BIO112: MOLECULAR BIOLOGY
AND GENETICS
Mr. Derrick Banda MSc, BSc
GENETICS AND HEREDITY
GENETIC TERMS
• Gene - a unit of inheritance that usually is directly responsible for one trait
or character.
• Allele - an alternate form of a gene. Usually there are two alleles for every
gene, sometimes as many a three or four.
• Homozygous - when the two alleles are the same.
• Heterozygous - when the two alleles are different, in such cases the
dominant allele is expressed.
• Dominant - a term applied to the trait (allele) that is expressed irregardless
of the second allele.
• Recessive - a term applied to a trait that is only expressed when the second
allele is the same (e.g. short plants are homozygous for the recessive allele).
• Phenotype - the physical expression of the allelic composition for the trait
under study.
• Genotype - the allelic composition of an organism.
• Punnett squares - probability diagram illustrating the possible offspring of a
mating.
Genetics terms you need to know:
• Gene – a unit of heredity; a section of DNA
sequence encoding a single protein
• Genes can be represented by letters in a particular
sequence and at particular spots (loci) on a
homologous pair of chromosome
• Genome – the entire set of genes in an organism
Genetics terms you need to know:
• Genotype - An organism's genetic makeup. Represented
by two letters, each representing an allele on homologous
chromosomes
• May be represented by a descriptive phrase
1. Homozygotes- having two identical genes (one from
each parent) for a particular characteristic. May be
homozygous dominant or homozygous recessive
2. Heterozygotes- having two different genes for a
particular characteristic.
• Phenotype-An
organism's
inherited
physical
characteristics. Determined by an organisms genotype
Genetics terms you need to know:
• Alleles represent alternative forms of a particular gene(two
genes that occupy the same position on homologous
chromosome). They have the same position on homologous
chromosomes and affect the same trait.
• Dominant allele – the allele of a gene that masks or
suppresses the expression of an alternate allele; the trait
appears in the heterozygous condition. Represented by a capital
letter.
• Recessive allele- an allele that is masked by a dominant
allele; does not appear in the heterozygous condition, only
in homozygous. Represented by a lower case letter.
• Locus – a fixed location on a strand of DNA where a gene or
one of its alleles is located.
Genetics terms you need to know:
• Monohybrid cross: a genetic cross involving a
single pair of genes (one trait); parents differ by a
single trait.
• P = Parental generation
• F1 = First filial generation; offspring from a genetic
cross.
• F2 = Second filial generation of a genetic cross
GENETICS
What is Genetics?
• Genetics is the study of heredity and variation
of inherited traits (characters).
Heredity is the tendency of offspring to
resemble their parents.
Variation is the tendency of offspring to vary
from their parents.
GENETICS
What is Genetics?
• Genetics – study of how traits are passed from
parent to offspring.
GENETICS
• The term ‘Genetics’ was coined by William
Bateson in 1905.
• Gregor Johan Mendel, an Austrian monk, is
known as the “Father of Modern
Genetics”.
• The modern concepts of Genetics took birth
from pioneering work on Pisum sativum
(Garden pea)
IMPORTANCE OF GENETICS
• Genetics occupies a central position in modern biology, so
its understanding is essential for all scholars of the life
sciences.
• The discipline has great impact on many everyday aspects
of human life. The food we eat and the clothes we wear
come from organisms improved by application of genetic
principles.
• The causes of important human diseases are being
discovered, and therapies developed, based on fundamental
genetic investigations.
• Increasingly, management of human health also depends on
genetic and genomic information. These impacts are
certain to grow over the coming decades, so genetics is a
growth field.
What is a GENE?
• A gene is a fundamental unit of heredity. A gene is a
small section of DNA within the genome that
contains the instructions for the production of a
specific protein.
• Gene contain the instructions for our
individual characteristics – like eye and hair
colour.
• DNA is the molecule responsible for the
inheritance of traits, and that this molecule is
divided into functional units called genes.
A GENE
• Each gene contains the information required to
build specific proteins needed in an organism.
TRAITS (CHARACTERISTICS)
• Traits are determined by the genes on the
chromosomes. A gene is a segment of DNA that
determines a trait.
CHROMOSOMES
• In the nucleus of every eukaryotic cell there are a number
of long threads called chromosomes.
• Most of the time, the chromosomes are too thin to be seen
except with an electron microscope. But when a cell is
dividing, they get shorter and fatter so they can be seen
with a light microscope.
CHROMOSOMES
• Human cells contain 46 chromosomes, which are in pairs. Sex
cells (sperm and ova) contain only 23 chromosomes. The 23
chromosomes comprise one from each pair.
• Inheritance of sex in humans
• Of the 23 pairs of chromosomes present in each human cell,
one pair is the sex chromosomes. These determine the sex of
the individual. Male have XY, female have XX. So the presence of
a Y chromosome results in male features developing.
DNA AND CHROMOSOME STRUCTURE
• Each chromosome contains one very long molecule of DNA. The
DNA molecule carries a code that instructs the cell about which kind
of proteins it should make. Each chromosome carries instructions for
making many different proteins.
DNA AND CHROMOSOME STRUCTURE
 Chromosomes are threadlike nuclear structures consisting of DNA
and proteins that serve as the repositories for genetic information.
GENE AND CHROMOSOME STRUCTURE
• Each chromosome is made up of a large number
of genes coding for the formation of different proteins
which give us our characteristics. The gene responsible for a
particular characteristic is always on the same relative
position on the chromosome.
HOMOLOGOUS CHROMOSOME
• In cells Chromosomes (and genes) occur in pairs called Homologous
chromosomes
• New combination of genes occur in sexual reproduction
• Fertilization from two parents
ALLELE AND LOCUS
• Locus: location of a gene on a
chromosome
• Allele: Different form of a gene
http://www.nwcreation.net/articles/images/genelocus.JPG
ALLELE AND LOCUS
• When the chromosomes are in pairs, there may be
a different form (allele) of the gene on each
chromosome.
ALLELE AND LOCUS
• When the chromosomes are in pairs, there may be
a different form (allele) of the gene on each
chromosome.
Relationship between allele, genotype, and
phenotype.
