Chapter 9: Genes, chromosomes
and patterns of inheritance 1
EL: To introduce genetics, with a
focus on chromosomes
Before we begin..
• Answer questions 1-4 on page 291
Genetic instructions
• Offspring receive genetic instructions from
their parents (or parent cells in the case of
unicellular organisms)
• In humans, these instructions are packaged in
gametes: the egg cells of a female and the
sperm cells of a male
• We will be examining how these instructions
come together to determine the
characteristics of the offspring
Prokaryotic Chromosomes
• In prokaryotes, a single circular chromosome is attached to the
plasma membrane at a specific point.
• When the cell divides by BINARY FISSION
• DNA molecule replicates
• The two copies are separated by the expansion of the plasma membrane
• Plasma membrane and cell wall furrow inwards to divide the cytoplasm
resulting in two daughter cells.
Binary = two, Fission = splitting
DNA
• Deoxyribonucleic acid (DNA)
is found within the nucleus of
eukaryotic cells
• Chromatin is a mass of
uncoiled DNA and associated
proteins called histones.
• When cell division begins,
DNA coils around the proteins
forming visible structures
called chromosomes.
Chromosome structure
Haploid cells
A cell with one set of chromosomes is called haploid (n) –
gamete cells are usually haploid
Diploid cells
A cell with two sets of chromosomes is called diploid (2n) – somatic cells
are diploid
Each matching pair is called “homologous” – they each contain the same
genes – however, the DNA sequence isn’t necessarily the same
Polyploid cells
A cell with more than two sets of chromosomes is called
polyploid. This is usually only found in plant cells.
Chromosome Numbers
• An organism of a particular species always has the same number
of chromosomes (e.g. humans have 46 chromosomes or 23 pairs)
• See table 9.1 page 292
Human chromosomes
• Diploid number = 46, Haploid number = 23
• The 22 matched, homologous pairs of autosomal
chromosomes are distinguished by:
– Relative size
– Position of centromeres
– Patterns of light and dark bands when stained
• The 23rd pair in a diploid somatic cell are the sex
chromosomes (N.B. In males these are NOT
homologous)
Human Karyotype
Karyotype: the display of the number, size and shape of
chromosomes from a cell
HUMAN FEMALE
HUMAN MALE
Autosomal chromosomes
Sex chromosomes
Activity
• In two groups, Complete Part A of activity 9.1
“Karyotypes” – one group will do figure 9.1C
and the other group will do Figure 9.1D.
Answer questions 1-5 i
• Re-visit Chapter 9 quick check questions 1-4
on page 291 (how did you go?) and complete
qu 5-8 on page 301, question 5 on page 336
Reflection
These are the questions you should answer each
lesson, preferably in writing
•What learning was new today?
•What learning was revision or built on what I
already know?
•What did I find most challenging and what
strategies will I put in place to help me?
•What percentage of the class did I spend on
task and how can I improve this if needed?
Chapter 9: Genes, chromosomes
and patterns of inheritance 2
EL: To introduce or revise mitotic cell
division in eukaryotic somatic cells
What do you remember?
• Before we begin the lesson, write down or
draw what you remember about eukaryotic
cell division
Cell Division in Eukaryotes
• A division of the
nucleus (mitosis)
followed by a
division of the
cytoplasm
(cytokinesis)
• To accomplish this
task, the cell passes
through a series of
discrete stages or
phases
http://www.youtube.com/watch?v=VlN7K1-9QB0
• Cells spend the majority of their
time (about 95%) in interphase.
• Cultured mammalian cells usually
divide once every 18-24 hours.
• The cell appears “to be at rest”.
Nothing could be further from the
truth!
• During interphase most cellular
contents are synthesised
increasing cell mass. It is a time of
cell growth, DNA replication and
metabolic activity.
• The genetic material in the
nucleus is in the form of
chromatin fibres. Discrete
chromosomes are not visible.
Interphase
• Interphase starts with G1 or
Growth 1; its the time for
the cell to grow and carry
out its biochemical activities.
The length of this phase is
highly variable between
cells, typically 8-10 hours.
• Some cells sit in G1 for
weeks, months, years. Cells
that are arrested in G1 are
said to be in a G0 state. Most
nerve cells never leave G0.
