•Protein 'packaging plant'
•Move materials within
the cell
•Move materials out of
the cell

Lysosome
•Digestive 'plant' for
proteins, fats, and
carbohydrates
•Transports undigested
material to cell membrane
for removal
•Cell breaks down if
lysosome explodes

Vacuoles
•Membrane-bound sacs for food
storage, digestion, and waste
removal
•Contains water solution
•Help plants maintain shape

Centrosome vs. Centriole
•The centrosome is the primary
microtubule-organizing centre
(MTOC) in animal cells, and so it
regulates cell motility, adhesion
and polarity in interphase, and
facilitates the organization of the
spindle poles during mitosis.
•Centrioles are paired
barrel-shaped organelles located
in the cytoplasm of animal cells
near the nuclear envelope.
Centrioles play a role in
organizing microtubules that
serve as the cell's skeletal
system. They help determine the
locations of the nucleus and
other organelles within the cell.

Chloroplast
•Usually found in plant cells
•Contains green chlorophyll
•Where photosynthesis takes place

Chromosome structure
Short arm
• A chromosome has two
arms separated by a
centromere.
• The ends of the arms are called
telomeres.
• The arms are divided in two, and
each part is known as a chromatid.
Long arm
Telomere
Centromere
Chromatids

• Different types of chromosomes can be distinguished according to the
position of the centromere:
Chromosome types
METACENTRIC
The centromere is
halfway along the
chromosome and
the two arms are
of the same
length.
Centromere
Centromere
ACROCENTRIC
The centromere is
very close to one
end, meaning that
one arm is much
shorter than the
other.
Centromere
SUBMETACENTRIC
One arm is longer
than the other. On
division they take on
an “L” shape as they
are pulled apart.
Centromere
TELOCENTRIC
The centromere is
at the end of the
chromosome, so
the chromosome
has only one arm.

Chromomere and Knobs
•the structural subunit of a
chromosome.
•The arrangement of
chromomere structure can aid in
control of gene expression.
•Maps of chromomeres can be
made for use in genetic and
evolutionary studies.

Phases of Mitosis Approximate duration
Prophase 30 to 60 minutes
Metaphase 2 to 10 minutes
Anaphase 2 to 3 minutes
Telophase 3 to 12 minutes

Three Major Types of Eukaryotic Life Cycles
a. Gametic or Terminal
b. Zygotic or Initial
c. Sporic or Intermediary

References:
•Bailey, R. (2020). Cell Theory: A Core Principle of Biology. Retrieved
from: https://www.thoughtco.com/cell-theory-373300
•Brooker, R. (2018). Genetics: Analysis & Principles. McGraw-Hill
International Edition
•Harris, Katherine (2019). Eukaryotic Life Cycles. Retrieved from:
https://bio.libretexts.org/Learning_Objects/Worksheets/Biology_Tut
orials/Eukaryotic_Life_Cycles#:~:text=We%20will%20now%20review
%20the,and%20gametic)%20in%20more%20detail.&text=The%20ga
metic%20life%20cycle%20is,are%20the%20result%20of%20meiosis.

Objectives:
⚫ Explain Gregor Mendel’s laws of
inheritance
⚫ Discuss the diﬀerence between genotypes
and phenotypes
⚫ Label the three types of genotypes
⚫ Draw and label a Punnett square
⚫ Predict outcomes of a Punnett square

Mendel
⚫ Modern genetics had its beginnings in an
abbey garden, where a monk named Gregor
Mendel documented a particulate
mechanism of inheritance.
⚫ He discovered the basic principles of
heredity by breeding garden peas in
carefully planned experiments.
⚫ His approach to science had been inﬂuenced
at the University of Vienna by one of his

Mendel’s work
⚫ In order to study inheritance, Mendel
chose to use peas, probably as they are
available in many varieties.
⚫ The use of plants also allowed strict control
over the mating.
⚫ He chose to study only characters that
varied in an ‘either-or’ rather than a
‘more-or-less’ manner.

Genetic crosses
⚫ To cross two different
pea plants, Mendel
used an artist’s brush.
⚫ He transferred pollen
from a true breeding
white flower to the
carpel of a true
breeding purple flower.

