Chapt 05

GENERAL BIOLOGY
HDL 121
GENETICS MENDELS LAW

PREPARED BY:MANEGA

SCHOOL OF MLT
FACULTY OF HEALTH SCIENCE

Learning Outcomes

After completing this lecture, students will be able to:
(a) Know some terminologies used in genetics
(b) Know the Mendel’s law
(c) Recall the Mendel’s experiments
(d) Describe the hybrid not in accordance to
Mendel’s law
(e) Explain genetic mapping

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Topics
© 2010 Cosmopoint

Topic Outlines

1.1. Terminologies

1.2. Mendel’s experiment
1.2.1 Garden pea plant
1.2.2 Monohybrid cross
1.2.3 Dyhybrid Cross
1.2.4 Result & conclusion from the experiment

1.3. Hybrid not in accordance to Mendel’s law
1.3.1 Codominance
1.3.2 Imcomplete dominance
1.3.3 Multiple alleles

1.4. Genetic Mapping

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© 2010 Cosmopoint


1.1. Terminologies

Introduction

 Allele: is one member of pair or series of different forms of a gene
found on the same locus on homologous chromosome
 Gamete: Mature male or female reproductive cell (sperm or
ovum) with a haploid set of chromosomes (23 for humans)
 Gene: part of DNA molecule found in the chromosome that
determines a polypeptide through which an inheritable trait is
expressed
 Genotype: The genetic constitution of an organism (only one or
two genes are considered at one time). Genotype can be
homozygous or heterozygous
 Dominant allele: A gene is said to be dominant if it expresses its
phenotype even in the presence of a recessive gene.

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1.1. Terminologies

 Phenotype: Observable trait / traits of an individual; arises from
interactions between genes, & between genes & the environment.
Phenotype determines individual structure, physiology & behaviour
that include followings:
(a) character that can be observed.eg. Colour
(b) character that can be felt. eg. Texture of the hair
(c) character that can be tested serologically. eg. Blood group
(d) quantitative character that can be measured including
intelligence using IQ test.

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1.1. Terminologies

 Heterozygote: a person possessing two different forms of a
particular gene, one inherited from each parent. A heterozygote is
also called a carrier (Eg. Pp, Tt)
 Homozygous: genotype of an individual that has any of a pair or
more of alleles considered are identical eg.
AA, aa, AABB, Aabb, aaBB or aabb
 Homozygote: diploid individual with two identical alleles at a given
locus.

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1.2. Mendel’s experiment

Mendel’s Experiment

Crossed garden peas in his monastery garden & analysed
the offsprings of these mating
Reasons:
(a) could be grown easily in large numbers
(b) had a short life cycle
(c) their pollination could be controlled
(d) their reproduction could be manipulated
(e) had easily observable characteristics.

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1.2.1 Garden pea plant

Pea plants

Have both male & female reproductive organs
Can either self pollinate / cross-pollinate with another plant

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Monohydrid cross

Established pure-breed stock for tall plants & a pure-breed
stock for short plants
Studied the inheritance of one trait, eg. Plant’s height

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Monohydrid cross

Gene – some DNA molecules that controls

Trait – Height (short @ tall)

Genotype

Homozygote (TT) Heterozygote (Tt) Homozygote (tt)

Phenotype – Tall Tall Short

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Cross-pollinated tall pea plants (TT) with each other

Parental generation (Genotype) TT X TT
Gamete T T
F1 TT
Phenotype All tall

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Cross-pollinated short pea plants (tt) with each other

Parental generation (genotype) tt x tt
Gamete t t
F1 tt
Phenotype All short

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Crossed tall plants with short plants

Parental generation (phenotype) tall plant short plant
Parental generation (genotype) TT X tt
Gamete T t
F1 Tt
Phenotype All tall plants

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Allowed plants in the F1 generation to self-pollinate (Self-
cross)

