Biology Form 5 Chapter 5 : 5.1 Inheritance

BIOLOGY FORM 5
CHAPTER 5
INHERITANCE

is the study of heredity
is the process in which traits are passed
from parents to offspring

Characters or Traits are resemblances
or differences which can be:
Seen e.g.
eye colour
flower colour
Tested for e.g.
blood groups
colour blindness

Gregor Mendel
(1822-1884)
Austrian monk who
formulated fundamental laws
of heredity in early 1860s
Experimented with peas
Over seven years, he made
crosses with 24,034 plants
Called the “Father of Genetics“

6
Mendel’s Experimental
Methods
Mendel hand-pollinated
flowers using a paintbrush
He could snip (cut) the
stamens to prevent
self-pollination
He traced traits through
the several generations

8
How Mendel Began?
Mendel
produced
pure
strains by
allowing the
plants to
self-
pollinate
for several
generations

11
Mendel stated that
physical traits are
inherited as “particles”
Mendel did not know
that the “particles”
were actually
Chromosomes & DNA
Particulate Inheritance

Let’s revise:
Genes:
 control the characteristics of living organisms
 are carried on the chromosomes

Chromosomes are in pairs, one from each
parent

Genes are in pairs
Genes controlling the same characteristics
occupy identical positions on homologous
chromosomes
The gene pairs control one characteristic
gene for
eye colour
gene for
nose shape
gene for
making insulin

The genes of a corresponding
pair are called alleles
Homologous chromosomes have the same
length and carry the same gene sequences
Alleles are alternative
forms of the same gene
Gene

Let’s take coat colour in mice as an
example
Mice can be:  Black  Brown

The allele for black fur is dominant to the
allele for brown fur
This combination of
alleles gives a
BLACK mouse
The dominant allele is expressed
The recessive allele is masked

Alleles are represented by letters
the alleles must have the same letter but
the dominant allele is always in capitals
Black mouse
(B – dominant allele)
Brown mouse
(b – recessive allele)

Possible combinations of alleles
A black mouse (BB) is crossed with a brown one (bb).
What will the offspring look like?
B
B
b
b
B
b
PURE-BREEDING organism – both alleles
are the same [BB and bb]

B
B
B
B
b
b
b
b
B
b
meiosis
meiosis
fertilisation
All offspring will
be black (Bb)
sperm mother cell
ovum mother cell
zygote

If two of the offspring (Bb)
thus produced are mated,
what type of offspring would
result?
Bb Bb
x
?

B
b
B
b
B
b
B
b
B
B
B
b
B
b
b
b
BB
Bb
Bb
bb
sperm mother cell
ovum mother cell
meiosis
Possible combinationsFertilisation
sperms
ova
zygotes

FIRST FILIAL GENERATION
(F1) the offspring produced
by a parental generation
xParents:
SECOND FILIAL
GENERATION (F2)
offspring of the F1
When two F1 offspring
mate, they produce the
F2

Homozygous & Heterozygous
HOMOZYGOUS – alleles on corresponding
positions of homologous chromosomes are
identical e.g. BB or bb
 HETEROZYGOUS – pairs of different alleles
are present on corresponding positions of
homologous chromosomes e.g. Bb

Genotype & Phenotype
genotype: describes the genetic make-up (all
of the genes) of an individual
homozygous dominant
heterozygous
homozygous recessive
phenotype: outward appearance of an
individual

27
 Genotype - gene combination for a
trait (e.g. RR, Rr, rr)
 Phenotype - the physical feature
resulting from a genotype (e.g. red,
white)

Which is the dominant allele?
Parents
(true breeding
parents)
F1 generation
F2 generation
Purple flowers White flowers

All plants have
purple flowers
Fertilisation
among F1 plants
(F1  F1)
3/4 of plants
have purple flowers
1/4 of plants
have white flowers
Alleleforpurplecolour[100% purpleinF1
generation]

Let us become familiar with terms learned
R represent round seed
r represent wrinkled seed
Round
What is the:
a) phenotype of a homozygous dominant plant?
b) genotype of a homozygous dominant plant?
c) genotype of a heterozygous plant?
RR
Rr

