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Inherited Change
Meiosis
Used in sexual reproduction to allow for
variation.
Homologous chromosome – chromosomes that
have the same genes but not the same alleles.
One from mum and one from dad.
Bivalent – when the homologous chromosomes
have replicated they join together by a chiasma
and this forms a bivalent made of 4 chromatids.
Crossing over – the non-sister chromatids that
lie next to each other in the bivalent may join
temporarily and then break off swopping some
of their genetic information.
Variation
Due to:
– Crossing over of non-sister chromatids in the
bivalent
– Random assortment of bivalents and sister
chromotids during metaphase I and II
– Random fertilisation
Definitions
Gene – part of a chromosome that codes
for a particular protein
Locus – position of a gene on a
chromosome
Allele – different forms of a gene. Eg. The
gene is eye colour but the allele is blue or
brown
Only one allele can be expressed most of
the time. Eg. Eye colour does not
constantly change even if you inherit two
different alleles from your parent.
This means that one is normally dominant
over the other and is shown in the physical
appearance of a person.
Recessive is the allele that does not show
in the physical appearance but it may
show in the next generation.
Codominant – when both alleles show in
the physical appearance. Eg. Red flowers
crossed with white flowers give pink
flowers.
Homozygous – alleles are the same on
both chromosomes.
Heterozygous – alleles are different on the
homologous chromosomes
Phenotype – physical appearance
Genotype – genetic make-up
Monohybrid crosses
Genetic crosses that involve just one
gene.
Eg. Eye colour
(Make sure the outline of the cross is written
out exactly as this gains the marks in an
exam)
Let B represent brown eyes. (parent is homozygous)
Let b represent blue eyes (parent is homozygous)
Parent phenotypes?
Parent genotypes?
Gametes?
F1 genotype?
F1 phenotype?
Ratio?
Dihybrid Crosses
Mendel
Looked at pea plants
Colour of seeds and round or wrinkled
Colour of flowers and tall or short
Dihybrid means showing two
characteristics in one genetic cross.
Round seeds are dominant over wrinkled
seeds
Yellow seeds are dominant over green
seeds
Let the dominent allele for shape be?
Let the recessive allele for shape be?
Let the dominant allele for colour be?
Let the recessive allele for colour be?
Parent Phenotypes?
Parent Genotype (pure breeding)?
Gametes?
(there are four
Possible)
F1 genotype (see punnett square)
F1 phenotype
Try June 2006 paper 4
Monohybrid phenotype outcomes
4 : 0 homozygous dominant crossed with
homozygous recessive
or homozygous dominant crossed with
heterozygous
3 : 1 heterozygous crossed with heterozygous
1 : 1 homozygous recessive crossed with
heterozygous
Codominance – heterozygous cross
heterozygous 1 : 2 : 1
Codominance
If both alleles show in the phenotype in the
heterozygous condition this is called
codominance.
Eg. Red and white alleles make pink.
Still use one letter though.
RR is red, rr is white, Rr is pink
Main example is blood groups
Sex Linkage
When genes are located on the either the X
chromosome or Y chromosome they are said to
be sex linked.
Eg. Ability to see particular colours and blood
clotting.
These are both found on the X chromosome.
Therefore a recessive allele will be more likely to
show in a male than in a female. As there is no
other X chromosome to mask it.
Haemophilia
When blood does not clot normally.
Sex-linked character caused by a recessive
allele carried on the X chromosome.
If the male has the recessive allele then he has
the disease.
The female only gets the disease if she inherits
two recessive alleles.
If the female is heterozygous she is known as a
carrier.
Let Xh represent haemophiliac
Let XH represent normal clotting
Parent Phenotype
Parent Genotype
Gametes
Test cross
If we don’t know the genotype of an
individual but we can find out by doing a
test cross.
This is a cross with a homozygous
recessive individual.
Let T represent tall
Let t represent dwarf
Parent phenotype
Parent genotype
Gametes
Tall
TT
Dwarf
tt
Chi – squared Test
To see if there is a difference between
numbers observed in an experiment and
the numbers given in an hypothesis.