• The decision to commit to
cell division is made when
the cell passes through the
first checkpoint at the end of
G1.
The G1 Phase
• Once committed to cell
division the cell enters the S
Phase – S stands for
synthesis.
• This is the time for DNA
replication. This typically
takes 6-8 hours.
• The S phase ends when the
DNA content of the cell has
doubled. The evidence for
this becomes obvious when
the chromosomes become
visible at the start of the M
Phase. Each chromosome is
now made up of two sister
chromatids.
The S Phase
• Once the genetic material
has doubled the cell now
enters G2 – Growth 2. This
phase is more fixed in its
timing usually 4-6 hours for
most cells.
• During this phase the cell
actively prepares for cell
division. It is a period of high
metabolic activity and
protein synthesis.
• The cell passes through
checkpoint at the end of G2
to ensure that all is ready
for the division of the
The G2 Phase
• The M phase encompasses a
division of the nucleus
(mitosis) and then a division
of the cytoplasm
(cytokinesis).
• This phase explains how the
two copies of the
chromosomal DNA formed
in S phase are separated and
partitioned into daughter
cells.
• The M phase lasts for less
than 1 hour. The M phase is
divided into various phases
that are characterised by
particular chromosome
behaviour.
The M Phase
The M phase summary
Mitosis can be divided into five
stages:
1.Interphase - cell performs all
its normal functions. Before
mitosis begins, DNA on
replicates
2.Prophase – Nuclear
membrane disappears
3. Metaphase - Spindle is
visible and helps chromatids
line up on equator
4. Anaphase - Chromatids get
pulled to opposite poles.
5. Telophase - Two nuclei
reform around the
chromatids. The cell then
divides (cytokinesis) into
two daughter cells.
Activity
• Use your pipe cleaners to model mitosis with
a partner using the information on the
upcoming slides
•
Prophase beings when the individual
chromosomes have condensed to become
discrete objects under the light microscope.
•
In the cytoplasm, adjacent to the nucleus,
the centrosomes, (duplicated in S phase)
move to opposite ends of the cell. Spindle
microtubules will form between these two
centrosomes.
•
Towards the end of prophase, the nuclear
envelop breaks down
•
The centrosomes are now at opposite ends of
the cell and growing spindle microtubules
enter the nuclear area and make contact with
the chromosomes.
•
Contact between a chromosome and spindle
microtubules occurs at a protein – DNA
complex region known as the kinetochore.
2. Prophase
The relationship between the centromere,
kinetochore and spindle microtubules.
3. Metaphase
• Chromosomes are now
maximally condensed and
lined up along the metaphase
plate.
• Chromosomes can now be
used in karyotype analysis.
• Metaphase occupies half the
time required for mitosis.
• The chromosomes appear
stationary, but each chromatid
is being tugged towards the
opposite poles by equally
strong forces.
• In animal cells the centrosome
contains a pair of centrioles.
4. Anaphase
• The centromere holding the two
chromatids abruptly separates.
• Each chromatid (now a single
chromosome) begins moving to opposite
spindle poles as the microtubules get
shorter and shorter.
• Anaphase is the shortest phase in mitosis
typically lasting only a few minutes.
5. Telophase
• Daughter chromosomes arrive
at the poles and revert to
extended fibres of chromatin.
• The spindle microtubules
disassembles and the nuclear
membrane forms around the
two groups of daughter
chromosomes.
• During this period the cell
usually undergoes cytokinesis
– an independent process –
that results in the division of
the cytoplasm.
Cytokinesis
Plant Cell
• Due to rigid cell wall, cytokinesis cannot constrict the plasma membrane inwards.
A new cell wall and plasma membrane is assembled across the cell plate.
Animal Cell
• Inward constriction of the plasma membrane results in cleavage furrow
during cytokinesis.
• The result of mitosis and cytokinesis are two new
daughter cells produced from one parent cell.
• The daughter cells contain the same (or virtually the
same) genetic information and the same number of
chromosomes as the parent cell.
Stage 5
Stage 4
Stage 1
Stage 2
Stage 3
http://www.youtube.com/watch?v=VlN7K1-9QB0
Checkpoints regulate the cell cycle
• The cell cycle is highly regulated by intracellular signalling
molecules and extracellular signalling proteins
Defective Cell Cycle Control Mechanisms
• When control mechanisms fail, uncontrolled cell
proliferation can produce a mass of cells called
a tumour. Tumours can be benign or malignant
(cancer).
