Unit 4.3 - Inheritance Flashcards
Heredity and genetics
the study of inheritable characteristics
phenotype
appearance/characteristics that are controlled by genes
genotype
total alleles possessed by the organism
alleles
different versions of the same gene
example of alleles
different alleles for different eye colours
what are the two possible types of alleles?
dominant and recessive
dominant allele
always expressed in the phenotype (represented by a capital letter)
what type of allele is always expressed in the phenotype and which type isn’t?
dominant is, recessive isn’t
recessive allele
only expressed in the absence of the dominant allele (represented by a lowercase letter)
alleles for the number of fingers gene
5, 6 (rare)
alleles for the freckles gene
present or absent
alleles for the rhesus blood group gene
positive, negative
what is the rhesus blood group gene?
gene that codes for a protein in the cell membrane of blood cells
how are some genes different to others expressed in the phenotype? give an example
some may be hidden and are part of our biochemical makeup so we have the test for them. for example, rhesus blood group
who was darwin influenced by for many of his ideas for the theory of evolution?
gregor mendel
why was it difficult for gregor mendel to come up with his laws?
the chromosomes hadn’t been discovered and nobody knew about meiosis etc
what did gregor mendel use to come up with his laws? how?
pea plants and he compared their colours and seed shapes to come up with the laws of inheritance
what type of laws did gregor mendel come up with?
the laws of inheritance
Mendel’s first law
an organisms characteristics are determined by factors which occur in pairs. only one of a pair can be found in a single gamete
Mendel’s second law
either member of an allelic pair may combine randomly with either of another pair
gene
a length of DNA on a chromosome normally coding for a specific polypeptide
locus
a specific position on a chromosome where a gene is located
alleles
different forms of the same gene
what is an allele always represented as?
a single letter
dominant allele and how its represented
will always be expressed in the phenotype when present
represented by a capital letter
recessive allele and how its represented
will be “hidden” when a dominant allele is present in the heterozygote. a recessive allele will only be expressed when it is homozygous
represented by a lower case letter
codominant
alleles that are equally expressed in a heterozygote (both versions of the gene expressed in the phenotype)
phenotype
the characteristic of an organism resulting from both its genotype (inherited) and the effects of the environment (its not just controlled by genes!)
what does a phenotype result from?
both an organisms genotype and its environment
genotype
the genetic make-up of an organism i.e - its alleles
always a pair of letters
what is a genotype always?
a pair of letters
homozygous
both alleles for a gene are identical
heterozygous
both alleles for a gene are different
F1
the first filial generation - the offspring of the parents in a genetic cross
F2
the second filial generation - the offspring of an F1 plant that is self-fertilised or a cross between twp members of the F1 generation
autosomes
chromosomes which are not sex chromosomes
sex chromsomes
chromsoomes which determine the sex of an individual organism (x and y)
what are the 2 types of sex chromosomes?
x and y
what are the two types of chromosomes?
autosomes
sex chromosomes
give an example and explain a codominant allele
snapdragons
by crossing a red flower with a white flower, a pink flower is obtained
therefore, both alleles have been expressed in the phenotype
homologous chromoeomes
carry the same genes at the same loci
what’s different between homologous chromosomes
there may be different alleles of the same genes
compare the dna from one chromatid to the other within one chromosome
the dna in each chromatid is identical
true breeding/pure breeding
homozygous for every gene
=offspring with the same phenotype
what happened when mendel crossed a pea plant which was true breeding for tall plants with a pea plant that was true breeding for short plants? what happened then when the F1 generation were crossed?
the f1 generation were all tall
the f2 generation showed a mix of tall and short plants
what are the steps to solving genetics problems?