B represent yellow seed
b represent green seed
What is the:
a) dominant allele for seed colour?
b) genotype of a homozygous recessive plant?
B
bb
c) genotype of a true breeding plant that produces
green seeds?
bb

32
Types of Genetic Crosses
 Monohybrid cross - cross involving a
single trait
e.g. flower color
 Dihybrid cross - cross involving two
traits
e.g. flower color & plant height

A - allele for purple flower colour
a - allele for white colour.
A pure breeding purple and a pure breeding white
flower are crossed.
What will the phenotype and genotype ratios be in the
F1 generation? purple - A – AA, Aa
white – a – aa
Parents: Purple x White
AA x aa
Gametes:
F1 generation: Aa Aa Aa Aa
A A ax a
F1 Phenotype: 100% purple
F1 Genotype: 100% heterozygous

The cross does NOT mean that FOUR offspring are
produced. It shows PROBABILITY.
Cross can be shown as:
Parents: Purple x White
AA x aa
Gametes:
F1 generation: Aa
A x a
F1 Phenotype: 100% purple
F1 Genotype: 100% heterozygous
IMPORTANT!!

Self-pollination occurs in one of the F1 plants.
What will the phenotype and genotype ratios
be in the F2 generation? purple – A – AA, Aa
white – a – aa
F1 generation: Purple x Purple
Aa x Aa
Gametes:
F2 generation:
xA a aA
AA Aa Aa aa
Phenotype- 3 purple : 1 white / 75% purple: 25% white( 3:1)
Genotype- 1 AA : 2 Aa : 1 aa (1:2:1)

Remember:
when both parents are heterozygous,
they produce offspring in 3:1 ratio
Pp Pp
x
Parents
Offspring

Gene diagram – Flower colour
Male female
RR rr
parent
gamete R R r r
Offspring
Rr RrRrRr
Genotype
Phenotype All red
Red – R
yellow – r

Male female
Rr Rr
parent
gamete R r R r
Offspring
genotype RR RrRrrr
Phenotype Red yellow red red
3 red : 1 yellow

Male female
Rr rr
parent
gamete R r r r
Offspring
genotype Rr Rrrrrr
Phenotype Red yellow yellow red
Red 50% yellow 50%

x
bb
50% black : 50% brown
x
100% black
bb
Bb
bbBb
Remember

Reginald Punnett (1875-1967)
In 1902, created the Punnett Square - a
chart which helped to determine the
probable results of a genetic cross
T t
T TT Tt
t Tt tt
Male
gametes
Female
gametes
Tt
Tt

Dihybrid Cross
Traits: Seed shape & Seed color
Alleles: R round r wrinkled
Y yellow y green
Pea plants seed A
Round
yellow
RRYY
Pea plant seed B
Wrinkled
green
rryy
Parental
Phenotype
RY ry
RrYy
All round and yellow seeds
Parental
genotype
gametes
F1 genotype
F1 phenotype

46
Dihybrid Cross
Traits: Seed shape & Seed color
Alleles: R round r wrinkled
Y yellow y green
RrYy x RrYy
RY Ry rY ry RY Ry rY ry
All possible gamete combinations
F1 x F1
gametes

49
Dihybrid Cross
RY Ry rY ry
RY
Ry
rY
ry

50
Dihybrid Cross
RRYY
RRYy
RrYY
RrYy
RRYy
RRyy
RrYy
Rryy
RrYY
RrYy
rrYY
rrYy
RrYy
Rryy
rrYy
rryy
Round/Yellow: 9
Round/green: 3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1 phenotypic
ratio
RY Ry rY ry
RY
Ry
rY
ry
Alleles: R round
r wrinkled
Y yellow
y green

51
Dihybrid Cross
Round/Yellow: 9
Round/green: 3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1

REMEMBER
52
MONOHYBRID CROSS eg (Rr x Rr)
Phenotype Ratio : 3:1
Genotype Ratio : 1:2:1
DIHYBRID CROSS eg (RrTt x RrTt)
Phenotype Ratio : 9:3:3:1