• When gametes unite during fertilization, the resulting zygote inherits two alleles for
each gene. One allele comes from each parent. The alleles an individual inherits
make up the individual’s genotype. The two alleles may be the same or different. As
shown in the Figure below, an organism with two alleles of the same type
(PP or pp) is called a homozygote. An organism with two different alleles (Pp) is
called a heterozygote.This results in three possible genotypes.
GENE-ENVIRONMENT INTERACTION
• Some traits are strongly influenced by genes, while other
traits are strongly influenced by the environment. Most
traits, however, are influenced by one or more genes
interacting in complex ways with the environment.
THE CELL CYCLE AND CELL DIVISION
• Every hour, about one billion (109) cells die and one
billion cells are made in your body.
• The Cell Cycle is used to allow the organism to grow,
and to replace cells as they grow and worn out.
THE CELL CYCLE
• The cell cycle is divided into two basic phases:
1. Interphase
2. M Phase (Mitosis phase)
AN OVERVIEW OF THE CELL CYCLE
• During interphase of Cell Cycle, the cell grows and
DNA is replicated.
• During the mitotic phase of Cell Cycle, the replicated
DNA and cytoplasmic contents are separated, and the cell
cytoplasm is typically partitioned by a third process of the
cell cycle called cytokinesis.
MITOSIS
• Mitosis occurs in the body cells and maintain the
diploid number of chromosome (2n).
• Diploid mean all the chromosomes in a cell occur in
pairs (one from male parent the other from the
female parent).
• Mitosis multiplies the number of cells and is a
method by which growth, replacement and repair of
cells occurs in eukaryotes.
MITOSIS
• Mitosis is a type of cell division in which one cell
(the mother) divides to produce two new cells
(the daughters) that are genetically identical to
itself.
AN OVERVIEW OF MEIOSIS
INTRODUCTION TO MEIOSIS
• Meiosis is a type of nuclear division that reduces the
number of chromosome from the diploid (2n) number to
haploid (n) number.
• In humans, the diploid number of chromosome is 46 and
is reduced to a haploid number of 23 by meiosis.
• Meiosis division occurs in reproductive cells
(gametes) such sperms and eggs.
FERTILIZATION RESTORES A DIPLOID
SET OF CHROMOSOMES
• During the formation of gametes, the number of
chromosomes is reduced by half, and returned to the full
amount when the two gametes fuse during fertilization.
• When a sperm and egg fuse during fertilization each
haploid gamete contributes one set of chromosome. As
such the diploid number is restored in the fertilized zygote.
MEIOSIS STAGES
• In meiosis, the starting nucleus is always diploid (2n) and the daughter
nuclei that result are haploid (n). To achieve this reduction in
chromosome number, meiosis consists of one round of chromosome
replication followed by two rounds of nuclear division.
Chromosomal Theory of Inheritance
The speculation that chromosomes might be the key to
understanding heredity led several scientists to examine
Mendel’s publications and re-evaluate his model in terms of
chromosome behavior during mitosis and meiosis.
 In 1902, Theodor Boveri observed that proper sea urchin
embryonic development does not occur unless
chromosomes are present. That same year, Walter Sutton
observed chromosome separation into daughter cells
during meiosis. Together, these observations led to the
Chromosomal Theory of Inheritance, which identified
chromosomes as the genetic material responsible for
Mendelian inheritance.
Chromosomal Theory of Inheritance
(a) Walter Sutton and (b) Theodor Boveri developed the
Chromosomal Theory of Inheritance, which states that
chromosomes carry the unit of heredity (genes).
Chromosomal Theory of Inheritance
• The Chromosomal Theory of Inheritance was consistent with
Mendel’s laws, which the following observations supported:
 During meiosis, homologous chromosome pairs migrate as discrete
structures that are independent of other chromosome pairs.
 Chromosome sorting from each homologous pair into pre-gametes
appears to be random.
 Each parent synthesizes gametes that contain only half their
chromosomal complement.
 Even though male and female gametes (sperm and egg) differ in size
and morphology, they have the same number of chromosomes,
suggesting equal genetic contributions from each parent.
 The gametic chromosomes combine during fertilization to produce
offspring with the same chromosome number as their parents.
Chromosomal Theory of Inheritance
• Despite compelling correlations between chromosome
behavior during meiosis and Mendel’s abstract laws,
scientists proposed the Chromosomal Theory of
Inheritance long before there was any direct evidence that
chromosomes carried traits. Critics pointed out that
individuals had far more independently segregating traits
than they had chromosomes.
• It was only after several years of carrying out crosses with
the fruit fly, Drosophila melanogaster, that Thomas Hunt
Morgan provided experimental evidence to support the
Chromosomal Theory of Inheritance.
PRINCIPLES OF GENETICS
• It is a common observation that seeds of mango trees
germinate to grow into mango plants, and dogs give birth
to puppies only and not into the young ones of any other
animal.
• Humans give birth to human beings. The tendency of
offsprings to inherit parental characteristics is termed as
‘heredity’ and the study of science of heredity and
the reasons governing the variation between the parents
and their offsprings, is called ‘Genetics’. Genetics also
seeks to answer questions like why two offspring of
same parents look different, why some people have dark,
and others have fair complexion. In other words, why is
there variation among individuals of the same kind.
HEREDITY AND VARIATION
• Whenever an infant is born in a family, the relatives begin to
wonder about the resemblance of the infant’s eyes, facial
features, complexion, colour of hair with those of the
parents, siblings and grandparents. The source of such
resemblances and differences are in the “genes” that are
passed down form parents to children and so on
generation after generation.
• This inheritance of genes is termed ‘heredity’ the study of
reasons of heredity is ‘Genetics’. New individuals develop
features according to the genes inherited by them from
their parents.
• The transmission of characters from one
generation to the next, that is from parents to
offsprings is known as heredity.
HEREDITY AND VARIATION
• It is further observed that siblings from same parents are unique and differ
from each other except the identical twins. Such differences are termed
variations.
• Variation means differences between parents and their offsprings
or between offsprings of same parents or between members of
the same population.
• Variation in a population is very important. It has survival value for the
population.
