3.4 slides inheritance

Inheritance (3.4)
IB Diploma Biology
Essential Idea: Genes are inherited
following different patterns

3.4.1 Mendel discovered the principles of inheritance with experiments in
which large numbers of pea plants were crossed.
Offspring inherit many traits from their parents,
but the specific details of inheritance eluded
scientists for centuries:
• Early theories assumed a simple blending of
traits of the two parents…
• Aristotle noticed that offspring sometimes
looked more like one parent than the other –
sometimes even more like a grandparent
• In 1866, Gregor Mendel published his pea plant
cross-breeding experiments that showed traits
being inherited in specific patterns
• His work was largely ignored until the early
1900s when it was rediscovered, replicated, and
became the foundation for modern genetics

Hybrid Parent Plant Cross Offspring Phenotypes Ratio
Tall stem x Dwarf stem 787 Tall : 277 Dwarf 2.84 : 1
Round seed x Wrinkled seed 5474 Round : 1850 Wrinkled 2.96 : 1
Yellow peas x Green peas 6022 Yellow : 2001 Green 3.01 : 1
Purple flowers x White flowers 705 Purple : 224 White 3.15 : 1
Mendel noticed that certain versions of a trait,
such as tall height, round seeds, yellow color,
and purple flowers would always show-up in a
cross with a purebred plant – He called these
DOMINANT versions of the gene, or ALLELE
Other versions of the trait only showed up in
hybrid crosses or when the ‘dominant trait’ was
not present in either parent – He called these
the RECESSIVE alleles http://ed.ted.com/lessons/how-mendel-s-pea-plants-
helped-us-understand-genetics-hortensia-jimenez-diaz

3.4.2 Gametes are haploid so contain one allele of each gene / 3.4.4 Fusion
of gametes results in diploid zygotes with two alleles of each gene that may
be the same allele or different alleles
Gametes are haploid and contain one copy of
each chromosome – and therefore one allele of
each gene:
When the male and female gametes fuse in fertilization,
the resultant diploid cell – called the Zygote – will have
two alleles of each gene, one from each parent
Many genes have two alleles, as Mendel observed – often
one is dominant, one recessive, producing three possible
genotypes:
• AA = Homozygous Dominant (dominant phenotype)
• Aa = Heterozygous (dominant phenotype)
• aa= Homozygous Recessive (recessive phenotype)

3.4.3 The two alleles of each gene separate into different haploid daughter
nuclei during meiosis
Since Meiosis involves a
reduction division, each haploid
gamete gets only one of the two
alleles that a parent has for each
gene. Which allele each sex cell
receives is random (i.e. if the
father is heterozygous for a gene,
half of his sperm will contain the
dominant allele and the other half
will have the recessive)

3.4.5 Dominant alleles mask the effects of recessive alleles but co-dominant
alleles have joint effects
Dominant alleles always show
their encoded trait, when
present in an organism (they
mask recessive alleles)
Recessive alleles only express
their encoded traits when no
other alleles are present
• Dominant alleles code for
functional proteins, while
recessive alleles code for non-
functional proteins
Codominant alleles can have
joint effects if both are
present* *Patterns of inheritance, like Codominance, that do not follow Mendel’s
observations are called Non-Mendelian inheritance patterns.

3.4.15 Construct Punnett grids (squares) for predicting the outcomes of
monohybrid genetic crosses.

A Mendelian monohybrid cross:
F1 Parent Genotypes: Tt x Tt
F1 Parent Phenotypes: Tall x Tall
Offspring Genotype Ratio:
1 : 2 : 1 (TT : Tt : tt)
Offspring Phenotype Ratio:
3 : 1 (Tall : Dwarf)*
*Just as Mendel observed!