F1 Tt X Tt

Gamete T t T t

F2 TT Tt Tt tt
Phenotype Tall Tall Tall short
Ratio 3 : 1

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Monohydrid cross

Height of plant must have been determined by certain
factors
Factors occur in pairs, because the offsprings of the F2
generation were both tall & short
 F1 generation must contain both tall & shorts factors
Two types of factor
(a) dominant: tall hides the effect of short
(b) recessive: short is recessive to being tall; hidden
by the dominant factor

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Mendel’s Laws of Inheritance

 Mendel first law of inheritance (The law of segregation)
(a) states that from only one parent only one factor (allele) is
passes from the parent to the offspring through the gamete.
(b) This law can be explained by meiosis. In garden pea that is
diploid, a heterozygous yellow seed (Yy) can only transmit one of
the alleles to each of its offspring. (Y is a dominant allele for yellow
seed coat whereas y is a recessive allele for green seed coat)
Parent (P1): Yy (yellow)

Gametes: Y y
(c) Each gamete can only obtain one allele from the parent because
meiosis reduces a diploid gamete mother cell to haploid gamete

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(d) Mendel used garden pea plants for his experiments. One of
the characters was seed colour. He started by crossing two pure
breeding strains; eg. One had yellow & the other had green
seeds. He then allowed the offsprings (F1 generation of first filial
generation) to self-fertilise (selfing) & the results are always the
same as follows:

P1: YY X yy
Phenotype: yellow green

Gametes: Y y

F1: Yy
Phenotypes: yellow

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(e) The F2 generation (second filial generation obtained by random
crossing or selfing of the F1 generation) has a ratio of ¾ of one character
and ¼ of the contrasting character, the classical Mendelian ration is 3:1

P2 (selfing): Yy X Yy
Phenotype: yellow yellow

Gamete: Y y Y y

F2: YY Yy Yy yy
Phenotype: yellow yellow yellow green
Genotypic ratio= ¼ YY : ½ Yy : ¼ yy

Phenotypic ratio= ¾ yellow : ¼ green

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1.2.3.Dihybrid Cross

Dihybrid cross

(f) He repeated his experiment using several contrasting
characteristics, which include tall & dwarf plants, round
& wrinkled seeds, inflated & constricted pods, red &
white flower. Therefore, he concluded that each plant
carried two factors through only one factor was
exhibited in F1. When selfed, the F1 would segregate
the factors & produced the characteristic ratio.

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Mendel second law of inheritance
(a) Dihybrid cross
(b) Mendel crossed pea plants that differed in 2
contrasting traits (pure breeding plants)
(c) He crossed a yellow plant (Y) with round seed (R) with
a green plant (y) with wrinkled seed (r)

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Mendel’s Second Law

Law of Independent Assortment
During gamete formation, segregation of the alleles of
one allelic pair is independent of the segregation of the
alleles of another allelic pair.

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F2: 9 yellow, round: 3 yellow, wrinkled: 3 green, round: 1 green, wrinkled
(9:3:3:1)

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Dihybrid
Punnet
Square

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Mendel confirmed the results of his second law by
performing a back cross where he crossed an F1 dihybrid
with a recessive parent.

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Conclusion on his 2nd experiment
(a) During gamete formation, segregation of the alleles
of 1 allelic pair is independent of the segregation of the
alleles of another allelic pair
(b) Genes that are on different chromosomes assort
independently

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Hybrib not accordance to Mendel Law

Codominance: when both alleles are fully expressed in the
heterozygous form.
Eg. Human MN blood typing
2 antigens, M & N, which are determined by a gene with 2
alleles, LM & LN
Individual with genotype LM LM will have only M antigen in
their RBC
LN LN: N antigen only
LM LN: M & N antigens

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Cross between LM LM and LN LN
P LM LM X LN LN
Gametes LM LN
F1 LM LN
(individual produces both antigens)
P LM LN X LM LN
Gametes LM LN LM LN
F2 LM LM LM LN LM LN LN LN
1 : 2 : 1

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Incomplete dominance: a blending of traits, condition
when neither allele is dominant over the other
Recognised by the heterozygote expressing an
intermediate phenotype relative to the parental
phenotypes
Eg. Red flowered plant is crossed with a white flowered
plant, all progeny will be pink.

