IB Genetics 2013: Topic 4 (SL and
HL)
Welcome to genetics!
Welcome to genetics!
• September 2012
Genetics: The science of difference
The science of heredity - the ‘genetic properties’ or
inherited features of an organism
• Molecular genetics: study of structure and function
of DNA and RNA
• Population genetics: study of genes within
populations, including gene frequency, the gene
pool and evolution
Essential definitions of genetics
THE GENOME
‘The entire genetic
information of an
organism’: the entire set
of genetic instructions
• The genome
• the announcement
• What have we got from
the genome?
The gene
• IB: ‘a heritable factor
that controls a
specific
characteristic’
• Heritable – passed on
through the generations
• Factor – locatable
sequence of nucleic acid
(DNA)
• Characteristic – ‘genetic
traits’
• an illustration
Some numbers…
• How many bases in the human
genome?
• By how many bases do each of
us differ?
• How many genes do we have?
(less than we thought!)
• How much of our DNA consists
of protein-coding genes?
• What does the rest of the DNA
do?
• Do all genes code for protein?
• Are all of our genes in the
nucleus?
• What does the rest of the
DNA do?
• Do all genes code for
protein?
• Are all of our genes in the
nucleus?
Genes are located in chromosomes
• ‘coloured bodies’
• Eukaryotic chromosomes
consist of DNA wound
around histone proteins
• Constriction point:
centromere
• When are chromosomes
visible in the cell?A fly-over
Chromosome 11...
Gene Locus
• The specific position of
a gene on a
chromosome
• All individuals of a
species will have the
same gene at the same
locus on the same
chromosome
• In other words, they are
homologous
Diploid/ haploid
Diploid/ haploid
Diploid
• Cells/organisms with paired
chromosomes
• Sexually reproducing
organisms are diploid
• All eukaryotic cells with
exception of gametes
• Contain 2n chromosomes, one
of each pair from each parent
• Human diploid number is 46,
i.e. 23 pairs of ‘homologous’
chromosomes
Haploid
• Gametes (sperm and egg
cells)
• Contain haploid number of
chromosomes – only one of
each chromosome
• Human haploid number is
23
Number of chromosomes
Zea Mays
Homo Sapiens
Mus musculis
Homologous chromosomes
• ‘similar in shape
and size’
• Carry the same
genes (not
necessarily the
same alleles for
that gene)
• NOT identical
Essential definitions: homologous chromosomes
• Human somatic cells have
22 homologous
chromosome pairs
(autosomes) and one nonhomologous pair (sex
chromosomes)
• One maternal/ one paternal
• Same size and structure
• Same genes at same loci
(not necessarily the same alleles
for that gene)
• NOT identical
Identifying chromosomes
(karyotyping)
Chromosomes are lined up
in their homologous pairs
according to:
• Size (1 largest – 22
smallest)
• Shape – position of the
centromere
• Specific banding (staining)
patterns using dyes
(Giemsa)
More later!
• and in more detail here
Allele
‘One specific form of a gene, differing from other alleles
by only a few bases and occupying the same gene locus
as other alleles of the gene’
‘Alternative’ versions of a gene
At the same gene locus
Alleles
Some ‘celebrity genes’
• BRAC1 and 2
Celebrity genes (2)
Mutations in the CTFR
gene cause cystic fibrosis
Celebrity genes (3)
APOE4 increases the
risk of Alzheimer’s
Disease
Celebrity genes (4): the oB Leptin gene
• Encodes a protein that plays a major role in
regulation of body mass and other hormonal
functions.
• Abnormalities in the Leptin gene are associated with
some types of obesity
Mutation
The basis of all
genetics
‘… the fuel for the
Darwinian factory’
‘Rarely pure and
never simple’
Gene mutation
• Definition: A random
change in genetic
material
• ‘COPYING ERRORS’ OF
DNA
• Various types of mutation
– base substitution,
insertion, deletion
• (point, frameshift – not
required by IB)
‘Mutation is the
destroyer and creator
of life’
‘Thanks to mutation,
we’re not all still in the
primordial soup’
Not all mutations cause disease
• ?silent mutations
• We have two copies of each gene – the functional
homologous chromosome may take over or
‘compensate’
Causes of mutation (1)
• Copying error during DNA replication (error made by
DNA polymerase and not corrected by the DNA
repair enzymes)
• Intra-cellular mutagenic chemicals (free radical
molecules)
• ?Gamma radiation?....
Causes of mutation (2)
•
•
•
•
•
•
Radiation
Chemicals
Viruses
Diet
Stress
Lifestyle
‘Hiroshima – the
most
cynical Biology
experiment of all
time’
Types of mutation
here's an animated summary...
Sickle Cell Anaemia – inherited base
substitution mutation
5% of the
world’s
population
carry the sickle
cell gene
An introduction to sickle cell disease
Sickle cell Hb
makes the RBC
collapse inwards
Life with sickle cell anaemia
• Shortened RBC lifespan –
16 days (instead of 35)
• Causes high mortality in
newborn
• Reduces lifespan by average of
30 years if untreated
• Low red cell count (anaemia)
• Abnormal red blood cells stick
in blood vessels – stroke,
jaundice, organ failure, lung
damage, chronic pain
• Even ‘carriers’ with only one
copy of the gene have mild
sublinical effects
Sickle Cell anaemia – Base substitution
mutation
• Base substitution (point)
mutation on chromosome
11
• Inherited during
conception (‘germ cell
mutation’)
• GAG becomes GTG
• Valine replaces glutamic
acid
• HbS (sickle cell Hb)
instead of HbA (normal
adult Hb)
• From the DNA learning
centre
• chromosome 11 flyover
A walk through sickle cell disease
‘Mutation is the fuel for the Darwinian
factory’
Map of malaria prevalence 1920
Map of sickle cell allele prevalence in
Africa
• Berkeley Uni link
• The power of evolution
Homework: Deepening your
knowledge on sickle cell anaemia
• Q. 10 – 15 in your
worksheet
Links on Sickle-cell
anaemia are on the Blog,
in the Genetics Section
‘Like begets like’…
Like Begets Like
Think about
physical
similarities
between you and
your ancestors
(mother, father,
grandparents,
siblings,cousins)