53
Question:
How many gametes will be produced
for the following allele arrangements?
Remember: 2n (n = # of heterozygotes)
1. RrYy
2. AaBbCCDd
3. MmNnOoPPQQRrssTtQq

54
Answer:
1. RrYy: 2n = 22 = 4 gametes
RY Ry rY ry
2. AaBbCCDd: 2n = 23 = 8 gametes
ABCD ABCd AbCD AbCd
aBCD aBCd abCD abCD
3. MmNnOoPPQQRrssTtQq: 2n = 26 = 64
gametes

55
Mendel’s Laws
1. Law of Dominance
2. Law of Segregation
3. Law of Independent assortment

56
Law of Dominance
In a cross of parents that are
pure for contrasting traits, only
one form of the trait will appear in
the next generation.
All the offspring will be
heterozygous and express only the
dominant trait.
RR x rr yields all Rr (round seeds)

MENDEL’S LAW
LAW OF SEGREGATION
Each individual characteristic
of a organism is determined
by a pair of allele.
The pairs of alleles
segregate during meiosis
Only one of each pair of
allele can be present in a
single gamete
58

MENDEL’S LAW
LAW OF INDEPENDENT ASSORTMENT
Two or more pair of alleles
will segregate or assort
independently of one
another during gamete
formation
62

63
Instead of
1 trait at a
time, let’s
look at
how 2
traits can
be passed
together.
Connection: Mendel’s Laws and Meiosis

Different Patterns of Inheritance
As we now
know, many
traits do not
follow
Mendelian
Inheritance
patterns.

Co-dominance
When both
alleles are
expressed
equally in the
heterozygous
individual.
A and B blood type
alleles are co-
dominant.
Black and orange
color in cats are co-
dominant.

BLOOD GROUPS
In humans, there are four blood types: A, B, AB & O

BLOOD GROUPS
sometimes a characteristic is controlled by
more than two alleles
e.g. three alleles control human blood:
 A, B and O
a person has two out of three alleles

BLOOD GROUP
(phenotype)
GENOTYPE
A IA IA or IA IO
B IB IB or IB IO
AB IA IB
O Io Io
The alleles for groups A & B are CODOMINANT
O is recessive

70
Codominance Problem
Example: homozygous male Type B (IBIB)
 x
heterozygous female Type A (IAi)
1/2 = IAIB
1/2 = IBi
Parents: IBIB x IAIO
Gametes: IB
F1 generation:
IB IA IO
IBIOIAIB IBIO
IAIB
Phenotype AB B AB B

71
Another Codominance Problem
Example: male Type O (ii)
x
female type AB (IAIB)
1/2 = IAi
1/2 = IBi
Parents: IoIO x IAIB
Gametes: IO
F1 generation:
IO IA IB
IAIOIAIO IBIO
IBIO
Phenotype A A B B

2) Two parents, one with blood group A and the
other with blood group B, have a child whose
genotype is homozygous.
a) Complete the diagram below to show how
this can happen. (5)
IoIo
Io Io
IAIo
IBIo
IBIA

b) What is the chance of these parents
producing a homozygous child? (1)
Parents: IAIO x IBIO
Gametes: IA
F1 generation:
xIO IB IO
IAIOIAIB IOIO
IBIO
25%
c) What is the blood group phenotype of the
homozygous child? (1) Blood group O

74
• The differences in human blood
are due to the presence or
absence of certain protein
molecules called antigens and
antibodies.
• The antigens are on the surface of
the red blood cells
• the antibodies are in the blood
plasma.
• The blood group you belong to
depends on what you have inherited
from your parents.
Why the different blood
groups?