• This is because if the environment changes, some individuals (variants) may
be able to adapt to new situations and save the population from dying out.
• Variation arises due to mutation or sudden change in the genes.
Variation also arises because genes get shifted and exchanged during
meiosis at the time of formation of gametes, giving rise to new gene
combinations. At fertilization, there is random mixing of paternal and
maternal chromosomes with different gene combinations. Such a source of
variation which is most common is called genetic recombination.
• Heritable Variations generally arise because of mutation and
recombination.
MENDEL’S EXPERIMENTS ON THE GARDEN PEA AND
PRINCIPLES OF INHERITANCE
• Sir Gregor Johann Mendel (1822 to 1884) was
Austrian monk who used garden pea (Pisum
sativum) for his experiments on plant breeding and
published his results in 1865.
• His work, however, was independently rediscovered
in 1900, long after Mendel’s death, by Tschermak,
Correns and DeVries. But since Mendel was the
first to suggest principles underlying
inheritance he is regarded as the founder or
father of genetics.
Gregor Johann Mendel
• Austrian Monk, born in what is now Czech Republic in
1822
• Son of peasant farmer, studied
Theology and was ordained
priest Order St. Augustine.
• Went to the university of Vienna, where
studied botany and learned the Scientific Methods.
• Worked with pure lines of peas for eight years
he
Gregor Johann Mendel
• With his careful experiments, Mendel uncovered the
secrets of heredity, or how parents pass characteristics
to their offspring.
• You may not care much about heredity in pea plants,
but you probably care about your own heredity.
Mendel's discoveries apply to people as well as to peas
— and to all other living things that reproduce sexually.
In this concept, you will read about Mendel's
experiments and the secrets of heredity that he
discovered.
• Prior to Mendel, heredity was regarded as a "blending"
process and the offspring were essentially a "dilution"of
the different parental characteristics.
Gregor Johann Mendel
• Gregor Johann Mendel was the first person who discovered the basic
principles of heredity during the mid-19th century. Hence, he is
known as the “Father of Modern Genetics”. He conducted
experiments in his garden on pea plants and observed their pattern
of inheritance from one generation to the next generation.
• Mendel laid the basic groundwork in the field of genetics and
eventually proposed the laws of inheritance. Law of Segregation, Law
of Independent Assortment and Law of Dominance are the three
laws of inheritance proposed by Gregor Mendel. These laws came
into existence from his experiments on pea plants with a variety of
traits.
• Mendel first studied the inheritance of one gene in the plant
through monohybrid cross. He considered only a single character
(plant height) on pairs of pea plants with one contrasting trait. Later,
he studied the inheritance of two genes in the plant through dihybrid
cross.
Inheritance Theories Before Mendel’s
• Mendel set out to address a fundamental issue of heredity: What
are the patterns of the transmission of traits from parents to
offspring? At the time, two hypotheses had been formulated to
answer this question:
1. Blending inheritance proposed that the traits observed in a
mother and father blend together to form traits in their offspring.
As a result, an offspring’s traits are intermediate between traits of
the mother and father. Blending inheritance predicted that when
black sheep and white sheep mate, their hereditary determinants
blend to give offspring the trait of grey wool.
2. Inheritance of acquired characters proposed that traits
present in parents are modified through use and then passed on
to their offspring in the modified form. Inheritance of acquired
characters predicted that if giraffes extend their necks by
straining to reach leaves high in the tops of trees, they transmit
this acquired trait to produce longer-necked offspring.
Blending Theory of Inheritance
• During Mendel's time, the blending theory of inheritance was popular. This is the
theory that offspring have a blend, or mix, of the characteristics of their parents.
• Mendel noticed plants in his own garden that weren’t a blend of the parents. For
example, a tall plant and a short plant had offspring that were either tall or short
but not medium in height. Observations such as these led Mendel to question the
blending theory. He wondered if there was a different underlying principle that
could explain how characteristics are inherited. He decided to experiment with pea
plants to find out. In fact, Mendel experimented with almost 30,000 pea plants over
the next several years!
Mendel’s model system: The pea plant
• Mendel carried out his key experiments using the garden pea, Pisum
sativum, as a model system. Pea plants make a convenient system for
studies of inheritance, and they are still studied by some geneticists
today. Useful features of peas include;
1.
2.
3.
Their rapid life cycle and the production of lots and lots of seeds.
Pea plants also typically self-fertilize, meaning that the same plant makes both the
sperm and the egg that come together in fertilization. Mendel took advantage of
this property to produce true-breeding pea lines: he self-fertilized and selected
peas for many generations until he got lines that consistently made offspring
identical to the parent (e.g., always short).
Pea plants are also easy to cross, or mate in a controlled way. This is done by
transferring pollen from the anthers (male parts) of a pea plant of one variety to
the carpel (female part) of a mature pea plant of a different variety. To prevent the
receiving plant from self-fertilizing, Mendel painstakingly removed all of the
immature anthers from the plant’s flowers before the cross.
Mendel’s model system: The pea plant
• Because peas were so easy to work with
and prolific in seed production, Mendel
could perform many crosses and examine
many individual plants, making sure that
his results were consistent (not just a
fluke) and accurate (based on many data
points).
Mendel’s Peas Experiments
•
1.
2.
3.
4.
5.
6.
7.
Mendel looked at seven traits or characteristics of pea plants. The seven characteristics that
Mendel evaluated in his pea plants were each expressed as one of two versions, or traits
Seeds can be round or
wrinkled
Seeds can have yellow or
green cotyledons.
Cotyledons refer to the
tiny leaves inside the seeds.
Flowers can be violet or
white
The seed pod can be full or
constricted
The seed pod can be
yellow or green
The flowers can occur
along the stem (in axial
pods) or at the end of a
stem (in terminal pods)
Stems can be long (6-7
feet) or short (less than 1
foot).
The seven (7) characteristics that Mendel
evaluated in his pea plants
Mendel’s experimental setup
• To research how characteristics are passed from
parents to offspring, Mendel needed to control
pollination.
• Pollination is the fertilization step in the sexual
reproduction of plants.
