General Biology 2 W3L3 Inheritance and Variations.ppt

© Boardworks Ltd 2004
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General Biology 2
Inheritance

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• All living organisms reproduce. Reproduction results in the
formation of offspring of the same kind. However, the resulting
offspring need not and, most often, does not totally resemble
the parent.
• Variations - Several characteristics may differ
between individuals belonging to the same species.
These differences are termed variations.
• Heredity - The mechanism of transmission of
characters, resemblances as well as differences, from
the parental generation to the offspring,is called
heredity.

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Genetics
The study of heredity, variations and the environmental factors responsible for
these, is known as genetics (from the Greek word genno = give birth).
The word genetics was first suggested to describe the study of inheritance and
the science of variation by prominent British scientist William Bateson.
Genetics
Classical
Genetics
Molecular
Genetics
Evolutionary
Genetics
Mendel's
principles, sex
determination,
sex linkage and
cytogenetics
Genetic material: its
structure, replication and
expression, as well as the
information revolution
emanating from the
discoveries of
recombinant DNA
techniques
mechanisms of
evolutionary change
or changes in gene
frequencies in
populations
(population
genetics).

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Mendelian Genetics
July 20, 1822 – January 6, 1884
Austria
Father of genetics

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Inheritance
Contents
All about alleles
Homozygous cross
Using a test cross
Non-Medelian
Heterozygous cross

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 the definition of certain genetic terms,
Inheritance introduction
To understand how inheritance works you need to know:
and how to use genetic crosses to determine
the characteristics of offspring.
In this unit, petal colour is used to show how characteristics
are inherited in offspring.

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Homologous chromosomes
In all living things, characteristics are passed on in the
chromosomes that offspring inherit from their parents.
Chromosomes are matched in pairs that contain
one chromosome inherited from each parent.
So are the genes in a matching pair of chromosomes
exactly the same?
chromosome from
female parent
chromosome from
male parent

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Each chromosome may have a different version of a gene.
Different versions of a gene, that code for different versions
of a characteristic, are called alleles.
The chromosomes in a matching pair contain the same type
of genes that code for the same characteristics.
Different versions of genes
version for
yellow petals
version for
red petals
gene for
petal colour
gene for
petal colour

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Pairs of alleles – homozygous
If the alleles in a matching pair are the same,
they are called homozygous alleles.
allele for
yellow petals
allele for
yellow petals
allele for
red petals
allele for
red petals
What colour are the flowers with these
homozygous pairs of alleles?

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Pairs of alleles – heterozygous
If the alleles in a matching pair are different,
they are called heterozygous alleles.
Which characteristic is expressed if alleles are different?
allele for
yellow petals
allele for
red petals
Some alleles are dominant to other forms of a gene
and will always be expressed.
Which is the dominant allele in this heterozygous pair?
Which is the recessive allele in this heterozygous pair?

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Representing alleles
Letters are used to represent different alleles.
The allele pair for each characteristic is called the genotype.
What colour are flowers with the genotype Rr?
A dominant allele is always a capital letter.
A recessive allele is always the corresponding small letter.
allele for
red petals R
allele for
yellow petals r
=
=

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Genotypes and phenotypes
The allele pair for each characteristic is called the genotype.
The physical expression of an allele pair is the phenotype.
What are the phenotypes of these genotypes?
genotype:
phenotype:
RR Rr
rr

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Genetic jargon
gene
allele
phenotype
genotype
Section of DNA that codes for a particular trait
or characteristic.
A different form of a gene that codes for a
different version of a characteristic.
A description of the pair of alleles present
for a characteristic.
The physical expression of the alleles.
What do these genetic terms mean?

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homozygous
heterozygous
recessive
dominant
Pair of alleles that produce a characteristic
that are the same, e.g. HH.
Pair of alleles that produce a characteristic
that are different, e.g. Hh.
An allele that will only be expressed when
both alleles are of this type, represented
by a lower case letter.
An allele that will always be expressed even
when there is only one of these alleles present,
represented by a capital letter.
Genetic jargon
What do these genetic terms mean?