Tracking Characteristics
⚫ Mendel tracked heritable
characters for 3
generations.
⚫ When F1
hybrids were
allowed to self-pollinate a
3:1 ratio of the 2 varieties
occurred in the F2
generation.

Mendel’s terminology
⚫ True breeding: When the plants
self-pollinate, all their oﬀspring are of the
same variety.
⚫ Hybridization: Mating, or crossing, of two
varieties.
⚫ Monohybrid cross: A cross between two
parents that breed true for diﬀerent
versions of a single trait.

Mendel’s terminology
⚫ P generation: True breeding parents.
⚫ F1
generation: (first filial) Hybrid offspring
of the P generation.
⚫ F2
generation: (second filial) Offspring from
the self-fertilization of the F1
hybrids.

Genotypes
⚫ The genotype refers to the entire set of
genes in a cell, an organism, or an
individual. A gene for a particular character
or trait may exist in
two forms; one is
dominant (A) and the
other is recessive (a).

The Punnett Square
⚫ Is a square grid used in genetics to calculate
the frequencies of the diﬀerent genotypes
and phenotypes among the oﬀspring of a
cross.

⚫ The male genotype is normally indicated
at the top and the female genotype is
indicated in the vertical margin.

A A
a Aa Aa
a Aa Aa
Punnett Square Example:
Genotypes and Phenotypes of
Offspring

⚫ homozygous dominant parents (PP x PP),
all oﬀspring will be homozygous dominant
polled individuals.

⚫ When crossing homozygous recessive
parents (pp x pp), all of the oﬀspring will be
horned (homozygous recessive)
individuals.

⚫ Crossing a heterozygous parent with a
homozygous dominant parent (Pp x PP), the
expected oﬀspring would occur in a 1:1 ratio of
homozygous dominant to heterozygous
individuals.
⚫ Phenotype: All oﬀspring would be polled.

⚫ We have a Angus Bull and a Charolais Cow.
⚫ The Angus (black) is Homozygous Dominant
⚫ The Charolais (white) is Homozygous
Recessive
⚫ How would you set up the Punnett Square?
Punnett Square Example

⚫ When crossing a homozygous dominant
parent with a homozygous recessive parent
(PP x pp), all oﬀspring would be
heterozygous and polled.

⚫ If two heterozygous parents are crossed (Pp x
Pp), one can expect a genotypic ratio of 1:2:1,
with one homozygous dominant polled, two
heterozygous polled, and one homozygous
recessive horned individuals.
⚫ The expected phenotypic ratio of oﬀspring
would be 3:1 (polled to horned).

Dominant and Recessive Genes
⚫ Dominant Genes
⚫ One gene overshadows the other.
⚫ Recessive Gene
⚫ The gene that is overshadowed by a
dominant gene.

Examples of Genotypes
⚫ There are three basic genotypes for a
particular
character:
⚫ AA = homozygous dominant
⚫ Aa = heterozygous
⚫ aa = homozygous recessive

Phenotypes
⚫ Phenotype is the physical
appearance or other
characteristic of an
organism as a result of the
interaction of its genotype
and the environment.
Some examples would
be:
•Size
•Shape
•Color

Three Different Inheritance Laws
by Mendel
⚫ Law of Segregation
⚫ Law of Independent Assortment
⚫ Law of Dominance

Law of Segregation
‘‘The two copies of each genetic factor segregate
during the development of gametes, to ensure that
each parent’s oﬀspring attains one factor.’’
OR
‘‘During the development of the gamete, each gene
is segregated in such a way that the gamete
consists of just one allele for that gene.’’

The fundamental principles of this law are
posited as follows:
⚫ There can be more than one type of allele
for a gene.
⚫ During the process of meiosis, when
gametes are formed, the allele pairs
segregate, i.e. they separate.
⚫ For the determination of a Mendelian trait,
two alleles are involved — one is recessive
and the other is dominant.

Law of Independent
Assortment
⚫ ‘‘Separate couples with alleles are transferred
separately from each other to the next
generation. As a result, gene inheritance does
not inﬂuence gene inheritance somewhere else at
one position in the genome.’’
OR
⚫ ‘‘This law described that alleles of various genes
that are distributed during gamete development
assort independently of each other.’’

