MODULE 5: HEREDITY
1: Reproduction - How does reproduction ensure the continuity of a
species?
 Mechanisms of reproduction that ensure the continuity of a species
o Animals (advantages of external and internal fertilisation)
Performed by
Gametes produced
Fertilisation chance
Frequency
Environment
Advantages
Internal
Terrestrial plants, birds,
mammals, reptiles
Many male, few female
High
More frequent
Water body, vulnerable to
external conditions
Higher fertilisation chance,
offspring protected
External
Marine organisms (fish,
sponges, amphibians)
Millions
Low
Less frequent
Female body/eggs, safe
until birth
Many offspring produced,
rapid colonisation, simpler
and faster
o Plants (asexual and sexual reproduction)
How it is performed
Cell division
Offspring
Parents needed
Advantages
Disadvantages
Asexual
Runners (outgrowth stem ->
new plant), budding, spores,
etc.
Sexual
Pollen lands on stigma, goes
through pollen tube to
ovary, fertilises ova. Can be
self-pollinated or crosspollinated.
Mitosis
Genetically identical to each
other and parent
One
Quickly produces many
offspring
Less variation—more risk
under environmental
change
Meiosis
Combination of info from
parents
Two
More variation
Complex, takes more time
and energy
o Fungi (budding, spores): Fungi can release spores, single cells
produced by mitosis that can grow into a new fungus. They can also
produce a replica of itself (via mitosis) that grows as a new organism,
attached to the parent.
o Bacteria (binary fission): This is when bacteria undergoes mitosis and
splits into two new organisms.
o Protists (binary fission, budding): See above. Protists are usually
algae/fungi.
 Features of fertilisation, implantation and hormonal control of
pregnancy and birth in mammals
o Fertilisation: Meiosis is undergone in the gonads, producing gametes
(female: ovum, male: sperm). The male deposits sperm into the female
reproductive system, where it travels into the oviduct, where
fertilisation occurs. When that happens, the two gametes fuse to have
46 chromosomes. It becomes a zygote, which grows by mitosis as it
travels to the uterus.
o Implantation: The zygote travels into the uterus and implants into its
wall. The blastocyst (zygote’s outer layers) grow into the endometrium
(uterus lining), which handles nutrition and waste for the first 2-4 weeks
until the placenta forms.
o Hormonal control of pregnancy and birth:
 Luteinising hormone: Triggers ovulation and development of
corpus luteum.
 Oestrogen: Main female sex hormone, triggers ovulation (release
of egg from ovary). Maintains and stimulates production of
other hormones, e.g. oxytocin.
 Human chorionic gonadotrophin: Released during implantation
-> keep the corpus luteum active -> ensure adequate
progesterone -> maintain endometrium.
 Progesterone: Released by corpus luteum during
implantation/first trimester to maintain endometrium, then by
the placenta to prevent contractions and miscarriage.
 Oxytocin: Released towards end of third trimester to stimulate
contractions, then milk production.
 Prolactin: Released during third trimester to stimulate lactation.
o Other pregnancy terms: Gestation (development of offspring in uterus),
oocyte (ovary cell, becomes ovum through meiosis), trimester (3
months).
 Impact of scientific knowledge on the manipulation of plant and
animal reproduction in agriculture
Reproductive Definition/example
technology
Artificial
Inserting semen from a
insemination selected male into a selected
female of same species.
E.g. used in cattle, sheep,
race horses.
Artificial
pollination
Dusting of pollen onto
female stigmas.
E.g. Mendel’s pea plants,
modern agriculture.
Wholeorganism
cloning
Producing a genetically
identical organism without
sexual reproduction.
E.g. plants: cuttings, graftings
E.g. animals: Dolly the sheep,
somatic cell nuclear transfer
(put body cell nucleus in
enuculeated egg cell, implant
into surrogate mother).
Advantage
Disadvantage
Can inseminate
many females
from one male,
transport is easier
and more cost
effective, can be
used in
conservation.
Simple and
inexpensive,
consistency in
growth
rates/food
quality, can
create new
varieties and
increase genetic
diversity (shortterm)
Organisms with
desired
characteristics
produced,
predictable
growth/yield,
conservation.
Reduces genetic
variation ->
susceptibility to
environmental
change
Decreases genetic
variation,
monocultures
increase risk of
disease/pests
Expensive with
limited advantages,
raises ethical issues,
poses health issues
to animals, reduces
genetic diversity
(identical plants are
all susceptible to the
same things).
2: Cell Replication - How important is it for genetic material to be
replicated exactly?