- choose a letter to represent the alleles (Capital = dominant, lower-case = recessive)
- the phenotype is the characteristic you see
- the genotype is always a pair of alleles (two letters)
- the gamete is always a single allele (one letter)
if the parent phenotypes are tall x short, give an example of the parent genotype followed by the gametes
TT x tt
T and T x t and t
describe the phenotype of F1
100% the dominant phenotype
when can an f2 cross happen?
when an f1 plant is self-pollinated (Selfing)
what is the phenotype that is expressed in the F1 generation
dominant
the dominant what is always expressed in the f1 generation?
phenotype
how do we end up with a short plant (not the dominant phenotype)?
both alleles have to be recessive
describe f1 offspring that have the genotype Tt and explain why you say this
heterozygous
(have 2 different alleles)
monohybrid ratio of an f2 cross
3 dominant phenotypes: 1 recessive phenotype
ratio of tall to short plants in an f2 cross
3 tall : 1 short
probability of tall and short plants in an f2 cross
75% tall
25% short
what is mendel’s first law also known as?
the law of segregation
give mendel’s first law in terms of genes
the characteristics of an organism are determined by facrors (genes) which occur in pairs. only on member of a pair of factors (genes) can be represented in a single gamete
which stage of meiosis creates the law of segregation?
metaphase 1
why is it metaphase 1 of meiosis that creates the law of segregation?
since homologous chromosomes - and hence allele pairs - are separated in meiosis 1
what does the law of segregation not take into account?
crossing over - it assumes that the alleles stay on the same chromosomes
how is each allele for a trait packaged following meiosis 1?
into a separate gamete
what is packages into separate gametes following meiosis 1?
each allele for a trait
monohybrid inheritance
the inheritance of a single gene
monohybrid cross
the inheritance of a single characteristic
how many characteristics are passed on during each cross and how many did mendel focus on for his experiments?
thousands during each cross
mendel focused on the inheritance of single characteristic in some of his experiments
examples of monohybrid crosses
flower colour
seed colour
what did mendel follow 2 of in some of his experiments?
the inheritance of 2 different characters
dihybrid cross
the inheritance of 2 different characteristics
example of a dihybrid cross + explain
seed colour and seed shape
each of these 2 genes (the gene for colour and the gene for shape) have 2 different alleles
how are f1 when we self-pollinate and why?
heterozygous since they have the genotype of both parents
why are f1 heterozygous?
they have the genotype of both parents
what is the ratio we get every time when we cross two heterzygotes?
9:3:3:1
when do we always get the 9:3:3:1 ratio?
when we cross two heterzygotes
how come genes assort in the gametes independently in a dihybrid cross?
they are on different chromsomes and independent assortment during meiosis occurs
how do genes assort in gametes? use the example of YyRr assorting into gametes
YR, Yr, yR, yr
how do group alleles in a dihybrid cross table?
group alleles of the same gene together
how many different possible gametes do we need to get the correct ratio in a dihybrid cross and how many squares is this in a punnet square?
4 different gametes
16 square punnet square
what is mendel’s second law of inheritance also known as?
the law of independent assortment
where do we see the chromosomal basis of mendel’s law?
2nd law of inheritance
what is mendel’s 2nd law of inheritance the basis of?
independent assortment in meiosis
what can we have because of independent assortment?
can have any combination of 4 alleles which will be evenly distributed
how come we can have any combination of 4 alleles which will be evenly distributed?
because of independent assortment
how can we prove that f1 is heterozygous? explain
cross the heterozygous with a homozygous recessive
the ratio of phenotypes will be 1:1:1:1
if it were homozygous, all would appear as the dominant one does
how many gametes are there if there’s only 2 alleles?
one
how can we determine whether the results of a cross differ significantly from the expected results?
can perform a statistical test known as the chi^2 test
how do we work out the expected numbers for the chi^2 test?
1 - add up each value for each phenotype
2 - divide by 16 (the number of possible results to an F1 cross)
3 - multiply the answer by the 9:3:3:1 ratio
what type of answers should the expected numbers come out as during the chi^2 test?
a similar set of results to the actual number
chi^2 formula
x^2 = sum of (O-E)^2/E
what is a statistical test designed for?
to test a null hypothesis
what is the null hypothesis in genetics?
there is no significant different between the observed and expected results
what does the chi^2 test test?
the null hypothesis of genetics
what do we do once we have a value for chi^2?
refer it to the chi^2 table
what do we normally have a probability level of in genetics and what does this mean?
