B4FA 2012 Nigeria: Principles of Genetics - Charles Amadi
Lecture Presented By Amadi Charles
National Root Crops Research
• A cursory look at siblings
immediately reveals similarities and
differences amongst them and
between them and their parents.
• Understanding the basis of these
similarities and differences and how
they are transmitted from one
generation to another is within the
purview of genetics.
• Genetics is the science of
heredity, dealing with
resemblances and differences of
related organisms resulting from
the interaction of their genes and
the environment (Online
• Gregor Mendel, through
a classical set of
experiments was able to
accurately describe the
based on the
assumptions of paired
units and random
transmission of the
units from parents to
• For this reason He is
called the Father of
Gregor Mendel (1822-1884)
Some Basic Principles
• Traits, or characteristics, are passed on
from one generation of organisms to the
• The traits of an organism are controlled
• Organisms inherit genes in pairs, one
gene from each parent
• Some genes are dominant, whereas
other genes are recessive
• Dominant genes hide recessive genes
when both are inherited by an organism
1. The Law of Dominance
2. The Law of Segregation
3. The Law of Independent
The Law of Dominance
• This law states that in a cross of parents that
are pure for contrasting traits, only one form
of the trait will appear in the next generation.
• Offspring that are hybrid for a trait will have
only the dominant trait in the phenotype.
• The trait whose appearance is suppressed in
the hybrid is said to be recessive.
The Law of Segregation
• This law states that during the formation
of gametes (eggs or sperm), the two
alleles responsible for a trait separate
from each other.
• Alleles for a trait are then "recombined"
at fertilization, producing the genotype
for the traits of the offspring.
• If we cross two tall hybrids with
genotype Tt, we will get both tall and
short plants in the ratio of 3 tall: 1 short
The Law of Independent
• Alleles for different traits are
distributed to sex cells (&
offspring) independently of one
Illustration of independent
RG Rg rG rg
• To illustrate independent
assortment, let assume
that the genotypes of
our parents are
• RrGg x RrGg
"R" = dominant allele for
"r" = recessive allele for
"G" = dominant allele for
"g" = recessive allele for
Non Mendelian Inheritance
• Not all genetic observations can be explained
and predicted based on Mendelian genetics.
• Other complex and distinct genetic
phenomena may also occur eg
– blood types,
– skin colour,
– Lower colour
– tuber yield etc.
• In some allele combinations, dominance
does not exist. Instead, the two
characteristics blend to form a new
character in the offspring.
• For instance, snapdragon flowers
display incomplete dominance in their
• There are two alleles for flower color:
one for white and one for red. But when
one allele for red is present with one
allele for white, the color of the
snapdragons is pink.
Illustration of Incomplete
Gametes W (white)
R (Red) RW (Pink)
• With codominance, a cross between
organisms with two different
phenotypes produces offspring with
a third phenotype in which both of
the parental traits appear together.
• For example a cross between Red
and White Parent will give rise to an
offspring that is red and white
Illustration of Codominance
• R = allele for red
W = allele for
• red x white --->
red & white
RR x WW --->
• In certain cases, more than two
alleles exist for a particular
• Even though an individual has only
two alleles, additional alleles may
be present in the population.
• An example of multiple alleles
occurs in blood type.
Human Blood Type
• In humans, blood groups are determined by a
single gene with three possible alleles: A, B, or
• Red blood cells can contain two antigens, A
• The presence or absence of these antigens
results in four blood types: A, B, AB, and O.
• Polygenic characters are controlled by
many genes at different locations on
• There is a gradual variation in the
character from one extreme to the
• An example of polygenic inheritance is
human skin color.
• A person with many genes for dark skin
will have very dark skin color, and a
person with multiple genes for light
skin will have very light skin color.
• A chromosome has many thousands of genes.
• It is common for a large number of genes to be
inherited together if they are located on the
• Genes that are inherited together are said to
form a linkage group.
• Gene linkage can show how close two or more
genes are to one another on a chromosome.
• The closer the genes are to each other, the
higher the probability that they will be inherited
• There are 23 pairs of chromosomes in human cells.
• One pair is the sex chromosomes. (The remaining 22
pairs of chromosomes are referred to as autosomes).
• The sex chromosomes determine the sex of humans.
• There are two types of sex chromosomes: the X
chromosome and the Y chromosome.
• Females have two X chromosomes; males have one X
and one Y chromosome.
• Typically, the female chromosome pattern is
designated XX, while the male chromosome pattern is
• Thus, the genotype of the human male would be 44 XY,
while the genotype of the human female would be 44
XX (where 44 represents the autosomes).
Sex linked Characters
• In humans, the Y chromosome is much shorter than
the X chromosome.