• Pollen consists of tiny grains that are the male sex cells,
or gametes, of plants. They are produced by a male
flower part called the anther.
• Pollination occurs when pollen is transferred from the
anther to the stigma of the same or another flower. The
stigma is a female part of a flower. It passes the pollen
grains to female gametes in the ovary.
Peas Can Be Self-Fertilized or Cross-Fertilized
a) Under normal conditions, garden peas pollinate themselves, or selffertilize.
b) Mendel developed a method of controlling matings to force crossfertilization, or outcrossing.
Mendel’s model system: The pea plant
• Pea plants are naturally self-pollinating. In selfpollination, pollen grains from anthers on one plant
are transferred to stigmas of flowers on the same
plant.
Mendel’s model system: The pea plant
• Mendel was interested in the offspring of two different parent plants,
so he had to prevent self-pollination. He removed the anthers from
the flowers of some of the plants in his experiments. Then he
pollinated them by hand with pollen from other parent plants of his
choice. When pollen from one plant fertilizes another plant of the
same species, it is called cross-pollination. The offspring that result
from such a cross are called hybrids. When the term hybrid is used in
this context, it refers to any offspring resulting from the breeding of
two genetically distinct individuals.
Mendel’s experimental setup
• Mendel began his work by obtaining individuals from
what breeders called pure lines or true-breeding
lines.
• A pure line consists of individuals that produce offspring
identical to themselves when they are self-fertilized or
crossed to another member of the same pure-line
population. For example, breeders had developed pure
lines for wrinkled seeds and round seeds. During two
years of trial experiments, Mendel confirmed that
individuals that germinated from his wrinkled seeds
produced only wrinkle-seed offspring when they were
mated to themselves or to another pureline individual
that germinated from a wrinkled seed.
Mendel’s experimental setup
• Once Mendel had established true-breeding pea lines with
different traits for one or more features of interest (such as
violet vs. white flowers colour), he began to investigate how
the traits were inherited by carrying out a series of crosses.
• Mendel began his single-trait crosses with individuals from
violet flowered and white flowered pure lines. The
individuals used in the initial cross are the parental
generation. Their progeny (offspring) are the F1
generation. F1 stands for “first filial”; the Latin roots
filius and filia mean “son” and “daughter,”
respectively. Offspring from a mating between two F1
individuals are called the F2 generation; an F2 mating leads
to an F3 generation, and so on.
Mendel's First Set of Experiments
• Mendel first experimented with just one characteristic of a pea plant
at a time. He began with flower colour. Mendel cross-pollinated
purple- and white-flowered parent plants. The parent plants in the
experiments are referred to as the P (for parent) generation.
Mendel's First Set of Experiments
•
•
The offspring of the P generation are called the F1 (for filial, or “offspring”) generation.
As you can see, all of the plants in the F1 generation had purple flowers. None of them had
white flowers. Mendel wondered what had happened to the white-flower characteristic. He
assumed some type of inherited factor produces white flowers and some other inherited
factor produces purple flowers. Did the white-flower factor just disappear in the F1
generation? If so, then the offspring of the F1 generation—called the F2 generation—
should all have purple flowers like their parents.
To test this prediction, Mendel allowed the F1 generation plants to self-pollinate. He was
surprised by the results. Some of the F2 generation plants had white flowers. He studied
hundreds of F2 generation plants, and for every three purple-flowered plants, there was an
average of one white-flowered plant.
Mendel's First Set of Experiments
• Mendel did the same experiment for all seven pea plant
characteristics.
• In each case, one value of the characteristic disappeared in
the F1 plants and then showed up again in the F2 plants. And
in each case, 75 percent of F2 plants had one value of the
characteristic and 25 percent had the other value.
• Based on these observations, Mendel formulated his first
law of inheritance.
• This law is called the law of segregation. It states
that there are two factors controlling a given
characteristic, one of which dominates the other,
and these factors separate and go to
different gametes when a parent reproduces.
Mendel’s Law of Segregation
• The Law of Segregation states that every individual
organism contains two alleles for each trait, and that
these alleles segregate (separate) during meiosis such
that each gamete contains only one of the alleles.
• An offspring thus receives a pair of alleles for a trait
by inheriting homologous chromosomes from the
parent organisms: one allele for each trait from each
parent.
• Hence, according to the law, two members of a gene
pair segregate from each other during meiosis; each
gamete has an equal probability of obtaining either
member of the gene.
Mendel’s Law of Segregation
• According to the law of segregation, only one of the two gene
copies present in an organism is distributed to each gamete (egg or
sperm cell) that it makes, and the allocation of the gene copies is
random. When an egg and a sperm join in fertilization, they form a
new organism, whose genotype consists of the alleles contained in
the gametes.The diagram below illustrates this idea:
Mendel’s Law of Segregation
• The four-squared box shown for the F2 generation is known as a Punnett square. To
prepare a Punnett square, all possible gametes made by the parents are written
along the top (for the father) and side (for the mother) of a grid. Here, since it is
self-fertilization, the same plant is both mother and father.
• The combinations of egg and sperm are then made in the boxes in the table,
representing fertilization to make new individuals. Because each square represents
an equally likely event, we can determine genotype and phenotype ratios by
counting the squares.
Mendel’s Law of Segregation
• Observing that true-breeding pea plants with contrasting traits gave rise to F1
generations that all expressed the dominant trait and F2 generations that expressed
the dominant and recessive traits in a 3:1 ratio, Mendel proposed the law of
segregation.
• This law states that paired unit factors (genes) must segregate equally
into gametes such that offspring have an equal likelihood of inheriting
either factor. For the F2 generation of a monohybrid cross, the following three
possible combinations of genotypes result: homozygous dominant, heterozygous, or
homozygous recessive. Because heterozygotes could arise from two different
pathways (receiving one dominant and one recessive allele from either parent), and
because heterozygotes and homozygous dominant individuals are phenotypically
identical, the law supports Mendel’s observed 3:1 phenotypic ratio.