Ocular albinism
Fabry’s Disease
Wiskott–Aldrich
Syndrome

Law of Segregation vs. Law of
Independent Assortment
“The law of segregation describes how alleles
of a gene are segregated into two gametes
and reunite after fertilization. The law of
independent assortment describes how
alleles of diﬀerent genes independently
segregate from each other during the
formation of gametes.”

Genetic Crosses
⚫ Monohybrid Cross
⚫ Dihybrid Cross
⚫ Test Cross
⚫ Back Cross
⚫ Reciprocal Cross

Back Cross
Purpose: to improve the breed
Ex: you wanted all breeds to be tall
P gen: Tt x tt Back Cross: tt x TT
T t
t Tt Tt
t Tt tt
T T
t Tt Tt
t Tt Tt

Law of Dominance
⚫ ‘‘Only one sort of the trait will show in the next
generation in a cross of parents which are pure
for diﬀerent traits. In the allele, children that
are hybrid for a trait will only show the
dominant characteristic, and children that are
not hybrid for a trait will show recessive traits.’’
OR
⚫ ‘‘It is stated that one factor in pair of traits
dominates while the other remains suppressed in
inheritance unless the two factors in the pair are
recessive. In the next generation of parents who
are pure for contrasting traits, there will be only
one type of trait.’’

Supplemental/Reference
Videos:
⚫ https://www.youtube.com/watch?v=OvmZ
6KcN5JE
⚫ https://www.youtube.com/watch?v=-FJWT
DqlvbU
⚫ https://www.youtube.com/watch?v=wrtLy
Lwt51o
⚫ https://www.youtube.com/watch?v=pv3Kj0
UjiLE
⚫ https://www.youtube.com/watch?v=a5GM
p9BPEkA
⚫ https://www.youtube.com/watch?v=H9BEt
dgvKQ4

References:
⚫ CK-12 Foundation (2021). Gene Linkage.
Retrieved from:
https://bio.libretexts.org/Bookshelves/Intr
oductory_and_General_Biology/Book%3A
_Introductory_Biology_(CK-12)/03%3A_G
enetics/3.10%3A_Genetic_Linkage#:~:text=
Linkage%20explains%20why%20certain%20
characteristics,hair%20with%20b%20rown%
20eyes.
⚫ Sethuraman et.al. (2011). Fabry’s disease.
The Lancet VOLUME 378, ISSUE
9798, P1254. Retrieved from:
https://www.thelancet.com/journals/lancet

MULTIPLE ALLELES
Genes which have more than two alleles
© 2007 Paul Billiet ODWS

Genes and their alleles
•About 30% of the genes in humans are di-allelic, that is they exist in
two forms, (they have two alleles)
•About 70% are mono-allelic, they only exist in one form and they
show no variation
•A very few are poly-allelic having more than two forms

Combinations
•Di-allelic genes can generate 3 genotypes
•Genes with 3 alleles can generate 6 genotypes

Genes and the immune system
•Poly-allelic alleles are usually associated with tissue types
•These genes are so varied that they provide us with our genetic finger
print
•This is very important to our immune system which must tell the
difference between our own cells (self) and invading disease causing
microbes (non-self)

The ABO blood system
• This is controlled by a tri-allelic gene
• It can generate 6 genotypes
• The alleles control the production of antigens on the surface of the red
blood cells
• Two of the alleles are codominant to one another and both are
dominant over the third
• Allele IA
produces antigen A
• Allele IB
produces antigen B
• Allele i produces no antigen

The ABO blood system
Genotypes Phenotypes (Blood types)
IA
IA ;
IA
i A
IA
IB
AB
IB
IB ;
IB
i B
ii O
Note:
• Blood types A and B have two possible genotypes – homozygous and
heterozygous.
• Blood types AB and O only have one genotype each.

Blood types and transfusions
•Blood types vary and your immune system recognises your own blood
type as being self
•Other blood types are recognised as non-self
•If a blood which is incompatible with your body is transfused it will
result in the agglutination of the foreign red blood cells

Antigens
© Biology Labs Online
© Bioformatica
refers to any substance that triggers an immune response.

Agglutination
© Dr Delphine Grézel, Ecole Nationale Vétérinaire de Lyon

Donor-recipient compatibility
Recipient
Type A B AB O
A
Donor B
AB
O
= Agglutination
= Safe transfusion
Note:
• Type O blood may be transfused into all the
other types = the universal donor.
• Type AB blood can receive blood from all the
other blood types = the universal recipient.