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 Multiple allele: genes may exist in more than 2 allelic forms
 Eg. ABO blood type
 Three different alleles for blood type:
(a) IA (Type A)
(b) IB (Type B)
(c) IO (Type O)

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 Only two of these alleles are presented in an individual
 They combine to form genotypes that result from codominance.

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An individual with blood type O mates with an individual
with blood type A.
P: IO IO X IA IA
Gametes: IO IA
F1: IA IO (blood type A)

Individual with IA IO genotype mate
F2: IA IA IA IO IO IO
1 : 2 : 1
Phenotype3 blood group A : 1 blood group O

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Genetic Mapping

 = chromosome mapping
 Determination of the position of a gene on a chromosome by the
means of recombination frequencies
 The percentage of recombinant phenotypes can be used to map the
chromosomes
 Why? – Direct relationship btw frequency of crossing-over & the
percentage of recombinant phenotypes

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If we want to determine the order of any three genes on
a chromosome, we can perform crosses that will
provide us the map distance between the three pairs of
alleles

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1.1. Hardy-Weinberg Law
Introduction

Population genetics is the study of genes in a
population
i.e. the study of Mendelian inheritance mathematically in a
population
The population in this context is a Mendelian
population, consisting of only one species of diploid
organisms, which reproduce sexually within a certain
geographical border
The study of population is important for the understanding
of evolution. Evolution is not the change of one individual
but that of a population over a long period of time
The study of population genetics reconciles the fact of
Darwin theory of evolution with that of Mendelian genetics 42 of 10
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Darwin theory of natural selection is based on
variation created by mutation in the form of different
genes / alleles. Individuals with certain combination
of alleles survive over the years bringing about
changes in a population
The change in allelic frequency caused by
environmental forces is evolution

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1.2. Principle

For easy calculation, a concept based on one gene locus is
treated at one time. So, gene pool is diagrammatically
represented as

A A a
A gene pool of A & a alleles 
A a a a

A a A

a A a A

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1.2. Principle


Darwin theory of natural selection is based on
variation created by mutation in the form of different
genes / alleles. Individuals with certain combination
of alleles survive over the years bringing about
changes in a population
The change in allelic frequency caused by
environmental forces is evolution

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1.2. Principle

Only one gene or locus is considered. That locus
consists of dominant & recessive alleles,
i.e. A & a alleles
The frequencies of alleles A & a depend on the
genotypic frequencies of AA, Aa & aa. Hence, if the
frequency of AA is very high, the frequency of A would
be high too.
From the frequencies of the alleles, the frequencies of
the genotypes of the next generation can be calculated
if we assumed that random fertilisation of the gametes
occurs.

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1.2. Principle
Concept of a Gene Pool

A gene pool is an aggregate of genes/gametes of a
Mendelian population from which the next generation is
produced
It can be considered as the total genetic information
possessed by reproductive members in a population
of sexually reproducing organisms.
Genes in the pool have dynamic relationships with one
another & with the environment around where the
organisms live
Environmental factors such as selection can alter
allelic frequencies & cause evolutionary changes in
the population
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1.2. Principle

Hardy-Weinberg Law

 States that after one generation of random mating, a population
will become in equilibrium
 i.e. the allelic & genotypic frequencies will not change from one
generation to the other
 However, equilibrium is only achieved depending on conditions /
assumptions as follows:
(a) the population must be large
(b) the mating must be random or panmitic
(c) there must not be any selection
(d) there must not be any migration
(e) there must not be any mutation
(f) meiosis must be normal

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1.2. Principle

Uses of the law / its formula

To study the changes of gene frequencies in a wild
population so that the direction & rate of evolution can
be determined.
To study the changes of gene frequencies in an
artificial population such as that of a herd of cattle / a
plantation of crop.
To plan for breeding programme so that a large
population of animals or plants can be manipulated to
produce more quantitatively and/or qualitatively

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