 Processes involved in cell replication
o Mitosis and Meiosis
Mitosis
Meiosis
1 division, produces two daughter cells
Daughter cells are identical to parent and
each other
Number of chromosomes maintained
Occurs in asexual reproduction and in
growth and repair
2 divisions, produces 4 gametes (sex cells)
Gametes are all different to each other
Number of chromosomes halved
Occurs to make gametes for sexual
reproduction
Meiosis 1: Interphase (diploid, chromatids are joined at centrometre) -> P1
(chromosomes condense, homologous chromosomes form bivalents, crossing over
occurs and genetic material is exchanged) -> M1 (spindle fibres align bivalents at
centre) -> A1 (bivalents are split, chromosomes move to opposite poles) -> T1 (cell
divides into two haploid daughter cells, nuclear membrane reforms)
Meiosis 2: P2 (chromosomes condense, nuclear membrane dissolves, centrosomes
move to opposite poles) -> M2 (spindle fibres align chromosomes) -> A2 (sister
chromatids separate and move to opposite poles) -> T2 (chromosomes decondense,
nuclear membrane reforms, cells divide into four haploid daughter cells)
Mitosis: Interphase (chromatids joined at centrometre to form chromosome) ->
Prophase (chromosomes condense, nuclear membrane dissolves) -> Metaphase
(bivalents lined up at centre by spindle fibres) -> Anaphase (chromosomes pulled
apart and become chromatids) -> Telophase (nuclear membrane reforms) ->
Cytokinesis (cytoplasm divides, two diploid daughter cells are made)
o DNA replication using the Watson and Crick DNA model (incl.
nucleotide composition, pairing and bonding)
 Nucleotide composition: DNA stands for deoxyribonucleic acid.
It is a double helix, a twisted ladder. DNA is made up of
nucleotides, each containing a sugar, a phosphate and a base.
The sugar and phosphate make up the backbone and the base
pairs the rungs.
 Pairing: DNA is made up of four nitrogen bases, held together
by hydrogen bonds. Adenine and Cytosine always pair, and
Guanine and Thymine always pair.
 DNA replication: Both strands of a DNA molecule are mirror
images of each other. The molecule is untwisted and ‘unzipped’
by enzymes, breaking the hydrogen bonds and forming a
leading and a lagging strand. Enzymes connect nucleotides with
complementary bases on the original strands to form two
identical DNA strands.
The leading strand is synthesised continuously (no breaks) in the
direction of the replication fork (where the strands separate). The
lagging strand is synthesised discontinuously (in fragments) in
the opposite direction by Okazaki fragments.
DNA replication occurs during interphase of meiosis and mitosis.
 Effect of the cell replication process on the continuity of a species
o DNA replication allows cells to be replicated through meiosis and
mitosis, which are essential processes in reproduction, as mitosis allows
simpler organisms such as bacteria to multiply through asexual
reproduction, while meiosis creates gametes which allow sexual
reproduction. So cell replication is essential to species continuity by
allowing asexual and sexual reproduction.
3: DNA and Polypeptide Synthesis - Why is polypeptide synthesis
important?
 Compare the forms in which DNA exists in eukaryotes and
prokaryotes
Prokaryotic
Found freely in cytoplasm
Naked (doesn’t bond w/ protein)
Compact genomes (little repetitive
DNA)
Contains plasmids (DNA molecule
separate of chromosomal DNA)
Circular
Eukaryotic
Found in nucleus
Bound to histone proteins for
strength/stability
Genomes have lots of non-coding +
repetitive DNA
No plasmids
Linear
 Model the process of polypeptide synthesis
o Transcription and translation
 Transcription: After DNA unzips, a gene’s DNA sequence is
copied (transcribed) to make an RNA molecule. The enzyme
RNA polymerase builds an mRNA molecule by pairing
nucleotides with complementary bases on the non-coding
strand (template strand)

The info is transcribed in codons/triplets (groups of three bases).
The mRNA strand is the same as the coding strand except uracil
replaces thymine.
The molecule begins to move away and transcription ends when
the enzyme reaches a stop codon. The introns (segment of a
gene that doesn’t code for proteins) are removed, leaving the
exons (codes for proteins/polypeptides). The mRNA then moves
out of the nucleus and onto a ribosome.
Translation: The ribosomes move along the mRNA, attaching
tRNA molecules by temporarily pairing anticodons (correspond
to codons) with corresponding codons, while another enzyme
makes peptide bonds between the amino acids (1 codon = 1
amino acid). The tRNA breaks off, leaving the chain of amino
acids—a polypeptide. The polypeptide may be joined by others,
then it is folded into its shape to form a protein.
o Importance of mRNA and tRNA in transcription and translation
mRNA is important to convey genetic information from DNA, serving as
a messenger and specifying the amino acid sequence of the DNA.
tRNA is important as it decodes an mRNA sequence into a polypeptide
chain and then a protein.
o Function and importance of polypeptide synthesis
Polypeptide synthesis forms polypeptides that fold to form proteins.