LETHAL ALLELES

Lucien Claude Marie
Julien Cuénot
Erwin Baur

Lethal Alleles
•Essential genes are those that are absolutely required
for survival
• The absence of their protein product leads to a lethal
phenotype
•Nonessential genes are those not absolutely required
for survival
•A lethal allele is one that has the potential to cause
the death of an organism
• These alleles are typically the result of mutations in
essential genes

LETHAL ALLELES
RECESSIVE LETHAL ALLELE
DOMINANT LETHAL ALLELE

•In a simple dominant/recessive relationship, the recessive
allele does not affect the phenotype of the heterozygote
• So how can the wild-type phenotype of the heterozygote be
explained?
•There are two possible explanations
• 1. 50% of the normal protein is enough to accomplish the
protein’s cellular function
• 2. The heterozygote may actually produce more than 50% of the
functional protein
• The normal gene is “up-regulated” to compensate for the lack of function
of the defective allele

•Conditional lethal alleles These alleles turn deadly only
when there is an external environmental aspect
involved.
• Temperature-sensitive (ts) lethals
• A developing Drosophila larva may be killed at 30 C
• But it will survive if grown at 22 C
• Favism
•Semilethal alleles
• Kill some individuals in a population, not all of them
• Environmental factors and other genes may help prevent the
detrimental effects of semilethal genes
• Ex: Hemophilia

•Synthetic lethal alleles are only lethal when paired with
a second mutation.

Penetrance of Lethal Genes
• What does penetrance mean in genetics?
- it means how often does someone with the genotype actually show the corresponding
phenotype
- Denoted by % or fractions of populations
• How do you determine gene penetrance?
Where:
• P(D|A) = penetrance
• P(D) = baseline risk (the lifetime risk of the disease in the general population)
• P(A|D) = allele frequency in cases
• P(A) = allele frequency in population controls
or

Environmental Influence on lethal genes
3 Environmental Factors that can affect genes:
• Food
• Drugs
• Exposure to toxins
Patterns of Exposure:
• Monophasic
• Polyphasic
• Possession by lethal-bearing males and females (sexual dimorphic) of
different temperature-sensitive stages.
• Occurrence of lethality by exposure to restrictive condition at any stage of
development.

1. Novel Phenotype
•A novel phenotype is a phenotype that is concerned with the unique
visual appearance of an organism as compared with its parents.
•There is complete dominance in both gene pairs. New phenotypes
result from interaction between dominants, and also from interaction
between both homozygous recessives, for example : comb shape in
poultry.

a locus (plural loci) is a specific, fixed position on
a chromosome where a particular gene or genetic
marker is located.

Environmental Influence on
Gene Expression

Definition of Terms:
•Penetrance
•Expressivity
•Pleiotropy
•Phenocopy

Penetrance and Expressivity
Penetrance
•Penetrance refers to the
probability of a gene or trait
being expressed.
2 classifications: complete and
incomplete/reduced penetrance

Expressivity
•Refers to variation in phenotypic
expression when an allele is
penetrant.
•Also refers to the range of signs
and symptoms that can occur in
different people with the same
genetic condition
2 Classifications: narrow expressivity
and variable expressivity

•Temperature Effects- temperature influence phenotypes because all
chemical activities depends on the kinetic energy of the reacting
substances which in turn depends on the surrounding temperature.
Example: evening primrose produces red flowers when grown at 23°C
and white flowers when grown at 18°C.
•Light Effects- energy provided by light and light itself is essential for
the growth and development of plants and animals.
Example: seedlings grown in the dark do not survive long because
chlorophyll is not developed even though there are genes for
chlorophyll development.

•Nutritional Effects- food provides energy for carrying out necessary
processes and materials that must be incorporated into necessary
structures. Depending upon its genotype, simple dietary changes can
lead to different phenotypes.
Example: the appearance of yellow fat in rabbits depends on the
genotype yy and green vegetables eaten. Elimination of green
vegetables also eliminates the appearance of yellow fat.

•Maternal Relations- In case of mammals where the direct body
relations between parent and progeny extend beyond fertilization,
interaction between the fetus and the maternal environment occurs.
Example: blood group incompatibilities between mother and offspring
produce special effects on the survival of particular genotypes.

