Transcript File

Inherited
Change
The X2 (chi-squared) Test
• If you look at a dihybrid cross between two heterozygous individuals,
then we would expect to see a 9:3:3:1 ration of phenotypes in the
offspring. However, this is still just a predicted probability!!!
• To determine whether the differences between the expected
numbers of each genotype and the observable numbers are due to
chance or something unexpected, we can use the X2 (chi-square)
test.
•
This statistical test allows for the comparison of the observed and
expected results to see if there is a significant difference between them.
• To carry out the X2 test, you need the following information:
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The observed numbers for each genotype [O].
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The expected ratio and the expected numbers for each genotype [E].
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The difference between the observed and the expected values. [O – E]
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The square of the difference [(O – E)2]
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The square of the difference divided by the expected values. [(O – E)2 /E]
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The value of X2 will be the sum of the previous bullet.
The X2 (chi-squared) Test
• Once the value of X2 is obtained, we need to figure out what the
value means. To do so, we must look at a table that relates X2 values
to probabilities. The table below gives the probability that the
differences
Degrees
Probabilty greater than
between our
of
0.1
0.05
0.01
0.001
expected and
Freedom
observed results
1
2.71
3.84
6.64
10.83
are due to
2
4.60
5.99
9.21
13.82
chance.
3
6.25
7.82
11.34
16.27
4
7.78
9.49
13.28
18.46
• In biological experiments, a probability of 0.05 is usually critical. If our X2
value represents a probability of 0.05 or larger, then we can be fairly certain
that the differences between our observed and expected results is due to
chance (Not significant)
• If the probability is smaller, then it is likely that the difference is significant and
consider what else may have gone wrong with the cross.
The X2 (chi-squared) Test
• Not only do we have to look at the probabilities, we should also look
at the degrees of freedom in the results. This value takes into account
the number of comparisons made.
• To work out the number of degrees of freedom, calculate the number
of classes date minus 1. (The classes of data is the number of possible
sets of phenotypes.
• Once the degrees of freedom are calculated, then you can look at
the correct portion of the table.
EXAMPLE:
144 offspring produced.
Purple, cut 86
green, cut 24
Purple, potato 26
green, potato 8
Mutations
• Most genes have different alleles that are made up of a sequence of
nucleotides, each with their own base. The different alleles originally
arose due to mutations.
• Mutations are unpredictable changes in the genetic material of an
organism.
•
A gene mutation is a change in the structure of a DNA molecule, which
will produce a different allele of a gene.
•
A chromosomal mutation is a change in the structure or number of whole
chromosomes in a cell.
• Mutations can occur with no obvious cause or because of an
environmental factor.
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All types of ionizing radiation (alpha beta, and gamma radiation) can
damage DNA molecule, altering the structure of the bases.
•
UV radiation has a similar effect by increasing the chances of mutations
(mutagen)
Mutations
• There are 3 different ways, in gene mutations, in which the sequence
of bases in a gene can be altered.
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Base substitutions occur when one base simply replaces another.
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Base additions occur when one or more extra bases are added to the
sequence.
•
Base deletions occur when one or more bases are lose from the
sequence.
• Base additions or deletions have a very significant effect on the
structure and function of the polypeptide that the allele codes for.
They are significant because they alter every codon that follows the
mutation in the DNA molecule. They are said to be frame shift
mutations.
• Base substitutions usually have no effect at all. They are said to be
silent mutations.
Sickle Cell Anemia
• Sickle cell anemia is caused by a base substitution. Haemoglobin is
composed of 2 alpha and 2 beta chains (polypeptides) The gene
which codes for the amino acid sequence in the beta chains is not
the same for everyone.
•
Most people have the sequence:
Val-His-Leu-Thr-Pro-Glu-Glu-Lys
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For some people the base sequence CTT can
be replaced by CAT to give:
Val-His-Leu-Thr-Pro-Val-Glu-Lys
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The change is sequence changes the
polypeptide, which does not affect the
haemoglobin molecule when it is combined
with oxygen. However, the unusual beta chains
causes the haemoglobin molecule to be less
soluble when it is not combined with oxygen
and it therefore becomes sticky.
Phenylketonuria (PKU)
• Phenylketonuria is a disease that is caused by an abnormal base
sequence that codes for an enzyme (phenylalanine hydroxylase).
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Phenylalanine hydroxylase helps to catalyze the conversion of
phenylalanine to tyrosine, which will then be converted to melanin.
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If phenylalanine hydroxylase cannot be made, then little melanin is
formed. Therefore, people with PKU had lighter skin and hair color.
• A bigger problem that people with PKU have is that too much
phenylalanine will accumulate in the blood and tissue fluid. This can
cause severe brain damage in children.
•
Brain damage can be prevented by testing before birth and placing the
child in a strict diet that does not contain phenylalanine. (NO
chocolate!!)
Gene Technology
• Gene technology (genetic engineering) allows scientists to change
the DNA in a cell.
• An example of how gene technology has been helpful is the use of
genetically altered bacteria to mass-produce human insulin. Scientists
succeeded in the early 1980s in inserting the gene for human insulin into a
bacterium.
1. mRNA carrying the code for insulin production from the β cells in the
pancreas was removed.
2. The mRNA is incubated with reverse transcriptase from a retrovirus.
However, in order for the insulin genes to stick onto other DNA, they
were given sticky ends using restriction enzymes (guanine nucleotides
in single stranded DNA at each end).
3. A vector (plasmid in this case) is used to insert the insulin gene in the
bacterium. The insulin gene was added to the plasmid by treating the
bacteria with enzymes that dissolve their cell walls. They were then
centrifuged, and the much smaller plasmids were cut using restriction
enzymes and sticky ends with cytosine added.
4. The plasmid and DNA are then mixed, where they pair together and
linked with DNA ligase to form recombinant DNA.
Gene Technology
5. The plasmids are now mixed with bacteria (E. coli). The portion of
bacteria that took up the plasmid was then separated from the others
using antibiotic resistance provided by another gene that was
introduced at the same time.
6. The genetically altered bacteria can now be cultured on a large
scale. The will secrete insulin, which will then be extracted, purified, and
sold to people.
• Gene technology has also been used to
synthesize other human protein hormones or
enzymes that can be used in the food industry.
• In addition, gene technology can introduce
genes into any organism. Gene therapy seems
promising in replacing defective genes in
humans with good genes. This can be very
helpful in treating genetic disorders.
• Human factor VIII is also genetically
modified in hamster cells.
Genetic Engineering