Proteins have many essential roles within our cells. See ‘structure and
function of proteins in living things).
o Assess how genes and environment affect phenotypic expression
Characteristics are determined by genes (by directly coding for certain
characteristics and features, e.g. eye colour) and the environment,
depending on its various features that affect the organism, such as
food and water availability. E.g. two plants growing in environments
with different quantities of sunlight, moisture, and nutrients will result
in different growth rates and yield.
Examples: The Himalayan Rabbit (low temperatures->black fur, high
temperatures->brown fur), the water buttercup (leaves above the water
are broad and lobed, leaves under the water are thin and finely divides)
 Structure and function of proteins in living things
o Primary structure: Chain of amino acids
o Secondary structure: Folded polypeptide -> forms structural proteins
(bone, muscle, etc.)
o Tertiary structure: Complex, 3D shape -> forms hormones (chemical
messengers) and enzymes (catalyse chemical reactions)
o Quaternary structure: Made of 2+ polypeptides -> form haemoglobin
(carries oxygen)
4: Genetic Variation - How can the genetic similarities and differences
within and between species compared?
 Conduct practical investigations to predict variations in the
genotype by modelling meiosis, incl. the crossing over of
homologous chromosomes, fertilisation and mutations
o Homologous chromosomes/alleles: Chromosomes come in pairs (one
from father, one from mother) called homologous pairs, containing
equivalent sets of genes, allowing different alleles (alternate forms of a
characteristic) to exist. One allele is often recessive while one is
dominant, and the dominant one is usually expressed over the
recessive one. (e.g. in Tt, T will be expressed)
Mendel’s pea plant experiments produced this model of inheritance.
o Fertilisation: When sex cells (haploid number: 23 chromosomes each)
fuse, they create a zygote (diploid number: 46 chromosomes).
Homologous chromosomes line up and separate at random during
meiosis, meaning that there are many possible gametes. Each gamete
gets one of the four chromatids shown below. Each gamete also
represents an allele.
o Crossing over: Occurs during prophase 1 of meiosis when homologous
chromosomes pair up. Maternal and paternal chromosomes of each
pair may tangle together and exchange segments of genes, making
new gene combinations.
 Model the formation of new genotypes produced during meiosis
o Interpreting autosomal, sex-linkage, co-dominance, incomplete
dominance, and multiple alleles
 Autosomal: Relating to chromosomes that aren’t sex
chromosomes, e.g. in Mendel’s pea plants. Each plant carries 2




genes for a characteristic, each an alternate form of the
characteristic (like tall and short). These genes are called alleles.
One allele is dominant (T) and expressed over the other (t),
which is recessive. If the two alleles for a characteristic are the
same, it is homozygous (TT, tt), if different, it is heterozygous
(Tt). Each parent passes one gene, so their offspring has two
genes for the characteristic.
The rhesus system only has two alleles, and is an example of
dominant and recessive alleles (Rh+ is dominant to Rh-).
Sex-linkage: Relating to sex chromosomes (like female XX and
male XY). Thomas Morgan, through fruit flies, found that sex
chromosomes often carry only 1 gene instead of 2 because the Y
chromosome is smaller and has less genetic material, and
doesn’t carry genes.
Many recessive conditions are mainly expressed in males
because a male only inherits one gene for a characteristic and it
is always expressed, even if it is recessive, because he cannot
inherit a dominant gene on his Y to mask its effect. (see punnett
squares)
Co-dominance: Two alleles are both dominant and both
apparent in the phenotype. An example of polygenic inheritance,
where the inheritance of a characteristic is controlled by 2 or
more genes.
Incomplete dominance: Two alleles blend together and show a
blended effect in the phenotype (e.g. white + red flower = pink
flower)
Multiple alleles: When there are over two alleles for a
characteristic. E.g. ABO blood groups (A and B are dominant
over O, A and B are co-dominant and = AB)
Blood group
Genotype
IAi or IAIA
Antigens (are
recognised by
antibodies)
A
Antibodies
(recognise antigens
for attack)
B
A
B
IBi or IBIB
B
A
AB
IAIB
A+B
None
O
ii
None
A+B
o Constructing/interpreting info from pedigrees and Punnett
squares


Punnett squares: Allows you to predict the phenotypes of the
offspring of two parents, using the genotypes of the parents.