5%
we can be confident to a limit of 5% that the results will be expected
how do we use the probability level of 5% on the chi^2 table?
use 0.05
how do we determine which degrees of freedom to use on a chi^2 table?
number of different categories (phenotypes) - 1
how do we get a critical value on a chi^2 table?
wherever the probability of 0.05 and the correct degree of freedom meet
what is the name for the value where the probability of 0.05 and the correct degree of freedom meet on a chi^2 table?
the critical value
what do we use the critical value of a chi^2 table for?
to decide whether we accept or reject the null hypothesis
how do we know whether to accept or reject the null hypothesis?
if out chi^2 value is below the critical value (Even by a little bit), we accept the null hypothesis
How do we test whether a parent has the correct genotype for breeding?
Cross it with a homozygous recessive
What is the only variety that will produce offspring with the same genotype as both parents?
Homozygous recessive
In which situation would Mendel’s principle of independent assortment hold true for all dihybrid crosses?
If each chromosome only had one gene locus
Dihybrid
Genes on different chromosomes
When does Mendel’s 2nd law apply?
When we’re looking at genes that are carried on different chromosomes
Why is Mendel’s 2nd law not perfectly applicable to reality?
In reality, each chromosome carries many different genes and these are inherited together during meiosis
Describe the genes that are on the same chromosome
Linked
When will independent assortment not separate genes?
If the genes we’re studying are on the same chromosome (linked)
What do linked genes not undergo?
Independent assortment
When does no exchange occur between genes?
During linkage, when there are two genes on a single pair of homologues
How many gametes can we get if genes are on separate chromosomes v.s when they’re on the same chromosome?
Separate = 4 gametes
The same = 2 gametes
What are the only gametes that we can get with autosomal linkage?
Only double dominant or double recessive
Autosomes
Non-sex chromosomes
The condition where genes are close together on the same chromosome
Linkage
Why can we only have double dominant or double recessive due to autosomal linkage?
Since these genes can’t be separated since they’re on the same chromosome
What are linked in some pea plants?
Genes for flower colour and pollen grain shape are linked
What are genes carried on the same if they’re linked?
Same autosome
Phenotype ratio for linked genes
3:1
What can’t we obtain when genes are linked and are on the same chromosome?
The usual 16 square punnet and 9:3:3:1 ratio
When don’t we get the usual 16 square punnet and 9:3:3:1 ratio?
When genes are linked (carried on the same autosome)
With linked genes, if we were to cross breed the parents, would we get intermediate phenotypes forming in the F2 generation?
Yes
What would we get if we were to cross breed parents that have linked genes in real life?
Would actually get intermediate phenotypes forming in the F2 generation even though they’re on separate chromosomes
Why would we get intermediate phenotypes forming in the F2 generation even though they’re on separate chromosomes if we were to cross breed parents with linked gens?
Due to crossing over between the homologous chromosomes
What is the ratio when crossing parents with linked genes when considering crossing over?
Still not a 9:3:3:1 ratio - closer to 3:1
Why is the phenotype ratio still 3:1 in the offspring of parents with linked genes when we consider crossing over?
Only a few of the intermediates form Because crossing over between genes is rare
What will most of the gametes be when crossing parents with linked genes?
Parental gametes
What are genes likely to do if they’re together on a chromosome?
Travel through time like this
Why do only a few intermediates form during the cross breeding of two parents with linked genes?
Because crossing over between genes is rare
What will most F2 phenotypes be when crossing parents with linked genes be?
The parental type
What is the small % that aren’t the parental type when crossing parents with linked genes due to?
Autosomal linkage
When does crossing over occur during meiosis?
Prophase I
What happens during crossing over of meiosis?
Homologous pairs of chromosomes come together and crossing over may occur between chromatids at points called chiasmata
Points at which crossing over occurs on homologous pairs of chromosomes
Chiasmata
What can happen if crossing over occurs between two genes?
It separates alleles that were previously linked and allows them to combine in new recombinant genotypes
Genotypes formed after crossing over between genes
Recombinant genotypes
How do recombinant genotypes form?
When crossing over occurs between two genes, separating alleles that were previously linked and allowing them to combine in new recombinant genotypes
Recombinant
The offspring phenotypes that are not the parental phenotypes
The offspring phenotypes that are not the parental genotypes
Recombinants
What for during crossing over?