• The equal segregation of alleles is the reason we can apply the Punnett square to
accurately predict the offspring of parents with known genotypes. The physical basis
of Mendel’s law of segregation is the first division of meiosis in which the
homologous chromosomes with their different versions of each gene are
segregated into daughter nuclei. This process was not understood by the scientific
community during Mendel’s lifetime
Monohybrid Cross and the Punnett Square
• When fertilization occurs between two true-breeding
parents that differ by only the characteristic being studied,
the process is called a monohybrid cross, and the
resulting offspring are called monohybrids.
• Mendel performed seven types of monohybrid crosses,
each involving contrasting traits for different characteristics.
• Out of these crosses, all of the F1 offspring had the
phenotype of one parent, and the F2 offspring had a 3:1
phenotypic ratio. On the basis of these results,
• Mendel postulated that each parent in the monohybrid
cross contributed one of two paired unit factors to each
offspring, and every possible combination of unit factors
was equally likely.
Monohybrid cross
• Parents differ by a single trait.
• Crossing two pea plants that differ in seed colour,
one yellow one green
Y = allele for Yellow
y= allele for green
YY = homozygous yellow seed
yy= homozygous green seed
YY yy
Punnett square
• A Punnett square, devised by the British geneticist
Reginald Punnett, is useful for determining probabilities
because it is drawn to predict all possible outcomes of all
possible random fertilization events and their expected
frequencies.
• We use the Punnett square to predict the genotypes and
phenotypes of the offspring.
Using a Punnett Square
STEPS:
1. determine the genotypes of the parent organisms
2. write down your "cross" (mating)
3. draw a p-square
Parent genotypes:
YY and yy
Cross
YY yy
Punnett square
4. “Split" the letters of the genotype for each
parent & put them "outside" the p-square
5. Determine the possible genotypes of the offspring
by filling in the p-square
6. Summarize results (genotypes & phenotypes of
offspring)
YY
 yy
Y
y
y
Yy
Yy
Y
Yy
Yy
Genotypes:
100% Yy
Phenotypes:
100% Yellow seed
Secret of the Punnett Square
• Key to the Punnett Square:
• Determine the gametes of each parent…
• How? By “splitting” the genotypes of each parent:
• If this is your cross
YY
The gametes are:
Y
Y

yy
y
y
Once you have the gametes…
Y
Y

y
y
Y
Y
y
y
Yy
Yy
Yy
Yy
Shortcut for Punnett Square…
• If either parent is HOMOZYGOUS
Y
Y

y
y
Y
Yy
• You only need one box!
y
Genotypes:
100% Yy
Phenotypes:
100% Yellow seed
Understanding the shortcut…
y
y
y
Y
Y
Yy
Yy
Yy
Yy
=
Genotypes:
100% Yy
Phenotypes:
100% Yellow seeds
Y
Yy
Monohybrid cross: F2 generation
• If you let the F1 generation self-fertilize, the next
monohybrid cross would be:
Yy
(Yellow)
Y
Y
y
YY
Yy

Yy
(Yellow)
y
Yy
yy
Genotypes:
1 YY= Yellow
2 Yy = Yellow
1 yy = Green
Genotypic ratio= 1:2:1
Phenotype:
3 Yellow
1 Green
Phenotypic ratio= 3:1
Monohybrid cross: F2 generation
• If you let the F1 generation self-fertilize, the next
monohybrid cross would be:
Monohybrid Cross and the Punnett Square
Another example: Flower color
For example, flower color:
P = purple (dominant)
p = white (recessive)
If you cross a homozygous Purple (PP) with a
homozygous white (pp):
PP

Pp
pp
ALL PURPLE (Pp)
Cross the F1 generation:

Pp
P
P
p
PP
Pp
Pp
p
Pp
pp
Genotypes:
1 PP
2 Pp
1 pp
Phenotypes:
3 Purple
1 White
Test cross
• When you have an individual with an unknown
genotype, you do a test cross.
• Test cross: Cross with a homozygous recessive
individual.
• For example, a plant with purple flowers can either
be PP or Pp… therefore, you cross the plant with
a pp (white flowers, homozygous recessive)
P ?  pp
Test cross
• If you get all 100% purple flowers, then the
unknown parent was PP…
p
p
• If you get 50% white,
50% purple flowers,
then the unknown
parent was Pp…
p
p
P
P
Pp
Pp
Pp
Pp
P
p
Pp
pp
Pp
pp
Mendel’s Law of Dominance
• The genotype of an individual is made up of the many
alleles it possesses. An individual’s physical appearance,
or phenotype, is determined by its alleles as well as by
its environment.
• The presence of an allele does not mean that the trait
will be expressed in the individual that possesses it.
• If the two alleles of an inherited pair differ (the
heterozygous condition), then one determines the
organism’s appearance and is called the dominant
allele; the other has no noticeable effect on the
organism’s appearance and is called the recessive
allele.
Mendel’s Law of Dominance
• Mendel’s law of dominance states that in
heterozygote, one trait will conceal the presence of
another trait for the same characteristic.
• For example, when crossing true-breeding Tall (T) plants
with true-breeding Dwarf (t) plants, all of the offspring
were Tall, even though they all had one allele for Tall and
one allele for Dwarf. Rather than both alleles contributing
to a phenotype, the dominant allele will be expressed
exclusively. The recessive allele will remain latent, but will be
transmitted to offspring in the same manner as that by
which the dominant allele is transmitted.
• The recessive trait will only be expressed by offspring that
have two copies of this allele, and these offspring will
breed true when self-crossed.
Mendel’s Law of Dominance
• The second law of inheritance maintains that when the two
genes of a pair, represent contrasting characters the
expression of one is dominant over that of the other.
• Thus if both genes of an allele are for tallness
(represented as TT) that is homozygous or one gene is
for tallness and another for dwarfness (Tt), that is
heterozygous, the pea plants will be tall. The opposite of
dominant gene is termed recessive gene. The recessive
feature (e.g. dwarfness of the plant) is expressed only when
both the genes of allele are in the homozygous condition
(tt).
• The law of dominance was found to be true in both
monohybrid and dihybrid crosses in cases of all the seven
characteristics studied by Mendel in the garden pea.