And if that difference is real or if it is due to
chance or sampling error.
Chi-squared =
Null hypothesis – where we assume there
is no difference between the expected and
the observed.
Which means our hypothesis (genetic
diagram) will be backed up by actual
results if we did the experiment.
Eg. If we said there should be a 3:1 ratio
then the actual result will be 9 white
rabbits and 3 pink ones.
To calculate the chi-squared value
first determine the number expected in
each category.
If the ratio is 3:1 and the total number of
observed individuals is 880, then
the expected numerical values should be
660 white and 220 pink.
Say that the actual cross between two
rabbits yields a population of 880 rabbitss,
639 with white fur and 241 with pink fur.
Now plug it into the equation:
The chi-squared value will be changed into
a probability value using a chi-squared
table.
The table contains degrees of freedom.
Degrees of freedom ≡ mathematical term
relating to the number of free variables in
the system. It is always (n – 1).
n is the number of categories or
phenotypes in a given example. In our
example 2 (white and pink) therefore
degrees of freedom would be 2 – 1 = 1
Use the chi-square distribution table to
determine significance of the value.
a) Determine degrees of freedom and
locate the value in the appropriate column.
b) Locate the value closest to your
calculated chi-squared value on that
degrees of freedom row.
Move up the column to determine the p
value.
Probability value means the probability that our
observed will match up to our expected every
time we do the practical.
State your conclusion in terms of your
hypothesis.
If the p value is p > 0.05, accept your
hypothesis.
– 'The deviation is small enough that chance alone
accounts for it. A p value of 0.6, for example, means
that there is a 60% probability that any deviation from
expected is due to chance only. This is within the
range of acceptable deviation.
If the p value is p < 0.05, reject your hypothesis,
– conclude that some factor other than chance is
operating for the deviation to be so great. For
example, a p value of 0.01 means that there is only a
1% chance that this deviation is due to chance alone.
Therefore, other factors must be involved.
So bigger than 0.05 backs the null
hypothesis.
Smaller than 0.05 means there is a
significant difference between the
hypothesis and the actual results so chuck
your hypothesis out and start again.
Eg. Pg 233 and 235
Variation
Phenotype results from the interaction between
the genotype and the environment.
The genotype determines the potential of an
individual but the environment determines to
what extent that potential is fulfilled.
Divided into
– Discontinuous – usually genetic only with an either or
outcome. Distinct groups.
– Continuous – environmental effects come in to play.
Groups merge in to one another.
Mutation and phenotype
Gene mutation is a change in the base
sequence of DNA
Alters primary structure of proteins
– Substitution - one base is swopped for
another
– Deletion – one base is removed
– Insertion – one base is added
– Duplication – one or more bases are repeated
– Inversion – a sequence of bases is reversed
examples
Sickle-cell anaemia – base substitution
Normal haemoglobin is:
– Val – His – Leu – Thr – Pro – Glu – Glu – Lys
Sickle-cell is:
– Val – His - Leu – Thr – Pro – Val – Glu – Lys
Valine is non-polar and hydrophobic
In low oxygen concentrations haemoglobin becomes
less soluble and crystallises
Sticky fibres then distort the red blood cell shape.
Cystic fibrosis
– Recessive mutation on chromosome 7
Normal gene codes for a chloride channel
protein called CFTR
In cystic fibrosis sufferers channel defective.
Result –
– as chloride ions do not flow out
– sodium ions rush in to balance charge and this
prevents water leaving the cell
– Mucus becomes thick and sticky.
– Affects lungs, pancreas, liver
– Mucus clogs ducts and passages
Down’s syndrome
– Chromosome mutation on 21
Sufferers have 47 chromosomes instead
of 46 as there is an extra copy of 21.
Called trisomy
Symptoms
– Mental retardation
– Short stature
– heart defects
– Coarse hair
Enviromental effect on phenotype
Height
Weight
Dark tips on rabbit ears and tails. Areas
exposed to cold temperatures cause a
pigmentation to be formed in those areas.