A Non-Mendelian monohybrid cross:
F1 Parent Genotypes: CRCW x CRCW
F1 Parent Phenotypes: Pink x Pink
Offspring Genotype Ratio:
1 : 2 : 1 (CRCR : CRCW : CWCW)
Offspring Phenotype Ratio:
1 : 2 : 1 (Red : Pink : White)
R

3.4.11 Inheritance of ABO blood groups
When surgeons started performing human
blood transfusions in the mid-1800s, they
were a very risky procedure. Doctors noticed
that sometimes the blood transfusion was
successful, while other times the transfused
blood clumped up (clotted) inside the patient
and killed them.
It was not until the early 1900s that scientists
discovered humans have different
glycoproteins on their red blood cells that
give them different blood types. Since the
immune system only recognizes certain
blood types, the immune cells of patients
who got mismatched blood would attack and
destroy the ‘foreign’ cells, causing clots.

3.4.11 Inheritance of ABO blood groups
In humans, ABO blood
group is determined
by a single gene on
Chromosome 9. ABO
blood type is an
example of ‘Multiple
Alleles’ and
Codominance:
The gene has three alleles:
IA: glycoprotein with A antigen (codominant)
IB : glycoprotein with B antigen (codominant)
i : normal glycoprotein (recessive)

3.4.16 Comparison of predicted and actual outcomes of genetic crosses using
real data
Mendel’s experiments and Reginald
Punnett’s models allow for the prediction
of genetic outcomes in offspring.*
However, predictions do not always match
actual outcomes. For example, a coin
tossed 100 times does not always produce
50 heads and 50 tails since each flip is a
random event
With this in mind, scientists can collect
large amounts of data to reduce the
impact of such random fluctuations – The
Law of Large Numbers (i.e. 10,000 coin
flips will show a closer to 50-50 split than
10 flips)
Scientists can also use statistical tests like
the T-test and the Chi-Squared test to
determine if results are significant, or due
to random chance!
*Mendel’s actual data were so close to
perfect 3:1 predicted ratios that many
esteemed scientists and mathematicians
have proposed that he might have
manipulated is results

3.4.6 Many genetic diseases in humans are due to recessive alleles of
autosomal genes
NOTE: An autosomal gene is a gene whose loci is
on an autosome, not a sex chromosome
A genetic disease is a disorder caused by a
gene, rather than microbes. In most cases, a
mutated allele causes a protein to be altered
which impairs normal cell / body function –
we have already seen an example of this in
Sickle Cell Anemia
Most disease-causing alleles are recessive –
meaning an individual must inherit both
copies of the disease allele to actually have
the disorder
Individuals can be carriers for these genetic
disorders, meaning they ‘carry’ one copy of
the recessive disease allele and one dominant
allele that gives them a normal phenotype.

3.4.6 Many genetic diseases in humans are due to recessive alleles of
autosomal genes

3.4.7 Some genetic diseases are sex-linked and some are due to dominant or
co-dominant alleles
• Huntington’s disease is caused by an
autosomal dominant allele:
• Inheriting just one mutated allele will
cause this deadly neurological condition
• Sickle Cell Anemia is caused by autosomal
codominant alleles:
• HbA HbA = Normal phenotype
• HbA HbS = Mild anemia phenotype with
malaria resistance
• HbS HbS = Full sickle cell phenotype
• Color-blindness is caused by a recessive
allele on the X sex chromosome (so the gene
is said to be ‘sex linked’):
• Since males only get one copy, they are more
likely to express the colorblind phenotype (a
female would need to inherit two colorblind
alleles to show this trait)

3.4.7 Some genetic diseases are sex-linked and some are due to dominant or
co-dominant alleles

3.4.13 Inheritance of cystic fibrosis and Huntington’s disease
Cystic Fibrosis
An autosomal recessive genetic disease
caused by a mutation of the CFTR gene
on Chromosome 7
The CFTR gene encodes the production
of a chloride membrane channel protein
involved in secretion of sweat, mucus,
and digestive juices
The mutated, disease allele produces a
non-functional membrane protein that
causes sticky mucus builds up in the
lungs, causing infection