Mendel’s Law of Dominance
• The dominant allele will hide the phenotypic effects of the recessive
allele in a heterozygous pair. The Law of Dominance it concerns the
expression of the genotype.
• The upper case letters are used to represent dominant
alleles whereas the lowercase letters are used to represent
recessive alleles.
Mendel’s Law of Dominance
• Mendel’s law of dominance states that: “When parents
with pure, contrasting traits are crossed together,
only one form of trait appears in the next
generation. The hybrid offsprings will exhibit only
the dominant trait in the phenotype.”
• In this law, each character is controlled by distinct
units called factors, which occur in pairs. If the
pairs are heterozygous, one will always dominate
the other.
• Law of dominance explains that in a monohybrid cross
between a pair of contrasting traits, only one parental
character will be expressed in the F1 generation and both
parental characters will be expressed in the F2
generation in the ratio 3:1.
Mendel’s Law of Dominance
• Law of dominance explains that in a monohybrid cross between a
pair of contrasting traits, only one parental character will be
expressed in the F1 generation and both parental characters will be
expressed in the F2 generation in the ratio 3:1.
Mendel’s Law of Dominance
• The Law of Dominance says that when an organism
is heterozygous for a trait, only the dominant allele will produce
a phenotype.
• Let's look at the allele pairs. The first cherry is homozygous for the
red allele and the second cherry is homozygous for the yellow
allele. The third cherry is heterozygous, meaning it has one red
allele and one yellow allele. Since this cherry is red, the Law of
Dominance would say that the red allele (A) is dominant because
only this allele produced a phenotype in a heterozygous organism.
Do Mendel’s Results Hold for Other Traits?
• Mendel was meticulous. He established that the results were general
and not restricted to one trait: He repeated the experiments with six
other traits.
Mendel’s law of independent assortment
• Law of independent assortment meaning
whereby that in the inheritance of two
features (each feature controlled by a pair of
genes), genes for the two different features
are passed down into the offspring
independently i.e. the segregation of one
pair of factors is independent of the
segregation of the factors belonging to any
other pair of factors or allelic pair.
Mendel’s Law of Independent Assortment
• Mendel’s law of independent assortment states that genes
do not influence each other with regard to the sorting of
alleles into gametes, and every possible combination of alleles for
every gene is equally likely to occur.
• Independent assortment of genes can be illustrated by the
dihybrid cross, a cross between two true-breeding parents
that express different traits for two characteristics.
• Consider the characteristics of seed color and seed texture for
two pea plants, one that has wrinkled, green seeds (rryy) and
another that has round, yellow seeds (RRYY). Because each parent is
homozygous, the law of segregation indicates that the gametes for
the wrinkled–green plant all are ry, and the gametes for the
round–yellow plant are all RY. Therefore, the F1 generation of
offspring all are RrYy
Hypothesis of independent assortment
• Alleles of different genes don't stay together when gametes form.
• Pure-line parents differing in two traits were crossed to produce a
dihybrid F1 generation. these F1 then were allowed to self-fertilize to
produce an F2.
Mendel’s Law of Independent Assortment
• The gametes produced by the F1 individuals must have one allele
from each of the two genes. For example, a gamete could get an R
allele for the seed shape gene and either a Y or a y allele for the seed
color gene. It cannot get both an R and an r allele; each gamete can
have only one allele per gene. The law of independent assortment
states that a gamete into which an r allele is sorted would be equally
likely to contain either a Y or a y allele. Thus, there are four equally
likely gametes that can be formed when the RrYy heterozygote is
self-crossed, as follows: RY, rY, Ry, and ry.
• Arranging these gametes along the top and left of a 4 × 4 Punnett
square gives us 16 equally likely genotypic combinations.
From these genotypes, we find a phenotypic ratio of 9 round–
yellow:3 round–green:3 wrinkled–yellow:1 wrinkled–green.
• These are the offspring ratios we would expect, assuming we
performed the crosses with a large enough sample size.
Mendel’s Law of Independent Assortment
Dihybrid cross
• A dihybrid cross is a breeding experiment between two
organisms which are identical hybrids for two traits. In
other words, a dihybrid cross is a cross between two
organisms, with both being heterozygous for two different
traits. The individuals in this type of trait are homozygous
for a specific trait. These traits are determined by DNA
segments called genes.
• In a dihybrid cross, the parents carry different pair of alleles
for each trait. One parent carries homozygous dominant
allele, while the other one carries homozygous recessive
allele. The offsprings produced after the crosses in the F1
generation are all heterozygous for specific traits.
Dihybrid cross
• Mendel took a pair of contradicting traits together for
crossing, for example colour and the shape of seeds at
a time. He picked the wrinkled-green seed and roundyellow seed and crossed them. He obtained only roundyellow seeds in the F1 generation. This indicated that
round shape and yellow colour of seeds are dominant
in nature.
• Meanwhile, the wrinkled shape and green colour of
seeds are recessive traits. Then, F1 progeny was selfpollinated. This resulted in four different combinations
of seeds in the F2 generation. They were wrinkledyellow, round-yellow, wrinkled-green seeds and roundgreen in the phenotypic ratio of 9:3:3:1
Dihybrid cross
• A dihybrid cross in pea plants involves the genes for seed color
and texture. The P cross produces F1 offspring that are all
heterozygous for both characteristics. The resulting 9:3:3:1 F2
phenotypic ratio is obtained using a Punnett square.