• Because of this shortened size, a number of sex-linked
• When a gene occurs on an X chromosome, the other
gene of the pair probably occurs on the other X
• Therefore, a female usually has two genes for a
• In contrast, when a gene occurs on an X chromosome
in a male, there is usually no other gene present on the
short Y chromosome. Therefore, in the male, whatever
gene is present on the X chromosome will be
• Examples of sex-linked conditions are Colour blindness
• The cell is the basic structural and
functional unit of all known living
• It is the smallest unit of life that is
classified as a living thing, and is often
called the building block of life.
• Most plant and animal cells are between
1 and 100 µm and therefore are visible
only under the microscope.
Diagram of Plant Cell
Diagram of Onion Cells.
Source Wikipedia Cell(Biology)
Diagram of a plant cell. Source:
Wikipedia Cell (Biology)
• A chromosome is a long, stringy
aggregate of gene that carries
heredity information (DNA).
• A chromosome has many thousands
of genes; there are an estimated
100,000 genes in the human
• Inheritance involves the transfer of
chromosomes from parent to
offspring through meiosis and
Structure of Chromosome
Source: National Institutes of Health
• Genes are segments of DNA
located on chromosomes. Traits
are passed from parents to
offspring through gene. Genes
contain the codes for the
production of specific proteins.
• An Allele is one of two or more
alternative forms of a gene at
corresponding sites (loci) on
homologous chromosomes, which
determine alternative characters
Deoxy Ribose Nucleic Acid (DNA)
• Deoxy Ribose Nucleic acid (DNA) is
the genetic material in most of the
• DNA is mainly found in the
chromosomes in the nucleus.
• It consists of smaller molecules
• Each nucleotide consists of a sugar,
phosphate group and a nitrogenous
TT = homozygous(pure)
Tt = heterozygous(hybrid)
tt = homozygous(pure)
The genes present in an organism
make up the genotype. That is the
genetic makeup of an organism.
• The manifested characteristic or
the physical appearance of an
• Examples of phenotypes are
– blue eyes
– brown fur
– striped fruit
– yellow flowers
Hazel EyeGreen eye
Steel Gray EyeElectric blue eye
Source Wikipedia from link www.obsidianbookshelf.com
Human Eye Colour
• The production of new living organisms by
combining genetic information from two
individuals of different types (sexes).
• In higher plants this usually involves:
– Pollination (Transfer of pollen grains from the
anther to the stigma of flower of a plant of the
– Fertilization (Mixing of the male and female
– Embryogenesis (embryo formation).
Diagram of a Flower
• Gametes are the reproductive or
sex cells produced in the sex
organs of a plant or animal.
• The male sex cell (Male gamete)
is known as sperm
• The female sex cell is known as
Symbols for Gametes
Symbol for female gamete Symbol for male gamete
• They physical appearance of an
individual, the phenotypic value
(P) is the combined result
– its genetic makeup, the genotype (G)
– the effects of the environment (E):
P = G + E
• Additive variation represents the cumulative
effect of individual loci, therefore the overall
mean is equal to the summed contribution of
• Dominance variation represents interaction
between alleles. If a trait is controlled by a
dominant allele, then both homozygous and
heterozygous individuals will display the same
Effect due to environment
Effects due to interaction
between genes and environment
• Interaction (I) between different
genes can modify the observed
• This is called epistasis, or non-
allelic interaction, distinguishing
it from dominance.
P = A + D + E + I
• The total phenotypic variation (V)
of a population is the sum of the
variation in additive (A),
(I), environmental (E)and gene-
environment interaction (GE)
VP = VA + VD + VI + VE + VGE
Why is this important?
• Being able to estimate how the total variance
is partitioned between genetic and
environmental effects is important to
quantitative geneticists trying to improve a
• If the proportion of variation is mostly due to
genetic effects (heritable), then selecting for
individuals that possess the desired genetic
value is a worthwhile investment.
• If however, the genetic variance is low (and
therefore the environmental variation has
more impact on phenotype), then a more
strategic approach would be to optimize
• A search of the internet for '' genetics
games or simulations" using google
search engine scored 25.6 million hits in
• This underscores the emerging tendency
to simulate the outcomes of genetic
studies using computer technology.
• Many reasons have been adduced for the
use of computer applications or
programs to simulate the results of
Why use of computer applications or
programs to simulate the results of
• Many generations of genetic research can
be carried out more quickly than with live
• It eliminates the drudgery associated with
carrying out field experiments
• Organisms do not need to be created or
• Simulators allow the application of class
lessons to real world situations.
• Complete crossing programs that are
impossible in live organisms can be carried
out rapidly at almost no cost.
• There are many computer simulation
and animation programs available
for genetic studies. Some of these
– “Hands On Genetics” programs
– Classical genetics simulator,
– PABSIM, etc.
• We will practice simulation with Drosophilab a