• Mutations in the genes that express regulatory
proteins accumulate. This leads to genetic
instability and the development of cancer.
Animations and web links
• http://www.biology.arizona.edu/CELL_BIO/tutorials
/cell_cycle/MitosisFlash.html
• http://www.johnkyrk.com/mitosis.html
Apoptosis
• Apoptosis is programmed cell death or “cellular suicide”. It is a key
event in many biological processes. Removal of the tadpoles tail.
• The process is a specific sequence of events that result in the
ordered dismantling of the internal contents of a cell.
• A key event is the activation of a series of enzymes called caspases.
• The pathway can be triggered by
– (1) death signals or
– (2) the withdrawal of survival factors.
• Mutations in genes that express proteins involved in apoptosis can
lead to various cancers.
• NoBiology2 p.34-5
http://wehi.edu.au/education/wehitv/apoptosis_and_signal_transduction/
Activity
• Create a cell cycle poster with all the stages or
mitosis mapped out
Reflection
These are the questions you should answer each
lesson, preferably in writing
•What learning was new today?
•What learning was revision or built on what I
already know?
•What did I find most challenging and what
strategies will I put in place to help me?
•What percentage of the class did I spend on
task and how can I improve this if needed?
Chapter 9: Genes, chromosomes
and patterns of inheritance 3
EL: To introduce or revise meiotic cell
division in eukaryotic gamete cells
Introduction to meiosis
• http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapter
12/animations.html#
• The formation of gametes
(i.e.sex cells) - sperm and eggsoccurs by a special type of cell
division called meiosis.
• The nuclei of sex cells contain
only half as many chromosomes
as the nuclei of all other cells (i.e.
haploid) – called reduction
division.
• When the nuclei of the sperm
and egg join during fertilisation,
the new cell then contains the
full complement of
chromosomes.
Meiosis
Meiosis
• There are two divisions in meiosis; the first division is meiosis 1
and the second is meiosis 2.
• The phases have the same names as those of mitosis. A
number indicates the division number (1st or 2nd):
– meiosis 1: prophase 1, metaphase 1, anaphase 1, and
telophase 1
– meiosis 2: prophase 2, metaphase 2, anaphase 2, and
telophase 2
• In the first meiotic division, the number of cells is doubled but
the number of chromosomes is not. This results in 1/2 as many
chromosomes per cell.
• The second meiotic division is like mitosis; the number of
chromosomes does not get reduced.
http://www.cellsalive.com/meiosis.htm
Meiosis I
Meiosis 2
Activity
• Use the jelly snakes to model meiosis with a
partner using the information on the
upcoming slides
Interphase
Interphase: Before meiosis begins, genetic
material is duplicated. There are two
homologous pairs of each chromosome (i.e.
cell is diploid).
• Duplicated chromatin
condenses. Each
chromosome consists of
two, closely associated
sister chromatids.
• Synapsis and crossingover occur during the
latter part of this stage:
two chromosomes of a
homologous pair may
exchange segments
producing genetic
variation.
Meiosis 1:
Prophase 1
Meiosis 1: Metaphase and Anaphase 1
• Metaphase 1:
Homologous
chromosomes align at
the equatorial plate.
• Anaphase 1:
Homologous pairs
separate with sister
chromatids remaining
together.
Meiosis 1: Telophase 1
• Telophase 1: Two daughter cells are formed
with each daughter containing only one
chromosome of the homologous pair
• After Meiosis 1, there is usually a brief
interphase
Meiosis 2
• Prophase 2: Spindle forms, DNA does not
replicate.
• Metaphase 2: Chromosomes align at the
equatorial plate.
Meiosis 2:
• Anaphase 2:
Centromeres divide
and sister
chromatids migrate
separately to each
pole.
• Telophase 2: Cell
division is complete.
Four haploid
daughter cells are
obtained.
Animations
• http://highered.mcgrawhill.com/sites/0072437316/student_view0/ch
apter12/animations.html#
• http://www.cellsalive.com/meiosis.htm
Mitosis vs Meiosis
Activity
• Complete qu 1-11 of activity 9.2 on pages 9091 of your activity manual (yes, you get to play
with play doh!)