Chiasmata
Points at which crossing over occurs?
Chiasmata
What is a homologous pair of chromosomes?
A bivalent of chromosomes in pairs
What form during crossing over?
Chiasmata
What will gametes contain following crossing over?
The single chromosomes
When are genes less likely to form recombinant chromosomes?
When they’re closer
What are closer genes less likely to form?
Recombinant chromosomes
When is the frequency of recombination low?
When genes are closer together
What are there more of with further apart genes and why?
More recombinant gametes since there are more opportunities for crossing over
When are the more recombinant gametes due to more opportunities for crossing over?
When we have further apart genes
What do more recombinant gametes lead to?
More offspring with recombinant phenotypes
What are unexpected combinations of phenotypes due to?
Genes being linked
What is responsible for less frequent phenotypes?
Crossing-over
What do linked genes lead to?
Unexpected combinations of phenotypes
What is a common explanation for rejecting a null hypothesis in a genetics experiment?
Linked genes
What are linked genes a common explanation for?
Rejecting a null hypothesis in a genetics experiment
If we have a pattern that doesn’t follow the 9:3:3:1 ratio, what is likely to be down to?
Autosomal linkage
what is codominance?
Codominance refers to inheritance patterns when both alleles in a heterozygous organism are equally expressed
What is it called when we have inheritance patterns where both alleles in a heterozygous organism are equally expressed?
Codominance
Examples of codominance
Coat colour in horses and cattle
Blood groups in humans
Explain codominance in horses and cattle
Red cot colour is co-dominant to white coat colour. Animals that are heterozygous are roan-coloured (red hair interspaced with white hair - not pink hair)
Explain codominance in human blood groups
The blood group AB is the result of two alleles, A and B, both being equally expressed in the phenotype, neither being dominant to the other
Which alleles are expressed equally in the phenotype with human blood groups and why?
A and B since neither are dominant to the other (codominance)
Which human blood group is recessive?
O
What are the codominant human blood groups and what are the recessive blood groups in humans?
A and B —> codominant
O —> recessive
Possible genotypes of blood type A
AA or AO
Possible genotypes of blood type B
BB or BO
Genotype of blood type AB
AB
Genotype of blood type O
OO
What do the sex chromosomes comprise of for females?
2 X chromosomes
What do the sex chromosomes comprise of for males?
X and Y
How is the X chromosome different to the Y chromosome?
The X chromosome is much longer than the Y chromosome
Why are sex linkage conditions more common in males?
Because the X chromosome is much longer than the Y chromosome
Explain why sex linkage conditions are more common in males due to the X chromosome being much longer than the Y chromosome
Because the X chromosome is larger, it carries more genes
If there’s a defective gene on the X chromosome, there’s no corresponding gene on the y-axis to cancel it out - the deflective gene isn’t counteracted by another dominant gene
Where are sex linked recessive alleles carried?
On the X sex chromosome
What are carried on X chromosomes?
Sex linked recessive alleles
When a sex linked recessive alleles expressed in females?
If both X chromosomes carry the allele (homozygous recessive)
Why are sex linked recessive alleles always expressed in males?
Since the Y chromosome does not have a homologous locus for the gene - there’s no second allele to be dominant over it
Example of a sex linked condition
Haemophilia
What is haemophilia an example of?
A sex linked condition
What is the haemophilia gene?
An X-linked recessive gene
Give an example of an X-linked recessive gene
The haemophilia gene
Unaffected carrier of haemophilia mother genotype
X^HX^h
Haemophilia gene notation
X^h
Unaffected by haemophilia father genotype
X^Hy
Give the genotypes for the following (haemophilia):
Unaffected son
Unaffected daughter
Affected son
Unaffected carrier daughter
X^Hy
X^HX^H
X^hy
X^HX^h
If women have one dominant copy of the normal allele, are they affected by haemophilia?
No
What does it take for a male to be affected by haemophilia? Why?
Have one copy of the recessive gene
There’s nothing along the y-chromosome to cancel it out
Describe the percentages and ratios when crossing a normal male and a carrier female of haemophilia
25% carrier female
25% normal female
25% normal male
25% haemophiliac male
1:1:1:1
What is sex linkage different to?
