There are three alleles for blood group given by the symbols I
A
, I
B
and I
O
. I
A
and I
B
are co-dominant giving blood group
AB or I
A
I
B
, and both dominant to I
O
.
3.5 Variation
Continuous variation (variation which is usually quantitive, it has a range of intermediate classes) is influenced by
genes and environment, resulting in a range of phenotypes between two extremes, e.g. height in humans
Discontinuous variation (variation which be classified as one thing or another, you are either blood group O, A, B or
AB, nothing else, for example) is caused by genes alone and results in a limited number of distinct phenotypes with no
intermediates
Mutation: a change in a gene or chromosome (*caused by mistakes in copying the DNA before cell division – pairing
with the incorrect base, or damage to DNA for example by radiation, or uneven distribution of chromosomes in cell
division).
Mutation is a source of variation for example in Down’s syndrome, where a parent’s chromosomes are unevenly
distributed in meiosis (e.g. one chromosome has 22 and the other has 24). In fertilisation, a zygote with a number of
chromosomes that is not 46 is created (e.g. 23 + 24 gives 47 chromosomes). This causes a variation in characteristics:
broad forehead, short neck, fold in eyelid, spots in iris, downward-sloping eyes, short nose, protruding tongue,
congenital heart defects and mental retardation.
Sickle cell anaemia: is a disease in which the red blood cell has a sickle shape instead of a round biconcave shape,
controlled by a recessive allele, which causes weakness, aching joints and poor circulation. The fact that it is recessive
means that a heterozygous person can be a carrier: they have the allele but it is not expressed. Being a carrier of
sickle cell anaemia makes you resistant to malaria (WIN!). In equatorial Africa, being sickle cell anaemic causes death,
malaria causes death, but the carriers have immunity to malaria and (according to the book:) have some symptoms of
anaemia, in severe cases they are very weak (but do not die).
Rate of mutation increases with:
-chemicals - tars in tobacco smoke, high concentrations of some preservatives and some plant control hormones
-radiation – gamma, ultraviolet and X-radiation can damage and cause mutations because they have an ionising
effect.
*These factors are called mutagens.
3.6 Selection
Artificial selection: is breeding the organisms with the valued characteristics together in order to try to produce
offspring which shares those useful characteristics (selective breeding). This can be used to produce organisms which
are more economically valued (i.e. they will earn you more money) for example Jersey cattle produce milk with a high
cream content, domesticated dogs are bread for appearance, or hunting or because they are aggressive therefore
make good guard dogs, and wheat has been bred so that all the stems are the same height to make harvesting easier
and the ears separate easily from the stalk (making collection of grains easier).
Natural selection: the greater chance of passing on of genes by the best adapted organisms.
Variation is natural or random changes in all living organisms. Variation leads to survival of the fittest since the
variations in certain organisms allow that organism to have an advantage over the others in its species in that area (*for
example the peppered moth which is white-bodied had a variation producing black-bodied peppered moths. During the
industrial revolution, pollution prevented lichen from growing on trees. The lichen previously camouflaged the peppered
moth. The trees became blackish from pollution so the black-bodied moth survived better from its predators since it
was now better camouflaged.) The surviving organisms reproduce, since they don’t get eaten up, so the variation has
caused the species to evolve. Evolution is caused by natural selection which is caused by a change in the
environment.
Example to know: Strains of antibiotic-resistant bacteria are developing as the use of antibiotics is increasing. In a
group of many, many bacteria, one might mutate to be resistant to the antibiotic, as a result it reproduces and the
others die making a new strain of bacteria, which is resistant to antibiotics.
*A selection pressure is something in the environment which causes only some organisms to survive (e.g. an
antibiotic)
*A survival advantage is a characteristic which allows the organism to survive instead of the others (e.g. resistance to
an antibiotic)
3.7 Genetic engineering