Download Variation Hardy

Variation
Organisms within the same species vary. Some of this variation is heritable, i.e. can be passed on from
the parents and some of it may be caused by the environment. Variation is often categorised as either
discontinuous (discrete) or continuous, depending upon whether the variation can be categorised into
definite groups or whether there is a continuum, with no definite categories.
Discontinuous Variation
Discontinuous or discrete variation is concerned with qualitative differences in the observable
characteristics or phenotype. Organisms can be placed into definite categories, with no intermediates.
Thus, Mendel’s peas were either tall or dwarf; you are either male or female; you have attached or
unattached earlobes; you are either rhesus positive or rhesus negative; you are blood group A, B, AB or
O. So, it is typical of dominance, recessive and codominant inheritance patterns.
Discontinuous variation is often controlled by a single gene, i.e. it is monogenic. In other cases, it may
be controlled by more than one gene, but they interact in an epistatic way. Notwithstanding this, the
alleles of individual genes have a large effect on the characteristic.
Continuous Variation
Continuous variation is concerned with quantitative differences in the observable characteristics or
phenotypes. There is a wide range of variation, a continuum, with no discrete categories. Examples
include: birth weight; height, mass and IQ in humans; milk yield in cattle; grain yield in cereals.
Typically, continuously varying traits are controlled by several genes, each of which adds a little to the
overall characteristic. Thus a tall individual is likely to have inherited a larger number of ‘tall genes’ from
its parents than a short person. As several genes are involved in producing the overall characteristic,
they are known as polygenic traits. The genes are not linked, i.e. they are not carried on the same
chromosome and so can be shuffled during meiosis. Even if they were on the same chromosome, crossover during meiosis could produce recombinations.
Phenotypic Variation
The phenotype or observed characteristic is determined partly by the genotype (the genes) and partly
by the environment. Thus, trees may grow straight and tall in a sheltered environment with good soil
but are stunted and deformed in an exposed, nutrient-poor environment, such as on the coast; pale skin
can tan when exposed to ultra violet light; people gain weight when overfed and under exercised.
However, the environmental influence is potentially greater with continuously varying characteristics.
Thus, an individual might have inherited the genes that give them the potential to become the next
Einstein, but lack of appropriate diet or mental stimulation and education may prevent them from
achieving this. Consider the following examples:
Stunted/Distorted Plants in Harsh Environments
These birch trees are in a sand dune system in Lancashire.
They are exposed to strong winds.
Q. List and explain the factors that might cause them to be
stunted and have distorted growth.
SJWMS Biology
Q. Suggest what might happen if cuttings were taken from these trees and grown in a sheltered
environment.
Q. Why use cuttings instead of seedlings?
Height in Humans
Height in humans is controlled by a large number of genes – tall people inherit a large number of tall
alleles from their parents whilst short people generally inherit fewer tall alleles. However, inheriting a
large number of tall alleles only gives the potential to become tall – adequate nutrition is essential if this
potential is to be realised!
Q. Studies in America have shown that second and third generation American Japanese and Chinese are
several inches taller than their parents or grandparents. Explain such findings.
Q. Height in the British population has increased over the past century, even though there is the same
genetic mix. In WW1, the North West had ‘bantam regiments’ where the normal minimum height of 5
foot 3 inches (63 inches) was reduced to 5 foot (60 inches). Suggest reasons to explain these
observations.
Coat Colour in Rabbits
A Himalayan rabbit has dark tips to its ears, nose, paws and tail. It has an
allele that allows melanin production, but only at low temperature.
Q. Explain the distribution of darker fur in this rabbit.
Himalayan, lop-eared rabbit
Q. The photograph of the lop-eared rabbit was taken after a prolonged period of high temperature.
Suggest an explanation for the colour of the ears and nose.
SJWMS Biology
Role of Variation in Selection
Whether it is natural selection or artificial selection, it is the heritable variation within a population that
the selection is operating upon, that brings about change or evolution. Thus, variation within rabbits for
coat colour (black, white or agouti) can bring about a selective advantage depending upon the
environmental conditions, e.g. white coat may be a good camouflage in snowy conditions whereas
agouti may be better at other times. This would be an example of natural selection. The development of
cattle varieties (e.g. Holstein-Friesian) that produce large yields of protein-rich milk is an example of
artificial selection.
Population Genetics and the Hardy–Weinberg Principle
The range of genetic variation within a population of interbreeding organisms is the gene pool. The
proportions of alleles tend to remain constant from generation to generation. This is the Hardy–
Weinberg Principle and can be used to predict the frequency of genotypes within a population if the
following conditions are valid:
 the population is large and mating is random
 there is no mutation
 there is no migration of genes into or out of the gene pool
 there is no selection in favour of or against a particular genotype
If a characteristic is determined by a single gene and there are two alleles, one dominant and the other
recessive, it is easy to calculate the frequency of the homozygous recessive. It is simply the number
showing the recessive characteristic as a proportion of the total population. Unfortunately, it is not so
simple to determine the frequency of the homozygous dominant or the heterozygous as these will look
the same!
If, p = the proportion having the dominant allele (A) and
q = the proportion with the recessive allele (a), then
p + q = 1.
Female
gametes
Since, in a large randomly mating population, the gametes will
have the alleles in the same proportion, it follows that:
 the proportion who are AA will be p x p = p2
 the proportion who are Aa will be 2 (p x q) = 2pq
 the proportion who are aa will be q x q = q2
Male gametes
X
A (p)
a (q)
A (p)
p2
pq
a (q)
qp
q2
Thus, p2 + 2pq + q2 = 1
It is probably best to follow a worked example to see how the Hardy–Weinberg equation can be used to
predict the proportion of genotypes in a population:
Rhesus Blood Group in Humans
Around 84% of Northern Europeans are Rhesus D positive whilst 16% are Rhesus D negative. The allele
that codes for the Rhesus D factor is dominant. If p = proportion of allele D and q = proportion of allele
d, it can be deduced that 16% or 0.16 of the population are homozygous recessive (dd) and that q2 =
0.16.
What is q? Take the square root of q2. So, q = 0.4.
As p + q =1, it follows that p = 1 – 0.4 = 0.6.
It is now possible to substitute values in the Hardy–Weinberg equation and calculate the proportions of
the population who are homozygous dominant (DD) and heterozygous (Dd):
p2 + 2pq + q2 = 1
0.36 + 0.48 + 0.16 = 1
SJWMS Biology
Cystic Fibrosis
Cystic fibrosis is a common inherited disorder. It is caused by a recessive allele that codes for an
abnormal channel protein, so that chloride ions are not transported properly and results in a thick, sticky
mucus. So, sufferers are homozygous recessive. If one person in 2,000 has cystic fibrosis, use the
Hardy–Weinberg formula to estimate the proportions of carriers in the population.
p is the proportion of the CF allele in the population
q is the proportion of the cf allele in the population
p + q = 1
p2 + 2pq + q2 = 1
q2 =
q =
p = 1–
So, 2pq (the carriers) =
Note: The Hardy–Weinberg principle assumes that certain conditions hold. Namely the population is
large and mating is random; there is no mutation; there is no migration or gene flow into or out of the
population; there is no selection operating. What would be the effect on proportions of alleles if these
conditions did not hold?
SJWMS Biology