• Quick check questions 9-11 pg 306
• Make a poster of the stages of meiosis
mapped out
Test revision
SAMPLE EXAM QUESTIONS
ANSWER = B
At the end of meiosis I females have two daughter cells and
meiosis II only occurs if and when fertilization occurs by a sperm
cell.
At that time both daughter cells divide to form 4 cells and of the 4
cells formed, 3 are discarded as polar bodies and the 4th cell
having an enhanced cytoplasmic component combines its nuclear
component with the sperm cell's nuclear component and crossing
over occurs to form the embryo which then begins to divide via
mitosis to become two cells, then four and so on.
An egg cell that is not fertilized is ovulated as a pair of daughter
cells and there is no formation of polar bodies, hence, the eggs
that are ultimately discarded at menstruation are not "finished"
eggs. They have not undergone meiosis II.
ANSWER = C
ANSWER = C
ANSWER = A
Reflection
These are the questions you should answer each
lesson, preferably in writing
•What learning was new today?
•What learning was revision or built on what I
already know?
•What did I find most challenging and what
strategies will I put in place to help me?
•What percentage of the class did I spend on
task and how can I improve this if needed?
Chapter 9: Genes, chromosomes
and patterns of inheritance 4
EL: To introduce inheritance and
some of its terminology
Test yourself
Write down what you think the following
terms mean:
Gene
Locus
Allele
Trait
Heredity
Genetics
70
Genetic Terminology
 Gene - Segment of DNA that codes for formation of a
protein
 Locus – Position of gene on a chromosome
 Trait - any characteristic that can be passed from
parent to offspring
 Heredity - passing of traits from parent to offspring
 Genetics - study of heredity
71
Alleles (page 309-311)
• A gene that controls one function can exist in
different forms. These different forms are called
alleles.
• Each different allele is identified by its specific
phenotypic action.
• Alleles are commonly represented by letters of the
alphabet.
o Eg. The gene LDLR controls blood cholesterol levels. Located
on chromosome 19, it has two allelic forms:
B = abnormally high cholesterol levels
b = normal range
Autosomal Genotype
• Remembering that nonsex chromosomes occur in
homologous pairs in the
diploid cell – there are two
copies of each gene.
• The double set of genetic
instructions present makes
up the genotype.
• The number of possible
genotypes depends on the
number of allelic forms of
the gene.
Genotype terminology
Homozygous (pure)
genotype - gene
combination involving 2
dominant or 2 recessive
genes
(e.g. RR or rr)
Heterozygous (hybrid)
genotype - gene
combination of one
dominant & one recessive
allele (e.g. Rr)
Phenotype
• The visible expression of the genotype is called
the phenotype. The expression may be a
physical, biochemical or physiological trait.
– Dominant trait: require only a single copy of the
responsible allele for its phenotypic expression
– Recessive trait: refers to a trait that is not
expressed in a heterozygote
– Co-dominant trait: both alleles in the
heterozygote are expressed in the phenotype
e.g. Genotype & Phenotype in Flowers
Genotype of alleles:
R = red flower
r = yellow flower
All genes occur in pairs, so 2 alleles affect a
characteristic
Possible combinations are:
Genotypes
RR
Rr
rr
Phenotypes
RED
RED
YELLOW
76
Genes and Environment Determine
Characteristics
77
The relationship between genotype
and phenotype is rarely simple!
Phenotype = Genotype + Environmental Factors
Hydrangeas: pink or blue?
Both plants have the pigment
for colour called anthocyanin.
In acidic soils (low pH) the
flowers are blue.
In alkaline soils (high pH) the
flowers are pink).
Same genotype – different
phenotype
Allele
Combination
AA
BB
AB
or
or
Ai
Bi
Phenotype
Multiple Alleles
A
• For some genes, three or more
alleles can be present in the
population.
B
AB
• You will only inherit two alleles
(one on each chromosome)
• The combination of any two
alleles determines the final
phenotype.
• The ABO blood groupings in
humans is an example.
ii
O
• Three alleles are involved in
controlling blood group IA, IB
and i.
Monogenic Traits
• Monogenic traits are due
to the action of a single
gene with two or more
allelic forms.