75
ABO BLOOD GROUP
Blood group A
 A antigens (on the surface of RBC)
 B antibodies (in blood plasma)
Blood group B
 B antigens (on the surface of RBC)
 A antibodies (in blood plasma)

76
Blood group AB
 A & B antigens (on the surface of RBC)
 No antibodies (in blood plasma)
Blood group O
 No antigens (on the surface of RBC)
 A & B antibodies (in blood plasma)

BLOOD GROUP ANTIGENS &
ANTIBODIES
BLOOD GROUP ANTIGEN
ON SURFACE
OF RBC
ANTIBODY
IN BLOOD
PLASMA
A A B
B B A
AB A and B None
O None A and B

79
Rhesus factor blood grouping system
 Rhesus = Rh Rh + ( dominant) Rh - (recessive)
 Rh + = has Rhesus antigen
(Cannot produce Rh antibody)
 Rh - = no Rhesus antigen
(Able to produce antibody if he or
she receives blood from a person
with Rh+ blood )
A person with Rh+ blood can receive blood from a
person with Rh- blood without any problems.

Rhesus Factor
 Rh - ( rh rh)
 Rh + (Rh Rh) or (Rh rh)
(Rh+ - dominant)

Rh inheritance
Mother Father
Rh – Rh +
( rh rh) X (Rh Rh)
Foetus : (Rh rh)
Rh+

Rh inheritance
Mother Father
Rh – Rh +
( rh rh) X (Rh rh)
Gamete rh Rh rh
Foetus : (Rh rh) (rhrh)
Phenotype Rh+ Rh-

Mother: Rh +ve, foetus : Rh –ve
No problem
Mother - antigen
in RBC unable to
diffuse through
placenta
Baby born alive
Rh +
Rh -

Rh -
Rh +
Mother: Rh – , Foetus: Rh +
Problem
 During delivery, baby’s blood in
placenta will mix with mother’s
blood
 1st baby will survive
 Rh antigen in RBC enter mother’s
blood system
 Mother’s lymphocytes stimulated
to produce Rh antibodies.

Rh -
Rh +
Mother: Rh – , Foetus: Rh +
Problem
 Rh Antibodies remain in mother’s
blood plasma
 Second pregnancy, if foetus also
Rh+
 Antibody from the mother’s blood
plasma diffuse into foetus blood
through placenta (leakage)
 Agglutination in foetus’ blood

Mom’s immune system recognizes that these cells are not like
hers, so the baby’s blood cells are attacked.
The same principle applies to rejected organ transplants and
blood transfusions.
Example of the immune system gone wrong…

90
Sex Chromosomes
The gender / sex of an individual is determined genetically by
the sex chromosomes.
XX = female, XY = male
All other chromosomes are called “autosomes”
Humans have 46 chromosomes
(44 autosomes+2 sex chromosomes)

91
Sex chromosomes
Every female cell has two
sex chromosomes which are
alike (XX)
Sex chromosome
XX
Human female: 44 + XX

92
Sex chromosomes
Every male cell has two
chromosomes which are
not alike (XY)
Human male: 44 + XY

93
Human male: 44 + XY
Human female: 44 + XX

94
cells in testis of male cell in ovary of female
44 +XY
44 +XX
there are two
types of sperm
22 + X
there is only one
type of ovum
22 + Y 22 + X 22 + X
44 +XY
44 +XY44 +XX44 +XX
Genotype: 44 +XX : 44 + XY
Female : Male
1 : 1
SEX DETERMINATION IN HUMANS

96
The type of sperm which fertilises
the ovum decides the sex of the
offspring
X sperm – baby girl Y sperm – baby boy

97
What is so different between the X and Y
chromosomes?
X- over 1000 genes identified
Y- 330 genes identified,
many are inactive
Y chromosome

Sex-linked genes are carried on the
sex chromosomes (X chromosome)
autosomes Sex
chromosomes
X X
X Y
Female
carries two
alleles of a gene
Male
carries one
allele of a gene

Sex-Linkage
red-green colour
blindness haemophilia
“3” or “8”??
examples of sex linked traits:

Males are more likely to suffer from
sex-linked diseases
Normal
A
Females
carry two alleles of a
gene. If one allele is
defective, female is
still normal as effect
is masked by the
normal allele.
A
Normal: A
Sick: a
SickPhenotypically
normal / carrier
A
A
a
a
aa
Normal Sick
Female: 3 choices
Male : 2 choices