Pedigree charts: Show inheritance patterns and allows
inheritance of genetic disorders to be followed. Problems usually
whether the condition is dominant or recessive and for the
genotypes of certain members.
Rules: If two affected have an unaffected child, the trait is
dominant. If two unaffected have an affected child, the trait is
dominant. If every affected person has an affected parent, the
trait is dominant.
Use data to represent the frequencies of characteristics in a
population to identity trends, relationships, limitations, etc.
o Examining frequency data
 Gene frequency: How often a certain allele for a gene occurs in a
population.
 Gene pool: Every allele of every gene in a population of a certain
species.
Hardy-Weinberg principle: In a sexually-reproducing population
with random matings, gene frequency will not change unless
certain changes/events are occurring.
 Changes: mutation, emigration (organisms leave, shrinking gene
pool), immigration (organisms enter population, increasing gene
pool), natural selection (see next point).
 Natural selection: Certain alleles are selected for or against,
depending on whether they increase or decrease an organism’s
survival chance. Advantageous alleles are passed on to offspring
who survive to reproduce and pass on the characteristic ->
frequency of this allele increases.
o Analysing single nucleotide polymorphism (SNP)
 Definition: Variation in a single nucleotide among a
species/population’s DNA. E.g. at a specific position on the DNA
strand, most people have an A, while some others have a C. This
variation means that there is a SNP in this position. Other than
SNPs, all humans have the same DNA.
 Effects: Most SNPs occur in non-coding DNA, usually having no
effect, but some can e.g. change the structure of tRNA
molecules. Other, occur in genes and can cause changes to a
nucleotide/protein (maybe stopping the protein from working),
while some don’t any effect at all.
 Applications: Can be used as markers to find disease-causing
genes, and to track the inheritance of disease in families. The
progression of a disease can also be dependent on certain SNPs,
the analysis of which can open up personalised treatment
options.

5: Inheritance Patterns in a Population - Can population genetic
patterns be predicted with any accuracy?
 Technologies used to determine inheritance patterns in a
population
o DNA sequencing
 Used to determine the sequence of bases in DNA
 Sanger method: Genes/DNA is isolated and replicated using PCR
(polymerase chain reaction), the sequence is graphed by a
computer.
 Maxim Gilbert method: Chemicals used to identify a specific
base, electrophoresis used to compare base patterns.
o DNA profiling
 Used to identity and compare individuals based on their DNA
sequence (and satellite DNA made up of STRs—short tandem
repeats—sections of DNA unique to every individual)
 A DNA sample is collected, DNA is isolated, PCR amplifies STRs,
gel/capillary electrophoresis detects difference in size.
 Applications: Paternity testing (comparing offspring’s DNA with
potential fathers), forensic investigations (identifying
suspects/victims with crime scene DNA)
 Investigate data analysis from a large-scale collaborative project to
identify trends, patterns and relationships
o Human Genome Project (HGP)
 A publicly funded international scientific project, aiming to
determine the DNA sequence of humans. The development of
technology accelerated progress greatly.
 Gave insight into human evolution/ancestry, allows for
improved/personalised disease treatments, allows determination
of disease inheritance
o Conservation population genetics
 Used to maintain biodiversity and genetic variation, as genetic
variation = organisms adapt to change = conservation of
species. Harmful alleles can be detected and bred out,
advantageous ones can be introduced.
 Methods used: field observation, sampling, statistical analysis,
DNA analysis (SNPs, haplotypes—groups of genes inherited
together, GWAS—genome wide association study).
o Population genetics to determine disease/disorder inheritance
 Enables scientists to better diagnose disease, study disease
inheritance and improve treatment options.
 Black urine disease—causes many issues. DNA sequencing/other
technology found this disease was caused by a mutation in a
specific gene.
o Population genetics in human evolution
 Anthropological genetics aims to explain causes of human
diversity (mutation, natural selection, genetic drift) and pathways
of evolution.
 Genetic evidence: Comparing human and chimp genomes,
accumulated differences can be used to determine how long
ago the species separated.
 Modern humans: By comparing SNPs of indigenous people from
different locations, we get clues about the patterns of migration
and interbreeding that occurred.
 Mitochondrial and Y-chromosomal DNA: Modern humans are all
descended from a wave of migration out of Africa 60,000 years
ago. We know this from mt-DNA, which is passed only from
mother to daughter. Studies of SNPs in mt-DNA shows that all
people are descended directly from a woman living in Africa
150,000 years ago. The same has been determined for males
from Y-chromosomal DNA.