Autosomal linkage
Explain what haemophilia is and what it does
Is an example of a sex linked condition
Affects the clotting of the blood
Blood doesn’t clot effectively with this genotype and there’s a risk of internal bleeding and bleeding around the joints
How can we show how a condition has been carried forward through the generations?
Use a pedigree chart
Pedigree chart
A chart that diagrams the inheritance of a trait or health condition through generations of a family
What did a pedigree chart prove for the royal family?
That haemophilia was carried down through the generations
What is the most reliable form of DNA evidence?
Mitochondrial DNA
Why is mitochondrial DNA the most reliable DNA evidence?
We inherit mitochondria from our mothers. It’s the mitochondria from the oocyte (not sperm) that ends up in the embryo.
Which mitochondria do embryos inherit?
The mitochondria from the oocyte, not the sperm
2 examples of sex linked conditions
Haemophilia
Duchenne Muscular Dystrophy (DMD)
What does DMD stand for?
Duchenne muscular dystrophy
What is DMD caused by?
A sex linked recessive allele of a gene that codes for the protein dystrophin
What is dystrophin?
A component of a glycoprotein that stabilises the cell membranes of muscle fibres
When do symptoms of DMD begin?
At a young age of around 2-3 years
Symptoms of DMD
Loss of muscle mass
Progressive muscle weakness
How is DMD inherited?
In the same way as haemophilia
Chance of having a son with DMD from a healthy father and a healthy carrier mother
1/4
What has been developed to help us treat DMD? Explain
Advances in gene therapy have meant that we can introduce healthy genes and molecular patches to repair the affected gene so that it produces a more healthy protein
What are the conditions that are not sex linked?
Autosomal
How are autosomal conditions different to sex linked conditions?
Will show a different pattern of inheritance
How can we indicate whether a condition is sex linked or autosomal and whether it is the result of a dominant or recessive allele?
Using pedigree diagrams/charts
How can you tell that a condition is sex linked from a pedigree diagram?
Since it will only occur in males, with the females either being carrier or unaffected
Describe the females in sex linked conditions
Carriers or unaffected
What type of condition only occurs in males?
Sex linked conditions
What are the two possible types of autosomal conditions?
Autosomal dominant
Autosomal recessive
Autosomal dominant condition
Only 1 copy of the harmful energy is needed to be affected
Autosomal recessive condition
Have to have 2 copies of the harmful gene to be affected by the condition
When is it not possible to determine the genotype of someone in the case of sex linked conditions? Why?
If a female child doesn’t have a child
We can’t determine whether they were homozygous and completely unaffected by the condition or were heterozygous (a carrier of the condition)
What was used to treat DMD before gene therapy?
Viruses
Problems with using a virus to treat a condition like DMD?
Cause immune responses against the virus
Virus can cause infection
Virus may affect other genes
Mutation
A change in the amount or the arrangement of the genetic material (DNA or RNA) in a cell
Describe mutations
Spontaneous random events
How are mutation rates usually?
Very low
In what situation are mutation rates higher?
In organisms with short life cycles and more frequent cell division, the rate of mutation is higher
What’s a good thing about mutations?
Mutations are the source of genetic variation which can result in evolution through natural selection
When exactly do most chromosomal mutations occur?
During crossing over prophase-I of meiosis or as a result of a non-disjunction during anaphase-I or anaphase-II of meiosis
Are mutations more likely in sperm cells or egg cells?
Sperm cells
Why are mutations more likely in sperm cells than in egg cells?
Since sperm cells are continuously produced during the life of the male
They’re therefore constantly dividing through mitosis and meiosis and mutations usually occur when DNA is copied for cell divisions
When do mutations usually occur?
When DNA is copied for cell divisions
At which stage of a male’s life are the chance of mutations even higher and why?
Older males
Since sperm cells are produced at an even higher rate than usually in older males, therefore there’s a higher chance of mutations
What’s more likely to mutate - DNA or RNA and why?
RNA
RNA is less stable than DNA
Why was Covid constantly mutating to produce new variants?
Since it was a virus and viruses only contain RNA, which is less stable than DNA and so is more likely to mutate
What is the only thing that viruses contain?