105
Features of Colour blindness
Colour blindness – Inability to differentiate
between red and green
o hereditary disease
o It is common in male but rare in female.
o Caused by recessive allele located on X
chromosome
o Colour blindness follows criss- cross inheritance
as transmitted from father to grandson through
daughter.
o It is never transmitted from father to son

107
XCXC - Normal female
XCXc - Carrier female
XCY - Normal Male
XcY - Affected male
XcXc - Affected female

108
Red-green colour blindness
Parental
Phenotypes Carrier Female x Normal Male
Genotypes XBXb XBY
Gametes
Offspring 1
Genotypes
Phenotypes
Normal Female : Carrier Female : Normal Male : Colour blind Male
1 : 1 : 1 : 1
XB
XbXB Y
XB
XBXb Y
XB
XB Xb Y

109
GENETIC PEDIGREE FOR COLOUR BLINDNESS

111
This disease is
appeared as a mutant
in Queen Victoria and
from her it was
transmitted to her
descendants.
“Royal disease”
XH XH - Normal female
XH Xh - Carrier female
XH Y - Normal Male
Xh Y - Affected male
Xh Xh - Affected female

113
XhY
XH XH
Xh Y
XHXH
XH YXHXh
XHY XHXh
UNAFFECTED MOTHER FATHER - HAEMOPHILIA
Carrier Normal Carrier Normal
Female Male Female Male

115
XHY
XH Xh
XH Y
XHXh
Xh YXHXH
XHY XHXh
CARRIER MOTHER UNAFFECTED FATHER
Normal Normal Carrier Haemophilia

117
XhY
XH Xh
Xh Y
XHXh
Xh YXHXh
XHY XhXh
CARRIER MOTHER FATHER - HAEMOPHILIA
Carrier Normal Haemophilia Haemophilia

119
XHY
Xh Xh
XH Y
XhXh
Xh YXHXh
XhY XHXh
MOTHER - HEMOPHILIA Normal FATHER
Carrier Haemophilia Carrier Haemophilia

121
XhY
Xh Xh
Xh Y
XhXh
Xh YXhXh
XhY XhXh
MOTHER - HAEMOPHILIA FATHER - HAEMOPHILIA
Haemophilia Haemophilia Haemophilia Haemophilia

122
HEREDITARY DISEASE
Genetic diseases that offsprings inherit
from their parents
Eg. Haemophilia, red-green colour blindness,
Duchenne muscular dystrophy are caused by
recessive allele on the X chromosome.
Eg. Cystic fibrosis, albinism, sickle cell anemia,
thalassaemia are caused by defective genes
found on the autosomes.

Sickle-cell Anaemia
 Caused by defective
allele for synthesis
of haemoglobin
 Autosomal gene
located on
chromosome
number 11

 This is due to the clumping of the abnormal
haemoglobin molecules in the red blood cell
 When blood oxygen is low the red blood cell
has the shape of a sickle
Sickle-cell
Anaemia

 They more likely to break, aggregate and
clog the blood capillaries
VIDEO
SICKLE CELL
DISEASE
http://www.yout
ube.com/watch?
v=R4-c3hUhhyc

Cystic fibrosis
 Caused by a lack of transport protein which
allows chloride ions to move across plasma
membranes
 Normally water will pass through the
plasma membranes after the chloride ions
passes

 Caused by cystic fibrosis gene
located on chromosome number 7

 Affected persons – frequent respiratory
infections

Thalassaemia
 Anaemia
 Red blood cells cannot carry enough oxygen.
Deficiency of iron.
 Caused by recessive gene
 Synthesis of abnormal haemoglobin in red blood
cells
 Passed down by parents who carry thalassaemia
genes in their cells

 Symptoms :
 Appear healthy at birth
 After two years – become pale, listless, fussy,
poor appetite
 Grow slowly
 Develop jaundice
 Treatment :
 Frequent blood transfusion
 Bone marrow transplant
VIDEO
http://www.youtube.com/watch?v=Ul7m_FNsd_c

Biology Form 5 Chapter 5 : 5.1 Inheritance

Biology Form 5 Chapter 5 : 5.1 Inheritance

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Biology Form 5 Chapter 5 : 5.1 Inheritance