• These traits show
discontinuous variation the members of the
population can be grouped
into a few discrete and
non-overlapping classes.
• E.g. blood types
Polygenic Traits
• Polygenic traits are due to the actions of many genes (and
their allelic forms). These traits show continuous variation
(e.g. height).
Human Sex Chromosomes
• Traits (genes) located on the sex chromosomes
– XX genotype for females
– XY genotype for males
• The X chromosome may carry up to 1500 genes. Genes
located on the X chromosome are said to be X-linked.
• Females have two alleles of a particular gene
whereas males have only one (hemizygous
genotype). This accounts for why many X-linked
diseases show up more frequently in males than in
females.
• The Y chromosome has less than 300 genes. Genes
located on the Y chromosome are said to be Y-linked.
Males are also hemizygous for Y-linked genes.
X Inactivation in Female Mammals
• Females who have two X
chromosomes but one is
subject to inactivation.
• 75% of alleles on one X
chromosome are switched
off in early embryonic
development.
• 15% remain activated with
another 10% altering their
activation state in different
females and in different cells
within the same female.
Activity
• In groups of 2, complete activity 9.3
• Complete Biochallenge 3 (pg 334) and Chapter
review qu 4, 6, 7 (pg 335&336)
84
Reflection
These are the questions you should answer each
lesson, preferably in writing
•What learning was new today?
•What learning was revision or built on what I
already know?
•What did I find most challenging and what
strategies will I put in place to help me?
•What percentage of the class did I spend on
task and how can I improve this if needed?
Chapter 9: Genes, chromosomes
and patterns of inheritance 5
EL: To learn how to use punnett
squares to determine inheritance,
with a focus on monhybrid crosses
Mendel’s Pea Plant
Experiments
87
Gregor Johann Mendel
• Austrian monk
• Studied the inheritance of traits
in pea plants
• Developed the laws of
inheritance
• Mendel's work was not
recognized until the turn of the
20th century
•
Between 1856 and 1863, Mendel cultivated and tested some 28,000
pea plants
•
He found that the plants' offspring retained traits of the parents
•
Called the “Father of Genetics"
88
Site of Gregor
Mendel’s
experimental
garden in the
Czech Republic
89
Particulate Inheritance
• Mendel stated that
physical traits are
inherited as “particles”
• Mendel did not know
that the “particles” were
actually chromosomes &
DNA
90
Reproduction in Flowering Plants
•Pollen contains sperm
–Produced by the stamen
•Ovary contains eggs
–Found inside the flower
Pollen carries sperm to the eggs
for fertilization
Self-fertilization can occur in
the same flower
Cross-fertilization can occur
between flowers
91
How Mendel Began
Mendel
produced pure
strains by
allowing the
plants to selfpollinate for
several
generations
92
93
94
Mendel’s Experimental Results
95
Did the observed ratio match the
theoretical ratio?
• The theoretical or expected ratio of plants
producing round or wrinkled seeds is 3 round :1
wrinkled
• Mendel’s observed ratio was 2.96:1
• The discrepancy is due to statistical error
• The larger the sample the more the results
approximate to the theoretical ratio
96
Generation “Gap”
• Parental P1 Generation = the parental generation
in a breeding experiment.
• F1 generation = the first-generation offspring in a
breeding experiment. (1st filial generation)
– From breeding individuals from the P1
generation
• F2 generation = the second-generation offspring in
a breeding experiment.
(2nd filial generation)
– From breeding individuals from the F1
generation
97
Following the Generations
Cross 2
Pure
Plants
TT x tt
Results
in all
Hybrids
Tt
Cross 2 Hybrids
get
3 Tall & 1 Short
TT, Tt, tt
98
Monohybrid Cross
• A trait determined by one gene with two or
more allelic forms.
