PATTERNS OF
INHERITANCE
JANUARY 21, 2013
INTERESTING FACT
• All the ova from which
the present human
poplation was derived
can fit into a 5-litre
bottle.
• All the sperm that
fertilized them could
fit in a……
• THIMBLE.
• For years similarities
between children and
parents  obvious.
• Male + female =
offspring
• But how?
• Aristotle 384-322 BC 
semen + menstrual fluid
 offspring X
• van Leeuwenhook 1677
 sperm in semen
contain miniature human
that implant in female to
be born X
• de Graaf 1670s 
discovered the
Graafian follicle from
which came the ovum
O
• Both parents contribute hereditary
characteristics and the offspring is a blend
of both.
• The offspring therefore has two sets of
genetic information but only one of any
pair of genes expresses itself.
MENDEL,1866
THE RUSSIAN MONK
• Mendel performed experiments
referring to the process of hybridization
in plants.
• He focused on the different forms in
which hybrid offspring appear and the
statistical relationship between them.
HIS EXPERIMENTS
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He chose the pea Pisum sativum for his
research because:
Several distinct varieties existed
Easy to cultivate
Self-pollinating (pure breeding)  same
characteristics gen. after gen.
Artificial cross-breeding varieties
possible  fertile hybrids.
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Mendel observed seven traits that are
easily recognized and apparently only
occur in one of two forms:
seed shape is (round or wrinkled)
seed color is (yellow or green)
flower color is (purple or white)
pod shape is (inflated or constricted)
pod color is (yellow or green)
flower position is (axial or terminal)
stem length is (long or short)
HIS SUCCESS
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He succeeded because of careful use of
the scientific method.
Familiarized self with the plants
Only one variable studied at a time
Meticulous methodology
Sufficient statistical data collected
Lack of genetic features such as
codominance and linkage
DEFINITIONS
• DNA- Deoxyribonucleic acid. This substance
is the back-bone of all genes and therefore
assists to make up chromosomes. DNA is
found in the nucleus of cells.
• RNA-Ribonucleic acid. This substance helps
to make new DNA and hence genes and
chromosomes. There are three types, one
found in the nucleus and two in the
cytoplasm of cells.
• Chromosomes: material found in the nuclei of cells,
are composed of DNA and protein and contain genetic
information in the form of genes.
• Genes: the basic unit of inheritance for
a given characteristic or trait.
• Alleles: contrasting forms of the same gene
found occupying the same locus (position) on
homologous chromosomes. They may produce
the same or different qualities.
• Homozygous: having two identical
alleles in corresponding positions on
homologous chromosomes.
• Heterozygous: having two contrasting
alleles in corresponding positions on
homologous chromosomes.
• Phenotype: the outward, visible
expression of a gene. E.g. hair colur,
eye colour, gender.
• Genotype: the inward, genetic make-up
of the organism, especially its alleles
(contrasting genes).
• Phenotype: the outward, visible
expression of a gene. E.g. hair colour,
eye colour, gender.
•
• Dominant: the allele which, if present, shows
its effect on the phenotype in both the
homozygous and heterozygous conditions.
Allele usually represented by capital letters.
• Recessive: the allele which only has an effect
on the phenotype of the dominant allele is
absent. Allele usually represented by
common letters.
• Incomplete Dominance is the expression
of alleles that are neither dominant or
recessive.
• These alleles mix together and result in
expression of a physical trait that is a
mixture of the two alleles.
• Example of Incomplete Dominance when you
cross a black mice (BB) with a white mice (WW),
the progeny of these mice is gray in color (BW).
The colors blend together and express the
phenotype of both alleles. This type of
dominance is similar to mixing paints. When you
mix one color with another color, the result is a
completely different color. Similarly, when one
allele is mixed with another allele they blend
together to give rise to a new phenotype.
Incomplete Dominance
• Co-dominance (existing together).
• When there are two alleles that share a codominant relationship, progeny will express both
alleles.
• For example, if a black cat (CbCb) is crossed with a
brown cat (CrCr), the kittens (CbCr) will be either brown
with black spots or stripes or black with brown spots or
stripes, that is, tabby cat. This means both the colors
are co-dominant in this case. Both alleles are
completely expressed and the kittens show both colors
at the same time.
Codominace
• As you can see, in codominance the
alleles express their individual traits
simultaneously. In case of incomplete
dominance, the alleles tend to blend in to
give rise to an altogether new phenotype.
• Examples of Codominance and Incomplete Dominance
Example 1
When one crosses a red snapdragon flower (RR) with a
white snapdragon flower (WW) the result will be as
follows:
RR X WW
will give rise to
RW
The flowers show incomplete dominance as the red and
white colors blend and express a completely new color,
that is, pink.
