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Mutations 2
Haploid and Diploid
Monoploidy
• A cell organism that has a functional genome consisting of one copy of
each chromosome, represented by 1n
• Male bees are monoploid (1n), while females and queens are diploid
• Monoploid does not mean haploid – male bees monoploidy status give
them a single operational set of chromosomes
• Haploid gametes are waiting (dormant) to be fertilised , which activates
them. This results in diploid female offspring
• Males are produced by parthenogenesis – a process by which entire
organisms are regenerated from a single egg cell without the need for
fertilisation
Monoploid and Diploid
• Monoploid exists in many organisms e.g. fungi,
fish, amphibians and reptiles
• Efficient, but majority of organisms are diploid –
why?
• Any defective alleles are masked by a functional
allele on a corresponding chromosome.
• In monoploid organisms cannot do this
Polyploidy
• Cell division during
meiosis results in haploid
cells
• When spindle fibers fail,
some cells contain double
the genetic information,
while others contain none
at all 
• See diagram p77
Polyploidy
• Polyploidy is common in flowering plants, ferns and
green algae
• Fruits and cereals have been genetically manipulated
to be polyploidy for human application e.g. seedless
fruits, larger sized fruit
• Diploid (2n) fusing with haploid (n) produces a triploid
organism (3n) – three of each type of chromosome
• Polyploidy is lethal in humans 
Aneuploidy
• A condition where there
is an addition or loss of
one chromosome from
a cell e.g. 2n + 1 or 2n –
1
• Results from nondisjunction
Non-disjunction
•
During meiosis, two homologous chromosomes,
instead of separating, both go into the same
gamete. The spindle fibers have failed.
•
This is known as non-disjunction.
•
As a result of non-disjunction some gametes have
two copies of a chromosome and others lack a copy.
•
After the first meiotic division the homologous pair,
instead of separating, have both remained in the
same cell.
•
After the second meiotic division the gametes
formed are of two types; half lack a chromosome
and half have two copies of the same chromosome.
Down’s Syndrome
• If non-disjunction occurs in a human egg cell then one or more
abnormal eggs may be formed
• This means the haploid number of these abnormal eggs is 24
• If this egg is fertilised the diploid number of the zygote is 47 instead of
46
• An extra copy of chromosome 21 is passed on as a result. This can be
clearly seen in a karyogram and is known as trisomy 21
• Sufferers of Down’s syndrome in general have learning difficulties,
heart problems and distinctive facial features
Non-disjunction of Sex chromosomes
• Females: Turners Syndrome
• Sufferer is short in stature and show a lack of sexual
development at puberty
• This non-disjunction results in only one X chromosome: XO
Non-disjunction of Sex chromosomes
• Males: Kleinfelter’s syndrome
• Sufferers are males with tests that remain very small. Few sperm are
produced therefore they are mostly infertile and low levels of testosterone
are produced leading to reduced development in puberty
• This non-disjunction results in only one the individual being XXY
Genetic Screening and Counselling –
FYI only!
• Genetic counsellors aim to assess the risk of
parents passing on genetic diseases to their
offspring so that they can make informed
decisions
• Two options are pre and postnatal screening
Prenatal Screening – FYI only!
•
Chorionic Villus Sampling (CVS)
– A high risk procedure done early
on in pregnancy
– Sample of developing placenta is
taken
•
Amniocentesis
• Amniotic fluid which foetus
bathes in is extracted
•
Cells obtained are photographed and
the karyogram is analysed for any
abnormalities
Genes and Body plans
How does a zygote become an embryo?
How do the different parts of an organism know
which way is “up”?
Learning Outcomes
Explain that the genes that control development of
body plans are similar in plants, animals and fungi, with
reference to homeobox sequences (HSW1);
What do homeobox genes do?
• Homeobox genes code for the production of
transcription factors
• These proteins can bind to a particular region
of DNA and cause it to be transcribed
– A single homeobox gene can switch on a whole
collection of other genes, regulating gene
expression
Homeobox genes
• Homeobox genes determine how an organism’s body
develops as it grows from a zygote into a complete
organism.
• They determine the organism’s body plan
• These sequences are highly conserved
• Homeobox genes have been discovered in animals,
plants and fungi
Homologous homeobox genes
• These are the sequences of 60 amino acids in the proteins
coded for by the homeobox genes Antp in a fruit fly and
HoxB7 in a mouse.
• All animals have homologous homeobox genes – they are
recognisably similar. Why is this significant?
Key words – homeobox, maternal effect and polarity
genes
What are they? What do they mean?
• Homeobox genes control the development of the body
plan of an organism, including the polarity (head and tail
ends) and positioning of the organs
• Maternal effect genes determine the embryo’s polarity
• Polarity genes Help to define the anterior and posterior
polarities within each embryonic segment of an
organism, such as in the fruit fly
Homeosis - when one part of a developing embryo becomes
anomalously transformed into another (mutation)
Name the type of gene, if mutated, gives rise to
dramatic changes in body plan
Homeobox gene
Drosophila melanogaster – the fruit fly
A model animal for the study of
genetics and gene regulation
The body of insects is segmented
This is obvious in larvae (e.g. maggots)
In adults these become specialised many develop special appendages
Head Segments - antennae
- labial palps
(mouth appendages)
Thoracic Segments - legs
- wings
- halteres
(balancing organs)
Abdominal Segments - no appendages
Identity of each segment is established in
the embryo
Mutations can destroy the identity of a
segment (more on this later)
Homeosis in Drosophila
Wings for halteres
Legs for antennae
These changes are the
results of mutations in
a set of genes
(homeosis)
In this instance, are
mutations a good thing?
Halteres for wings
Homeobox genes have been found in:
 worms
 snails
 starfish
 fish
 mice
 humans
Summary
The body plan of the drosophila fly is controlled by the homebox genes.
These contain transcription factors that regulate other genes so that the body
plan develops normally. Transcription factors act like on/off switches for gene
expression.
Some of these genes are maternal-effect genes. They will determine the
polarity of the embryo. This means which end is the front and which is the
back.
Segmentation genes are another group of genes that determine the dorsal
and ventral ends of specific segments of the fly’s body.
Sex Determination
• SRY gene on the Y chromosome directs the
development of male characteristics in
mammals
• SRY gene activates DNA to express the genetic
programme for testes development
• Consequently, testosterone and penis
development as a result
X Chromosome Inactivation
• X Chromosome inactivation restores the proper
concentration of proteins within cells of females
and the process that leads to further variation in
gene expression
• Two X chromosomes produce twice the chemicals
required – disrupts biochemistry
• X inactivation is random, therefore increases
variation