RNA (no DNA)
What does a mutation change in an organism?
The volume, arrangement or structure in the DNA of the organism
What can mutations affect?
A single gene or a whole chromosome
List the ways in which mutations can happen
Gene or point mutation
Chromosome mutation
Aneploidy
Polyploidy
What are the most common types of mutations that can happen?
Gene mutations
What is a gene mutation also known as?
A point mutation
What is a gene/point mutation?
A change in a single nucleotide (a single letter in the genetic code)
List the types of gene mutations
Addition (or insertion)
Subtraction (or deletion)
Substitution
Duplication
Inversion
Addition (or insertion) gene mutations
A base is added
What happens is an addition gene mutation happens in 3 places?
An extra amino acid is added to the polypeptide chain at translation
Subtraction (or deletion) gene mutations
A base is deleted
What happens if a subtraction gene mutation happens in 3 places?
The polypeptide has one fewer amino acids when translated
Substitution gene mutation
A different base is incorporated
Duplication gene mutation
The same base is incorporated twice
Inversion gene mutation
Adjacent bases on the same DNA strand exchange position
What is it called when Adjacent bases on the same DNA strand exchange position?
Inversion (type of gene mutation)
What types of gene mutations are frame-shift mutations?
Addition and subtraction point mutations
What are addition and subtraction point mutations examples of?
Frame shift mutations
What happens during frame-shift mutations?
All codons after the mutation will be affected
What happens as a result of frame-shift mutations?
All codons after the mutation will be affected so every amino acid in the polypeptide from the mutation onwards will be affected
Explain why frame-shift mutations can have big effects on the shape of proteins
In frame-shift mutations, all codons after the mutation will be affected. As a result, every amino acid in the polypeptide from the mutation onwards will be affected. Changing even one amino acid in the polypeptide can have a big affect on the final protein structure. For example, when going from one amino acid to a chemically different one, such as from polar to non-polar, it will have a big effect on the folding of the protein.
Why would changing an amino acid in a polypeptide sequence change the structure of a protein?
For example, when going form a polar to a non-polar amino acid, it will have a big effect on the folding of the protein
Explain how frame shift mutations occur during subtraction mutations
Since all of the bases shift to the left as we can’t have a gap
Explain how frame shift mutations occur during addition mutations
Since all of the bases move to the right from where the new bases is added, affecting every codon after the mutation
What are the 4 different effects of single base mutations in DNA code?
Silent
Missense
Nonsense
Read through
What do silent mutations usually involve?
The last letter of the codon
Explain what silent mutations are and how they come about
Since the genetic code is a degenerate code (more than one codon per amino acid), some mutations may not actually affect the amino acid formed. So, it’s a silent mutation since the same amino acid is generated.
How can the genetic code be described?
A Degenerate code
Why is the genetic code described as a degenerate code?
Since there’s more than one codon per amino acid
What is a silent mutation?
When the change in DNA from the mutation forms the same amino acid (since the genetic code is a degenerate code)
Missense mutations
A mutation that changes the amino acids that are incorporated into a protein
The mutation that changes the amino acids that are incorporated into a protein
Missense
Nonsense mutations
A mutation that introduces a stop codon into the genetic code and prevents a protein from being made completely. A shorter polypeptide forms as a result.
The mutation that introduces a stop codon into the genetic code and prevents a protein from being made completely
Nonsense
What forms as a result of a nonsense mutation?
A shorter polypeptide
Read through mutation
A mutation that converts a normal stop codon into an encoding amino acid, so the polypeptide gets too long
The mutation that converts a normal stop codon into an encoding amino acid, so the polypeptide gets too long
Read through
What happens as a result of a read through mutation?
The polypeptide gets too long
Mutation that ends up producing the same amino acid
Silent
What do all of the effects of single base mutations in the DNA code all change?
The nature of the polypeptide and the protein formed
What will an affected mother have in her genotype for a dominant autosomal condition?
Only one harmful gene
What will an affected mother have in her genotype for a recessive autosomal condition?
Two harmful genes
If the cross is autosomal recessive and both parents have the condition, describe the children
All affected
Why will not all of the children be affected by an autosomal dominant cross of two affected parents?