• Example 2
When cream-colored mare (CC) is paired with a brown
colored horse (BB) it will result in a pony that is tan in
color. This is an incomplete dominance example where
the alleles blend to give a new phenotype.
CC X BB
this results in an offspring that is tan in color
CB
What would be the phenotypes from CB x CB
• Example # 3
When a red flower is crossed with white flower it results
in flowers with red spots on white background or white
spots on red background.
R1R1 X R2R2
the resultant flowers are
R1R2
This is an example of codominance where both red and
white color are expressed in the resultant flowers.
• Genes, like to play around and express
different phenotypes in progeny.
• It is very difficult understanding genes at
times.
• Most often we observe complete
dominance, where one allele completely
takes over another allele.
• Epistasis
MONOHYBRID
INHERITANCE
• For monohybrid inheritance Mendel stated
that:
• The characteristics of an organism are
determined by internal factors which occur
in pairs. Only on of a pair of such factors
can be represented in a single gamete.
Monohybrid Inheritance
• Inheritance is the process by which certain
characteristics or traits are passed on from
generation to generation.
• Monohybrid inheritance is the analysis of only
one of these traits at a time.
• It is very simple to do.
• A diagram called a Punnett Square is
usually used in order to work out
monohybrid inheritance.
• We say that we perform a monohybrid
cross.
• Definitions to definitely remember when
performing these crosses are:
• Dominant
• Recessive
• Genotype
• Phenotype
• Co-dominance and incomplete
dominance.
• We will be using monohybrid crossing to
predict the genotypic and phenotypic ratios of
off-spring born with:
• Different genders (male or female)
• Albinism
• Sickle-cell
• Haemophilia
• Night-blindness
• Blood types
• Finally, we will be able to trace defects,
diseases or traits throughout family
trees when we have mastered the
monohybrid cross.
• Along the way we will pay attention to
ratios that are expected for certain
crosses.
Performing Monohybrid
Crosses
STEPS
• 1). Choose letters to represent the
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alleles/genes.
2). Determine the genotypes of the parents.
3). Determine the available gametes.
4). Draw Punnett Square.
5). Assign gametes.
6). Fill in Punnett Square.
7). Analyze results for phenotypic and
genotypic ratios.
Mating for Albinism
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Albino X Albino
Albino X Homozygous black (normal)
Albino X Heterozygous black (normal)
2 Hetero normals
Homozygous normal X Hetero normal
2 Homozygous normals
Mating for Tallness
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Short X Short
Short X Homozygous tall
Short X Heterozygous tall
2 Hetero tall
Homozygous tall X Hetero tall
2 Homozygous tall
Eye Colour
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2 Homozygous brown
2 Heterozygous brown
Homo brown X Hetero brown
Homo brown X Green
Hetero brown X Green
2 Green eyed parents
Inheritance of Gender
• Females have 2 X chromosomes (XX).
• Males have one X and one Y
chromosome (XY).
• Let us do a monohybrid cross to
determine how sex is passed on from
parents to of-spring.
Sex-Linked Genes
• Sex-linked genes are genes that are only found
on the longer region of the X chromosome and
so will be absent from the Y chromosomes in
males.
• When performing crosses involving sex-linked
genes, the letters representing the alleles are
attached to larger letters that represent the
chromosomes, so that we will be able to tell if we
are dealing with a man or woman.
• In sex-linkage, women are called carriers, but
not men.
Mating for Haemophilia
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Normal parents
Haemophiliac parents
Haem mom x Haem dad
Haem mom x normal dad
Carrier x haemophiliac
Carrier x normal
Mating for Colour-blindness
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Normal parents
Colour-blind parents
Colour blind mom x normal dad
Colour blind dad x normal mom
Carrier mom x normal dad
Carrier mom x colour-blind dad
Co-dominance
• A form of inheritance in which both alleles are
equally shown. Blood typing is a great
example. AB blood is the codominant
relationship between the A protein and B
protein
both
expressing
themselves
completely. AO (type O allele means there is
no protein), A is dominant and you see type A
phenotype. BO is the same except you see
the B phenotype. Type O is recessive
• 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.
Mating for Sickle Cell
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2 parents with sickle cell disease
2 parents with sickle cell trait
Sickle cell disease X Sickle cell trait
Sickle cell disease x normal
Sickle cell trait x normal
Incomplete dominance
• A form of inheritance in which the
heterozygous alleles are both expressed,
resulting in a combined phenotype. The
one example that most books give is seen
in some flower colors. A red and a white
allele gives pink. If it were codominance,
you would see the red and white colors
mixed yet separate. Incomplete
dominance is most commonly found in
plants.
• With incomplete dominance, a cross
between organisms with two different
phenotypes produces offspring with
a third phenotype that is a blending of the
parental traits.