3.4.13 Inheritance of cystic fibrosis and Huntington’s disease
Huntington’s Disease
An autosomal dominant genetic disease
caused by a mutation of the HTT gene on
Chromosome 4
The function of the normal huntingtin protein
is still being studied, but the dominant allele
causes progressive neurodegeneration
beginning between ages 30 and 50.
Life expectancy is ~20 years after the onset of
symptoms.
Due to the late-onset nature, most parents
have already had children before they become
symptomatic…

3.4.8 The pattern of inheritance is different with sex-linked genes due to
their location on sex chromosomes.

3.4.12 Red-green color-blindness and hemophilia as examples of sex-linked
inheritance

3.4.17 Analysis of pedigree charts to deduce the pattern of inheritance of
genetic diseases
The basics of Pedigrees:
• Sex of Individual:
• Male = Square
• Female = Circle
• Presence of Trait:
• Shading = Affected
• Unshaded = Unaffected
• Half-shade = Carrier
• Rows represent generations
• Autosomal Dominant or Recessive?
• Recessive
• Genotype of Max? (AA, Aa, or aa)
• aa
• Genotype of Ryan? (AA, Aa, or aa)
• Aa

genetic diseases
Sex-Linked, Recessive:
• Trait is able to skip generations
• Males are predominantly affected
Autosomal, Recessive:
• Trait is able to skip generations
• No major sex-bias in expression
Autosomal, Dominant:
• Trait cannot skip generations
• No major sex-bias in expression

genetic diseases

genetic diseases
Medical researchers have identified more than
4,000 human genetic diseases so far.
Examples to know include Cystic fibrosis,
Huntington’s disease, Sickle cell anemia, and
Hemophilia
Yet most people are unaffected by genetic
diseases because most are caused by very rare,
recessive alleles
Today, cheap and fast genetic tests can allow
parents to determine if they carry genetic diseases
before they make decisions about having children

3.4.10 Radiation and mutagenic chemicals increase the mutation rate and
can cause genetic disease and cancer.
Alleles for a gene differ by just a few nitrogen
base letters (SNPs). New alleles are formed by
mutation – a random change in the base
sequence of a gene
High-energy radiation, including Gamma rays, UV
light, and X-rays, can increase mutation rate
Some chemicals – called mutagens – can also
increase mutation rate, such as mustard gas in
WWI and chemicals in tobacco smoke
Mutations in oncogenes (genes that regulate the
cell cycle) can lead to cancer
Mutations in genes of gametes can be passed on
to children, possibly causing a genetic disease

3.4.14 Consequence of radiation after nuclear bombing of Hiroshima and
Nagasaki and the nuclear accident at Chernobyl.
The Bombing of Hiroshima & Nagasaki
Massive release of high-energy,
radioactive isotopes into the
environment
150,000 – 250,000 people died
directly or within a few months of
the bombing of the two Japanese
cities in 1945
Long-term survivors showed
slightly higher rates of tumor
formation but no statistically-
significant increase in fetal
mutations

The Chernobyl Nuclear Accident
Massive release of high-energy,
radioactive isotopes into the
environment – 400 times more
than Hiroshima!
1986 – Nuclear reaction explodes
in Chernobyl, Ukraine
All plant workers quickly killed by
fatal does of radiation and clouds
of radioactive isotopes were
spread across Europe

Effects of nuclear fallout:
• Miles of forest land killed,
including most animals and plants
• Lynx, owls, wild boar, and other
select species began to thrive in
the ‘exclusion zone’
• Bioaccumulation caused high
levels of radioactive isotopes in
fish as far away as Germany and
Scandinavia
• More than 6,000 cases of thyroid
cancer attributed to accident

Scientists estimate that the ‘Exclusion Zone’
– 19 square miles of land around the
former power plant – will not be safe for
human habitation again for 20,000 years…

Bibliography / Acknowledgments
Bob Smullen

3.4 slides inheritance

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3.4 slides inheritance