Dihybrid crosses
• Matings that involve parents that differ in two genes
(two independent traits)
• For example, flower color:
P = purple (dominant)
p = white (recessive)
and stem length:
T = tall
t = short
Dihybrid cross: flower color and
stem length
TT PP  tt pp
(tall, purple)
(short, white)
tp
Possible Gametes for parents
TP
and t p
tp
tp
tp
TP
TtPp
TtPp
TtPp
TtPp
TtPp
TtPp
TtPp
TtPp
TP
TtPp
TtPp
TtPp
TtPp
TP
TtPp
TtPp
TtPp
TtPp
TP
F1 Generation: All tall, purple flowers (Tt Pp)
Dihybrid crosses: flower color and
stem length (shortcut)
TT PP  tt pp
(tall, purple)
(short, white)
Possible Gametes for parents
T P
TP
tp
t p
F1 Generation: All tall, purple flowers (Tt Pp)
Tt Pp
Dihybrid cross F2
If F1 generation is allowed to self pollinate, Mendel
observed 4 phenotypes:
Tt Pp  Tt Pp
(tall, purple)
(tall, purple)
TP
Possible gametes:
TP Tp tP tp
TP
Tp
tP
tp
Tp
tP
tp
TTPP TTPp TtPP
TTPp TTpp TtPp
TtPp
Ttpp
TtPP
TtPp
ttPP
ttPp
TtPp
Ttpp
ttPp
ttpp
Four phenotypes observed
Tall, purple (9); Tall, white (3); Short, purple (3); Short white (1)
Dihybrid cross
9 Tall
purple
TP
3 Short
white
tP
Tp
TtPp
Ttpp
tP
TtPP
TtPp
ttPP
ttPp
tp
TtPp
Ttpp
ttPp
ttpp
purple
Phenotype Ratio = 9:3:3:1
1 Short
white
tp
TTPP TTPp TtPP
TTPp TTpp TtPp
TP
3 Tall
Tp
Dihybrid cross: 9 genotypes
Genotype ratios (9):
1
TTPP
2
TTPp
2
TtPP
4
TtPp
1
TTpp
2
Ttpp
1
ttPP
2
ttPp
1
ttpp
Four Phenotypes:
Tall, purple (9)
Tall, white (3)
Short, purple
(3)
Short, white (1)
Mendel’s law of independent assortment
• Members of one gene pair segregate independently
from other gene pairs during gamete formation”
• Genes get shuffled – these many combinations are
one of the advantages of sexual reproduction
Mendel’s law of independent assortment
• The physical basis for the law of independent assortment also lies in
meiosis I, in which the different homologous pairs line up in random
orientations. Each gamete can contain any combination of paternal
and maternal chromosomes (and therefore the genes on them)
because the orientation of tetrads on the metaphase plane is random
GENE MUTATION
• Gene Mutation it is a change in the sequence of a
gene that changes its functions. These changes may be
harmful, beneficial, or have no effect (neutral) on the
individual or cell.
• Mutation may arise during replication and/or
recombination.
• Mutation could be spontaneous or induced by
chemical and physical mutagens.
• Mutation may occur in either somatic or germ cells.
• Mutations that occur in germ cells may be passed to
subsequent generations.
GENE MUTATION
• Mutation can result in changes in the proteins that are
made.This can be a bad or a good thing.
SPONTANEOUS AND INDUCED GENE MUTATIONS
Mutation can be caused by:
i. Spontaneous methods
ii. Induction through physical or chemical means
•
•
•
•
SPONTANEOUS MUTATIONS
Spontaneous mutations arise naturally.
Spontaneous mutations are rare and occur
without any reason.
It arises due to metabolic errors, replication
errors or due to development errors.
Larger genes are more prone to spontaneous
mutation because the chance of replication
error is higher in larger genes.
INDUCED MUTATIONS
• Induced Mutations by physical or chemical mutagens.
i. Chemical: Many chemical mutagens, some
exogenous, some man-made, some environmental, are
capable of damaging DNA. Many chemotherapeutic
drugs and intercalating agent drugs function by
damaging DNA. e.g. nitrous acid and base analogs.
ii. Physical: Gamma rays, X-rays, even UV light can
interact with compounds in the cell generating free
radicals which cause chemical damage to DNA.
MUTAGENS
• What is a mutagen? A mutagen is a substance or agent
that causes change of the DNA sequence. This change of
the DNA sequence is known as mutation.
• Any agent causing mutation is called mutagen. Mutagens
can be physical, chemical, or biological.
• The ability of a substance to induce the changes in the base
pairs of DNA or mutation is known as mutagenicity.
• Mutagenesis is the process that result in change of DNA
sequence.
MUTAGENS
• Mutagens can be physical mutagens, chemical
mutagens, or biological mutagens.
TYPES MUTAGENS
PHYSICAL MUTAGENS
• Mutation are normally very rare. However, exposure to radiation and
some chemicals, such as tar in tobacco smoke, increases the rate of
mutation.
• Exposure can cause uncontrolled cell division, leading to the
formation of tumours (cancer).
• Exposure of gonads (testes and ovaries) to radiation can lead to
sterility or to damage to genes in sex cells that can be passed on to
children.
PHYSICAL MUTAGENS
• Physical mutagens are X-rays and UV light.
• X-rays, gamma rays, cosmic rays are ionizing radiation
which ionizes water of the cell to release hydroxyl free
radical (OH). The hydroxyl radical is a powerful oxidizing
agent. Hydroxyl radical oxidises the phosphodiester
bond of DNA. Higher dose of X-rays can even
causes death of an organism.
• UV light is a non-ionizing radiation. It causes the
formation of thymine dimer (Pyrimedine dimer). If two
thymine occur together in one strand of DNA, UV light
causes fusion to form thymine dimer.
Consequences of Radiation Exposure
CHEMICAL MUTAGENS
• Three types of chemical mutagens are found.
i.
Intercalating agent: The chemical intercalate or slip in between
two base pair in Double stranded DNA helix and hence alter the
shape of DNA at that position. Chances of error during replication
is higher at this position causing mutation. Examples; Acridine
orange, ethidium bromide, proflavin
ii. Base analogs: The shape of these chemicals are similar to that of
normal nitrogen bases. So during replication these molecules are
incorporated instead of normal nitrogen bases and hence causes
mutation. Example; 2-aminopurine is analogue to Adenine, 5bromourcail is analogue to thymine
iii. Reacting chemicals: These chemical mutagens reacts directly
with the nitrogenous bases of DNA and chemically modify the DNA
causing mutation. Example; Nitrous acid react with
nitrogenous bases and remove amino group from purine
and pyriminine.
TYPES OF MUTATION
• Mutation can be;
i. Base substitutions (Point) mutationsOne base replaced by another in a sequence.
ii. Insertion
or
deletion
mutations
(Frameshift) – Where bases are deleted or
inserted in a sequence.