2. Sex
•Sex-limited traits
The traits limited to only one sex due to anatomical differences are
called a sex limited trait. Such trait affects a structure or function of
the body of males or females only. These traits are controlled by
sex-linked or autosomal genes. For example:
(i) Genes for milk production in dairy cattle affect only cows.
(ii) Beard growth in humans is limited to men. A woman does not grow
a beard herself but she can pass the genes of heavy beard growth to
her sons.

2. Sex
•Sex-linked traits
Sex-linked traits are determined by genes found on the X and Y
chromosomes.

2.1 X-linked Dominant Genes
• If a single copy of the X-linked dominant allele is present, the individual will
express the trait of interest.
• X-linked Dominant Genes in Females
Since females have two copies of the X chromosome, a single X-linked dominant
allele is sufficient for the female to express the trait. For example, a female who is
XA
XA
or XA
Xa
will express the dominant trait as they have at least one copy of the
XA
allele. In contrast, a female who is Xa
Xa
will not express the dominant trait.
• X-linked Dominant Genes in Males
A male has only one X chromosome; therefore, if a male is XA
Y, they will express the
dominant trait. If the male is Xa
Y, they will not express the dominant trait

2.2 X-linked Recessive Genes
• In contrast to X-linked dominant genes, X-linked recessive alleles are masked by
a dominant allele. Therefore, a dominant allele must be absent for the X-linked
recessive trait to be expressed.
• X-linked Recessive Genes in Females
Females have two X-chromosomes; therefore, both X chromosomes must have
the X-linked recessive allele for the trait to be expressed.
• X-linked Recessive Genes in Males
Since males only have one X-chromosome, having a single copy of the X-linked
recessive allele is sufficient to express the X-linked recessive trait

2.3 Y-linked Genes
•In Y-linked genes, the genes are found on the Y chromosome. Since
only males have a Y-chromosome, only males will express the trait of
interest. Furthermore, it will be passed from father to son only.

3. Substrates
•A substrate is a molecule acted upon by an enzyme. A substrate is
loaded into the active site of the enzyme, or the place that allows
weak bonds to be formed between the two molecules.
•Example: Lactose

Probability and Statistical
Testing

Ratio
•The genotypic/phenotypic ratio is the ratio depicting the different
genotypes/phenotypes of the offspring from a test cross. It
represents the pattern of offspring distribution according to
genotype/phenotype of an organism.

Basic Laws of Probability in Genetics
•Multiplication/ Product Law (and)
•Addition/ Sum Law (or)

Multiplication/ Product Law
•What are the odds of flipping a coin 5 times and getting tails in every
flip?
½ x ½ x ½ x ½ x ½
= 1/32

Multiplication/ Product Law cont’d.
•What are the odds of rolling “snake eyes” on two pair of dice?
1/6 x 1/6
= 1/36

*Work 1 letter at a time
½ x ½ x ½
= 1/8

Addition/ Sum Law
•What are the odds of flipping a coin getting either heads or tails?
½ + ½
= 2/2 or 1

Addition/ Sum Law cont’d.
•What are the odds of rolling a two or a five on six-sided dice?
1/6 x 1/6
= 2/6 or 1/3

*Work 1 letter at a time.
* Work on each possible offspring.
•Offspring 1:
•Offspring 2:

+
1/4x ½ x ½ + 1/4x ½ x ½
1/16 1/16
= 2/16 or 1/8

Example:
•Null Hypothesis (H₀): There is no significant difference between
observed and expected frequencies (i.e. genes are unlinked)
•Alternative Hypothesis (H₁): There is a significant difference between
observed and expected frequencies (i.e. genes are linked)

Step 1: Identify the hypotheses
A chi-squared test seeks to distinguish between two distinct possibilities
and hence requires two contrasting hypotheses:
•Null Hypothesis (H₀): There is no significant difference between
observed and expected frequencies (i.e. genes are unlinked)
•Alternative Hypothesis (H₁): There is a significant difference between
observed and expected frequencies (i.e. genes are linked)

What is binomial distribution?
•a frequency distribution of the possible number of successful
outcomes in a given number of trials in each of which there is the
same probability of success.