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Laws of Inheritance
Mendel proposed three laws:
•Law of Dominance
•The Law of Segregation
•Law of Independent
Assortment

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Mendelian Inheritance
Law of Dominance
• Every gene has two alleles for a
trait.
• One allele is dominant and it will
always express.
• One allele is recessive and it will
be suppressed by the presence
of the dominant allele.
Tall X Dwarf
TT X tt
TT, Tt = Tall
tt = dwarf

Tt Tt
T T
Tt tt
Tt tt
T
t
t
t
Gamete
F1 Generation
F2 Generation
Genotypic ratio-1:1
Phenotypic ratio-1:1

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Law of Segregation
• During gamete formation, every paired alleles
separate (segregate) randomly so that each
gamete receives one allele or the other.
• The two alleles of a gene present in the F1 do not
mix with each other; they separate and pass into
different gametes in their original form producing
two different types of gametes in equal proportion.
• The pair of alleles of each parent separate and
only one allele passes from each parent on to an
offspring
• Which allele in a parent's pair of alleles is
inherited is a matter of chance.

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• Segregation of alleles
occurs during the process of
gamete formation (meiosis)
• Randomly unite at
fertilization
• Mendel crossed tall and
dwarf varieties of garden
peas.
• All progenies of F1
generation were tall (Tt).
Law of Segregation

TT tt
T T t
Tt
Tt
Tt
Tt
Tall, male Dwarf female
Parents
Gamete
F1 Generation
All Tall (heterozygous)

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Law of Independent Assortment
• This law states that allele pairs
separate independently during
the formation of gametes..
• This means that traits are
transmitted to offspring
independently of one another.
• Mendel crossed plants that differ
in two pairs of alleles e.g., round
yellow seeds (YYRR) and
wrinkled green seeds (yyrr).
• F1 hybrids were round and yellow
seeds, as expected (YyRr).

YYRR yyrr
YR yr
YyRr
Yellow- round
seed
×
YyRr
YyRr ×
¼ (YR, Yr, yR, yr)
¼ (YR, Yr, yR, yr)
F1 Generation
Gamete
Parents
Gamete
Yellow round seed Green wrinkled seed

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Law of Independent Assortment
• When F1 were self-
fertilized, four F2
phenotypes were observed.
• This results closely fit a ratio
of 9 : 3 : 3 : 1.
• It verified the ratio in which
the different gametes of the
F1 hybrid were produced.
YyRr X YyRr

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Biological Significance of Mendel’s Laws
• It showed the pattern of inheritance of the character from
parent to the offsprings.
• The Law of Independent Assortment has laid the foundation of
inheritance which claimed that different genes assorted
themselves independently of one another during gamete
formation and had established the relationship between
dominant and recessive gene type.
• It also provided the reason for variation in offspring transferred
from the parents to the offspring in the form of gene (factor)
present in chromosome of gamete.

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Contents
All about alleles
Homozygous cross
Using a test cross
Non-Mendelian
Heterozygous cross
Inheritance

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x
phenotype:
genotype: RR rr
Homozygous means that both alleles of a gene are the same.
Red is the dominant allele for these flowers, so the alleles
for petal color are red = R , yellow = r.
RR x rr – crossing homozygous parents
What are the possible offspring of a cross between a
homozygous red flower and a homozygous yellow flower?

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RR x rr
R R r r
parental genotype:
gametes:
r
r
R
R
Rr Rr
Rr
Rr
F1 offspring
genotype:
RR x rr – F1 offspring
What are the phenotypes of the F1 offspring?
?