Punnett Square
Used to help solve
genetic problems
100
101
P1 Monohybrid Cross
• Trait: Seed Shape
• Alleles: R – Round
r – Wrinkled
• Cross: Round seeds x Wrinkled seeds
RR
x
rr
r
r
R
Rr
Rr
R
Rr
Rr
Genotype: Rr
Phenotype: Round
Genotypic
Ratio: All alike
Phenotypic
Ratio: All alike
102
P1 Monohybrid Cross Review
 Homozygous dominant x Homozygous
recessive
 Offspring all Heterozygous (hybrids)
 Offspring called F1 generation
 Genotypic & Phenotypic ratio is ALL ALIKE
103
F1 Monohybrid Cross
• Trait: Seed Shape
• Alleles: R – Round
r – Wrinkled
• Cross: Round seeds x Round seeds
Rr
x
Rr
R
r
R
RR
Rr
r
Rr
rr
Genotype: RR, Rr, rr
G.Ratio: 1:2:1
Phenotype: 3 Round &
1 wrinkled
P.Ratio: 3:1
104
F1 Monohybrid Cross Review
 Heterozygous x heterozygous
 Offspring:
25% Homozygous dominant RR
50% Heterozygous Rr
25% Homozygous Recessive rr
 Offspring called F2 generation
 Genotypic ratio is 1:2:1
 Phenotypic Ratio is 3:1
105
…And Now the Test Cross
• Mendel then crossed a pure & a hybrid from his F2
generation
• This is known as an F2 or test cross
106
F2 Monohybrid Cross (1st)
• Trait: Seed Shape
• Alleles: R – Round
r – Wrinkled
• Cross: Round seeds x Round seeds
•
RR
x
Rr
R
r
R
RR
Rr
R
RR
Rr
Genotype: RR, Rr
Phenotype: Round
Genotypic
Ratio: 1:1
Phenotypic
Ratio: All alike
107
F2 Monohybrid Cross (2nd)
• Trait: Seed Shape
• Alleles: R – Round
r – Wrinkled
• Cross: Wrinkled seeds x Round seeds
•
rr
x
Rr
R
r
r
Rr
Rr
r
rr
rr
Genotype: Rr, rr
Phenotype: Round &
Wrinkled
G. Ratio: 1:1
P.Ratio: 1:1
108
F2 Monohybrid Cross Review
 Homozygous recessive x
heterozygous(hybrid)
 Offspring:
50% Homozygous rr
50% Heterozygous Rr
 Phenotypic Ratio is 1:1
109
Test crosses
• Can be used to determine if an individual of dominant
phenotype is homozygous or heterozygous
• There are two possible testcrosses:
Homozygous dominant x Homozygous recessive
= All heterozygous dominant
Hybrid x Homozygous recessive
= Mix of dominant and recessive phenotypes
Monohybrid cross Practice
Problems
111
1. Breed the P1 generation
• tall (TT) x dwarf (tt) pea plants
t
t
T
T
112
2. Breed the F1 generation
• tall (Tt) vs. tall (Tt) pea plants
T
t
T
t
113
1. Solution:
tall (TT) vs. dwarf (tt) pea plants
t
t
T
Tt
Tt
produces the
F1 generation
T
Tt
Tt
All Tt = tall
(heterozygous tall)
114
2. Solution:
tall (Tt) x tall (Tt) pea plants
T
t
T
TT
Tt
t
Tt
tt
produces the
F2 generation
1/4 (25%) = TT
1/2 (50%) = Tt
1/4 (25%) = tt
1:2:1 genotype
3:1 phenotype
115
Results of Monohybrid Crosses
• Inheritable factors or genes are responsible for
all heritable characteristics
• Phenotype is based on genotype
• Each trait is based on two genes, one from the
mother and the other from the father
• True-breeding individuals are homozygous
(both alleles) are the same
116
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)
117
Law of Dominance
118
Law of Segregation
• 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.
119
Applying the Law of Segregation
120
Activity
• Activity 9.4 page 95-98 (NOB AM)
Reflection
These are the questions you should answer each
lesson, preferably in writing
•What learning was new today?
•What learning was revision or built on what I
already know?
•What did I find most challenging and what
strategies will I put in place to help me?
•What percentage of the class did I spend on
task and how can I improve this if needed?
Chapter 9: Genes, chromosomes
and patterns of inheritance 6
EL: To explore dihibrid crosses
Law of Independent Assortment
• Alleles for different traits are distributed
to sex cells (& offspring) independently of
one another.
• This law can be illustrated using dihybrid
crosses.
124
Dihybrid Cross
• A breeding experiment that tracks the inheritance
of two traits.