Sample Problems:
If a couple has 7 children, what
is the probability of them
having 3 girls and then having 4
boys?
a. 1/2
b. 1/144
c. 1/128
d. 1/256
e. None of these
If a couple has 7 children, what
is the probability of them
having 3 girls and 4 boys?
a. 1/2
b. 21/128
c. 70/128
d. 35/128
e. None of these

If a couple has 7 children, what is the probability of them having 3 girls and 4
boys?
a. 1/2
b. 21/128
c. 70/128
d. 35/128
e. None of these
Step 1: Substitute values to the formula.

boys?
a. 1/2
b. 21/128
c. 70/128
d. 35/128
e. None of these
Step 2: Expand the factorials
3!= 3·2 ·1
4!= 4 ·3 ·2 ·1
7!= 7 ·6 ·5 ·4 ·3 ·2 ·1

boys?
a. 1/2
b. 21/128
c. 70/128
d. 35/128
e. None of these
Step 3: Solve the equation

What is normal distribution?
•It is a type of continuous probability distribution in which most data
points cluster toward the middle of the range, while the rest taper off
symmetrically toward either extreme. The middle of the range is also
known as the mean of the distribution.
•Properties of a normal curve:
• It is smooth and symmetric.
• Its highest point occurs over the mean of the entire population.
• It never touches the x axis.
• The total area under the curve is 1.

References:
• BiologyOnline (2023). Antigen definition. Retrieved from:
https://www.biologyonline.com/dictionary/antigen
• BiologyDictionary (2023). Substrate definition. Retrieved from:
https://biologydictionary.net/substrate/
• BioNinja (2023). Chi-square Test. Retrieved from:
https://ib.bioninja.com.au/higher-level/topic-10-genetics-and-evolu/102-inheritance/chi-squared-t
est.html
• Boundless (2022). Lethal Inheritance Patterns . Retrieved from:
https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biol
ogy_(Boundless)/12%3A_Mendel's_Experiments_and_Heredity/12.02%3A__Patterns_of_Inheritanc
e/12.2F%3A_Lethal_Inheritance_Patterns#:~:text=recessive%20lethal%3A%20an%20inheritance%2
0pattern,some%20altered%20non-lethal%20phenotype
• Khillar, S. (2020, December 13). Difference Between Antibody and Antigen Test.
• Lobo, I. (2008) Mendelian ratios and lethal genes. Nature Education 1(1):138
• Lobo, I. (2008) Genetics and Statistical Analysis. Nature Education 1(1):109

References:
• MedlinePlus (2021). What are reduced penetrance and variable expressivity? Retrieved
from:
https://medlineplus.gov/genetics/understanding/inheritance/penetranceexpressivity/
• Penetrance (2021). Retrieved from:
https://www.cureffi.org/2016/10/19/estimation-of-penetrance/#:~:text=P(D%7CA)%20%
3D,allele%20frequency%20in%20population%20controls
• Penetrance and Expressivity (2021). Retrieved from:
https://bio.libretexts.org/Courses/Lumen_Learning/Book-_Biology_for_Majors_I_(Lumen
)/14%3A_Module_12-_Trait_Inheritance/14.12%3A_Penetrance_and_Expressivity#:~:text
=Penetrance%20refers%20to%20the%20probability,fingers%20and%2For%20toes).
• Saini, V.P. (2020). Gene Interactions ppt. Retrieved from:
https://basu.org.in/wp-content/uploads/2020/03/Gene-interactions.ppt

References Continued…
•StudySmarter (2023). Retrieved from:
https://www.studysmarter.us/explanations/biology/heredity/sex-link
ed-traits/

Supplemental Videos:
•https://www.youtube.com/watch?v=MKD1hHVNgcA
•https://www.youtube.com/watch?v=ec2ELp514Ns
•https://www.youtube.com/watch?v=WXPBoFDqNVk
•https://www.youtube.com/watch?v=kgRhwTmy-nI

GENETICS REVIEWER.pdf

Recommended

Recommended

More Related Content

What's hot

What's hot (20)

Similar to GENETICS REVIEWER.pdf

Similar to GENETICS REVIEWER.pdf (20)

Recently uploaded

Recently uploaded (20)

GENETICS REVIEWER.pdf