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F1 genotypes:
RR x rr – F1 phenotypes
Rr Rr Rr Rr
F1 phenotypes:
The possible offspring of a cross between two homozygous
parents are always heterozygous and so the dominant
characteristic is always expressed in this generation.
RR x rr
parental genotype:
R= round
r = wrinkled

Homozygous cross activity

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Contents
All about alleles
Homozygous cross
Using a test cross
Non-Mendelian
Heterozygous cross
Inheritance

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The offspring (Rr) from the first cross (RR x rr) are called
the F1 generation. What happens in a cross between
these offspring?
Both parent plants are now heterozygous, so the alleles
in each plant are different.
F1 generation
genotype:
phenotype:
X
Rr Rr
Rr x Rr – crossing heterozygous parents

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Rr x Rr – F2 offspring
R r
R
r
RR Rr
Rr rr
What are the phenotypes of the F2 offspring?
?
parental genotype: Rr x Rr
R r R r
gametes:
F2 offspring
genotype:

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Rr x Rr – F2 phenotypes
F2 genotypes: RR Rr Rr rr
F2 phenotypes:
In the F2 generation, 3 of the 4 possible offspring are round.
Only one offspring shows the recessive phenotype.
When two heterozygous parents are crossed, the possible
offspring will always show a 3:1 ratio in favour of the
dominant phenotype.
parental genotype: Rr x Rr
R= round
r = wrinkled

Heterozygous cross activity

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Contents
All about alleles
Homozygous cross
Using a test cross
Non-Mendelian
Heterozygous cross
Inheritance

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What is a test cross?
A test cross allows you to find out if an organism showing
a dominant characteristic is homozygous or heterozygous
for the dominant allele.
For example, the genotype of a red flower could be:
RR or Rr
What could you cross a red flower with to find its genotype?

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A test cross is carried out between the flower of unknown
genotype and another flower whose genotype is known.
For example, a yellow flower can only have the genotype rr
because it’s recessive.
So, the test cross is:
x
?
(RR or Rr)
rr
Carrying out a test cross

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Test cross – 2 types
If the red flower is homozygous (RR) then the cross is the
same as the first cross (RR x rr). All of the offspring will be
heterozygous and have red petals.
What about the other possible cross between a heterozygous
red flower (Rr) and yellow flower (rr)?
rr
x
?
(RR or Rr)

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Test cross offspring
R r r r
gametes:
offspring
genotype:
r r
R
r
Rr Rr
rr rr
What are the phenotypes of these offspring?
?
Rr x rr
parental genotype:

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Test cross results
F2 genotypes: Rr Rr rr rr
F2 phenotypes:
A cross between a heterozygous parent and a recessive
parent yields different types of offspring in a 1:1 ratio.
Rr x rr
parental genotype:

Test cross activity
Brown-eyed person can have homozygous or
heterozygous alleles. Unlike the blue-eyed person
which can only have heterozygous alleles.

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Contents
All about alleles
Homozygous cross
Using a test cross
Non-Mendelian
Heterozygous cross
Inheritance

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Non-Mendelian Inheritance

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FLASH 5 – Blood groups
Co-dominance in Human Blood
How can co-dominance exist in the Blood Type
Groups?

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Co-dominance cross – phenotypes
IA, IB, and IAIB are dominant, while ii is recessive alleles.

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Co-dominance cross – phenotypes
offspring
genotypes:
offspring
phenotypes:
IA and IB parent alleles are dominant which are considered
as co-dominance.
Phenotypes of offspring can have 50% AB blood type and
50% A type.
parental genotype: IAIA, and IBi
IA IB i
IAIB IAIB IAi
IAi

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Non-Mendelian

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Other Non-Mendelian Inheritance

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Assignment
1. Give 3 other examples of the following non-mendelian inheritance with pictures:
(show the Punnett square)
a. Incomplete Dominance
b. Co-Dominance
c. Pleiotropy
d. Epistasis
e. Polygenic Traits
f. Sex-linked Traits / Disease
Reminder:
Asynchronous Task will be posted in our LMS. Make sure to check it to keep you updated
and to submit it before the deadline ( Tuesday 9:00 am).

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