• Mendel’s “Law of Independent Assortment”
– a. Each pair of alleles segregates independently
during gamete formation
– b. Formula: 2n (n = # of heterozygotes)
125
Question:
How many gametes will be produced for the
following allele arrangements?
• Remember: 2n (n = # of heterozygotes)
•
1.
RrYy
•
2.
AaBbCCDd
•
3.
MmNnOoPPQQRrssTtQq
126
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
127
Dihybrid Cross
• Traits: Seed shape & Seed color
• Alleles: R round
r wrinkled
Y yellow
y green
•
RrYy
RY Ry rY ry
x
RrYy
RY Ry rY ry
All possible gamete combinations
128
Dihybrid Cross
RY
Ry
rY
ry
RY
Ry
rY
ry
Try filling in the punnet square and work out ratios
129
Dihybrid Cross
RY
RY RRYY
Ry RRYy
rY RrYY
ry
RrYy
Ry
rY
ry
RRYy
RrYY
RrYy
RRyy
RrYy
Rryy
RrYy
rrYY
rrYy
Rryy
rrYy
rryy
Round/Yellow:
Round/green:
9
3
wrinkled/Yellow: 3
wrinkled/green:
1
9:3:3:1 phenotypic
ratio
130
Dihybrid Cross
Round/Yellow: 9
Round/green:
3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1
131
Test Cross
• A mating between an individual of unknown
genotype and a homozygous recessive individual.
• Example: bbC__ x bbcc
•
•
•
BB = brown eyes
Bb = brown eyes
bb = blue eyes
•
•
•
CC = curly hair
Cc = curly hair
cc = straight hair
bC
b___
bc
132
Test Cross
• Possible results:
bc
bC
b___
C
bbCc
bbCc
or
bc
bC
b___
c
bbCc
bbcc
133
Summary of Mendel’s laws
LAW
DOMINANCE
SEGREGATION
INDEPENDENT
ASSORTMENT
PARENT
CROSS
OFFSPRING
TT x tt
tall x short
100% Tt
tall
Tt x Tt
tall x tall
75% tall
25% short
RrGg x RrGg
round & green
x
round & green
9/16
3/16
3/16
1/16
round seeds & green pods
round seeds & yellow pods
wrinkled seeds & green pods
wrinkled seeds & yellow pods
134
Activity
• In pairs, complete activity 9.5 pg 99- 100 (NOB
AM)
• Independently, complete chapter review qu 6,
8 (pg 336&337)
135
Reflection
These are the questions you should answer each
lesson, preferably in writing
•What learning was new today?
•What learning was revision or built on what I
already know?
•What did I find most challenging and what
strategies will I put in place to help me?
•What percentage of the class did I spend on
task and how can I improve this if needed?
Chapter 9: Genes, chromosomes
and patterns of inheritance 7
EL: To learn about dominance
relationships, sex-linked characteristics
and how to undergo pedigree analysis
Incomplete Dominance
• F1 hybrids have an appearance somewhat in
between the phenotypes of the two parental
varieties.
• Example: snapdragons (flower)
red (RR) x white (rr)
r
r
R
• RR = red flower
• rr = white flower
R
138
Incomplete Dominance
r
r
R
Rr
Rr
R
Rr
Rr
produces the
F1 generation
All Rr = pink
(heterozygous pink)
139
Incomplete Dominance
140
Codominance
• Two alleles are expressed (multiple alleles) in
heterozygous individuals.
• Example: blood type
•
•
•
•
1.
2.
3.
4.
type A
type B
type AB
type O
=
=
=
=
IAIA or IAi
IBIB or IBi
IAIB
ii
141
Codominance Problem
• Example:
homozygous male Type B (IBIB)
x
heterozygous female Type A (IAi)
IA
i
IB
IAIB
IBi
IB
IAIB
IBi
1/2 = IAIB
1/2 = IBi
142
Another Codominance Problem
• Example: male Type O (ii)
x
AB (IAIB) female type
IA
IB
i
IAi
IBi
i
IAi
IBi
1/2 = IAi
1/2 = IBi
143
Codominance
• Question:
If a boy has a blood type O and his sister
has blood type AB, what are the
genotypes and phenotypes of their
parents?