BASE SUBSTITUTION (POINT MUTATION)
1. Base Substitutions
• Is when one or more bases are replaced by another in a
sequence.
• Base substitution mutations that occur in DNA sequences are
either;
i. Silent
ii. Missense
iii. Nonsense
BASE SUBSITUTION MUTATION
i. Silent: If a base substitution occurs in the third
position of the codon there is a good chance that
a synonymous codon will be generated. Thus the
amino acid sequence encoded by the gene is not
changed and the mutation is said to be silent.
BASE SUBSTITUTION MUTATION
ii. Missense: When base substitution results in the
generation of a codon that specifies a different
amino acid and hence leads to a different
polypeptide sequence.
BASE SUBSTITUTION MUTATION
iii. Nonsense: When a base substitution results in a
stop codon ultimately terminating translation and
most likely leading to a nonfunctional protein.
FRAMESHIFT MUTATIONS
2. Frameshift (deletions and insertions) mutations
•
•
•
-A mutation that causes all the nucleobases following
it to be shifted.
Deletions or insertions of nucleotides may results
in a shift in the reading frame or insertion of a stop
codon.
Several nucleotides are inserted or deleted into a
gene. These mutations may shift the reading frame of
translation, resulting in a completely different amino
acid sequence after mutation site.
These mutations tend to have serious effects on
protein functionality.
FRAMESHIFT (DELETION) MUTATION)
i. Deletion - A deletion results when one or more
base pairs are lost from the DNA. If one or two
bases are deleted the translational frame is altered
resulting in a non-functional product.
FRAMESHIFT (INSERTION) MUTATION)
ii. Insertions -The insertion of additional base
pairs may lead to frameshift depending on
whether or not multiples base pairs are
inserted.
SOMATIC AND GERM-LINE MUTATIONS
• In multicellular organisms, depending on the cells
that are affected by the mutagen, mutations can be
classified as follows:
1. Somatic mutations
2. Germ-line mutations
SOMATIC AND GERMLINE MUTATIONS
1. Germline mutations – occur in gametes and can be passed
onto offspring (every cell in the entire organism will be affected)
2. Somatic mutations – occur in a single body cell and cannot be
inherited (only tissues derived from mutated cell are affected)
SOMATIC AND GERMLINE MUTATIONS
SOMATIC MUTATIONS
• An alteration in DNA that occurs after conception.
Somatic mutations can occur in any of the cells of the
body except the germ cells (sperm and egg) and
therefore are not passed on to children.
• Numerous types of somatic mutations may not be
manifested to affect an individual due to the reparative
and compensative processes of the body. However, a
somatic mutation that alters the cell division patterns
of the cell can eventually result in the formation
of cancerous cells or tissue.
SOMATIC MUTATIONS IN CANCER
GERMLINE MUTATIONS
• Germline mutations occur in gametes or in the
reproductive cells that produce gametes or sex cells.
• Germline mutations is typically passed down from
one or both parents to the child. Germline mutations
are heritable, meaning they have the ability to be passed
down from generation to generation.
• For example, BRCA gene mutations are a common
cause for families with a strong history of breast or
ovarian cancer. Hereditary cancer testing, such as
BRCA gene testing, has been popular in the western
world for a long time.
EFFECTS OF MUTATIONS
• Mutations can be lethal or non-lethal and
also, these can be inheritable as well as
non-inheritable.
EFFECTS OF MUTATIONS
• Beneficial Mutations
• Some mutations have a positive effect on the organism in which they
occur. They are called beneficial mutations. They lead to new
versions of proteins that help organisms adapt to changes in their
environment. Beneficial mutations are essential for evolution to occur.
They increase an organism’s changes of surviving or reproducing, so
they are likely to become more common over time. There are several
well-known examples of beneficial mutations. Here are just two:
• Mutations in many bacteria that allow them to survive in the
presence of antibiotic drugs. The mutations lead to antibioticresistant strains of bacteria.
• A unique mutation is found in people in a small town in Italy. The
mutation protects them from developing atherosclerosis, which is the
dangerous buildup of fatty materials in blood vessels. The individual in
which the mutation first appeared has even been identified.
EFFECTS OF MUTATIONS
• Harmful Mutations
• Random change in a gene's DNA is likely to result in a protein that
does not function normally or may not function at all. Such mutations
are likely to be harmful. Harmful mutations may cause genetic
disorders or cancer.
• A genetic disorder is a disease caused by a mutation in one or a
few genes. A human example is cystic fibrosis. A mutation in a single
gene causes the body to produce thick, sticky mucus that clogs the
lungs and blocks ducts in digestive organs.
• Cancer is a disease in which cells grow out of control and form
abnormal masses of cells. It is generally caused by mutations in genes
that regulate the cell cycle. Because of the mutations, cells with
damaged DNA are allowed to divide without limits. Cancer genes can
be inherited.
EFFECTS OF MUTATIONS
• Some of the diseases which are the outcome of
mutation are
i. Retinoblastoma or retinal tumors in children,
ii. Tay-Sachs disease
iii. Phenylketonuria
iv. Color-blindness
v. Cystic fibrosis
vi. Xeroderma pigmentosa
EFFECTS OF MUTATIONS
• Xeroderma pigmentosa is a condition in which
thymine dimerization from exposure to UV light is
not repaired. Exposure to sunlight results in skin.
EFFECTS OF MUTATIONS
• Many mutations can actually lead to various diseases.
Certain mutational diseases are inheritable and occur due
to mutation in the germ cell. One such disease is sickle cell
anemia, which occurs due to a single missense mutation at
codon 6 of the β-globin gene in germ cells. This mutation
results in the replacement of the glutamic acid at position 6
in the normal protein by valine. This modification severely
affects the oxygen-carrying protein, i.e., hemoglobin. The
mutated hemoglobin has a highly reduced oxygen-carrying
property and erythrocytes become rigid resulting in the
painful passage of the blood cells and even blockade in the
capillaries and tissue damage. Interestingly, the defective
erythrocytes are resistant to malaria and thus this mutation
has been maintained in the African population.
The comparison between the Sickle Cell
and the Normal Red blood cells
END OF LECTURE!
THANK YOU