• boy - type O (ii) X girl - type AB (IAIB)
144
Codominance
Answer:
IA
IB
i
i
IAIB
ii
Parents:
genotypes = IAi and IBi
phenotypes = A and B
145
Pedigree Charts
Autosomal Dominant Pattern
• Both males and females are
affected.
• All affected individuals have at
least one affected parent.
• Once the trait disappears from
a branch of the pedigree, it
does not reappear.
• In large samples equal
numbers of males and females
affected.
• A heterozygote will show the
trait.
Autosomal Recessive Pattern
• Both males and females
may be affected.
• Two unaffected parents
can have an affected
child.
• The trait may not be
present in all generations.
• The trait is only expressed
in the homozygous state.
X-linked Dominant Pattern
• The male will pass on
the trait to all of his
daughters, but not sons.
• A female will pass the
trait to both her
daughters and sons.
• Every effected person
has at least one parent
with the trait.
X-linked Recessive Pattern
• All the sons of a female
with the trait will be
affected.
• All the daughters of an
affected male will be
carriers of the trait.
• In a large sample more
males that females
show the trait.
Sex-linked Traits
Example: Eye color in fruit flies
Sex Chromosomes
fruit fly
eye color
XX chromosome - female
Xy chromosome - male
151
Sex-linked Trait Problem
• Example: Eye color in fruit flies
(red-eyed male) x (white-eyed female)
XrY
x
XrXr
• Remember: the Y chromosome in males does not
carry traits.
Xr
Xr
•
•
•
•
•
RR = red eyed
Rr = red eyed
rr = white eyed
Xy = male
XX = female
Xr
Y
152
Sex-linked Trait Solution:
Xr
XR
XR
Xr
Y
Xr Y
Xr
XR
Xr
Xr Y
50% red eyed
female
50% white eyed
male
153
Female Carriers
154
The Dihybrid Cross: Two possibilities
• If genes are on
different
chromosomes they
act independently of
each other
• E.g. YY RR
• If genes are on the
same chromosome
they are linked
• E.g AB/ab
155
Linkage of gene loci
• Linked genes = genes which are on the same
chromosome
– E.g. RH gene that controls rhesus blood type
(represented by D & d) and EL1 gene that determines
shape of RBCs (represented by E & e)
• Linked genes are written differently to non-linked
genes
– DE/de
156
Linked genes and crossing over
• If genes are close together on chromosome,
there is LESS chance of them being separated
during crossing over
• If crossing over DOESN’T occur, the gametes
produced are said to be parental OR non
crossover gametes
157
No crossing over between linked genes
If no crossing over occurs, the parental combinations of alleles will be preserved.
Metaphase 1
Synapsis
Linked genes and crossing over
• If genes are far apart on a chromosome, there
is MORE chance of them being separated
during crossing over
• If crossing over DOES occur, the gametes
produced are said to be recombinant OR
crossover gametes
159
Crossing over between linked genes
During synapsis, crossing over may occur between the paternal and maternal
chromatids that give rise to recombinations of alleles that are different from
the parental combinations.
Synapsis
Detecting Linkage
• To detect linkage, perform a test cross
between a double heterozygote (i.e.DdEe) and
a double homozygous recessive (i.e. ddee)
– DdEe x ddee
GAMETES
DE
de
De
dE
de
DdEe
ddee
Ddee
ddEe
• If the two genes are NOT linked, the genes will
assort independently and the outcome of the
test cross will be a genotypic ratio of 1:1:1:1
161
Detecting Linkage
• If two genes are linked, we get a different ratio
of offspring
Gametes
DE
de
dE
De
de
DE
de
de
de
dE
de
De
de
There will be four classes of offspring but the proportions of these will
not be equal. Instead, there will be an excess of offspring from parental
gametes and a deficiency of offspring from recombinant gametes.
Genotypic ratio:
few : few : 1 : 1
Activity
• Complete activity 9.6 (pg 100 – 104 NOB AM)
and submit
• Complete biochallenge 2 (pg 334) and chapter
review qu 9, 12, 13, 14 (page 337)
163
Reflection
These are the questions you should answer each
lesson, preferably in writing
•What learning was new today?
•What learning was revision or built on what I
already know?
•What did I find most challenging and what
strategies will I put in place to help me?
•What percentage of the class did I spend on
task and how can I improve this if needed?