Since for the offspring we use the usual method where we need two copied of the harmful gene for the female to be affected, even if it’s a dominant autosomal cross initially
Why is a different protein formed when a mutation occurs?
Due to a change in the genetic code
Why are frame shift mutations often fatal for the cell?
The protein structure can change so much that it no longer functions
What are frame shift mutations fatal for?
The cell
When can homologous pairs of chromosomes form?
When you have even numbers of chromosomes
What can form once homologous pairs of chromosomes have formed?
Viable gametes
What are able to form if you have even numbers of chromosomes?
Homologous pairs of chromosomes
What do all mutations do and why?
Change the nature of the polypeptide and the protein formed due to a change in the genetic code
What type of mutation synthesises the dystrophin protein? How do you know this?
The nonsense mutation
It isn’t long enough
What is the issue when a nonsense mutation occurs in the dystrophin protein?
It isn’t long enough
What can be done about the distrophin protein not being long enough due to a nonsense mutation?
Gene therapy can lead to a ribosome making a longer protein
Which stage of protein synthesis is affected most by mutations?
Translation
What would happen if a mutation happened during transcription?
It wouldn’t occur again and again
Explain what will happen if DNA is changed by a mutation during replication
It will change the base sequence and this will be copied every time transcription happens into the mRNA. This will be copied into translation.
Where else can mutations happen aside from genes?
Introns
How come mutations in introns are more common than we think?
It would have no affect on the protein structure
Why do base sequences vary more in introns?
They are not always corrected by proof-reading systems
Mutations occur more frequently
Why do mutations occur more frequently in introns than in genes?
Since they’re not always corrected by proof-reading systems
What can mutations affect?
Protein synthesis
What can mutations affecting protein synthesis change?
The phenotype of an organism
Do all mutations affect the phenotype?
No
What do gene(point) mutations affect?
A single base in a gene
What do chromosomal mutations affect?
Many genes
2 words to describe mutations
Random
Spontaneous
What do mutagens do?
Make mutations more likely
Whatis increased by mutagens?
The rate of mutation
Give examples of mutagens
Ionising radiations - gamma radiation, UV, X-rays
Certain chemicals - polycyclic hydrocarbons in cigarette smoke, chemicals in air pollution
Explain why mutagens increase the rate of mutation
Since they damage the DNA, and as the cell repairs the DNA, mutations can happen. The more damage to DNA caused by mutagens the more likely mutations are to happen.
Carcinogens
Mutagens that increase the likelihood of the development of cancer by affecting genes that lead to the formation of cancers
What does cancer rise from?
DNA mutations in cells
How do carcinogens increase the likelihood of the development of cancer?
By affecting genes that lead to the formation of cancers
What can cause DNA mutations?
Heredity
Radiation or chemicals
Spontaneous errors during DNA duplication
Cancer
Cells which divide uncontrollably through mitosis and form a tumor
How does a cancer spread?
The cells can break away and travel to other parts of the body via the lymphatic system
What happens every time a cancerous cell divides?
Forms genetically identical offspring
What has to occur for a normal cell to turn into a cancerous cell?
A mutation has to occur in specific genes that are involved in cell division, which are tumor suppressor genes and proto-oncogenes
Types of genes that a mutation has to occur in in order for a normal cells to turn into a cancerous cell
Tumor suppressor genes
Proto-oncogenes
What do tumor suppressor genes do?
Code for proteins that stop the cell from dividing in an uncontrolled way
What has to happen to tumor suppressor genes for a cancer to form?
Have to be switched off
What has to happen to proto-oncogenes for a cancer to form?
Be switched on
What happens when proto-oncogenes are switched on and what does this do?
The proto-oncogenes become oncogenes, which stimulate the cell to divide in an uncontrolled way
How do oncogenes form and what do they do?
When proto-oncogenes are switched on, they form oncogenes
Oncogenes stimulate the cell to divide in an uncontrolled way
Which two conditions have to be met for a cell to become cancerous?
Both the tumor suppressor genes need to be switched off and the proto-oncogenes need to be switched on within the same cell
Within what do both the tumor suppressor genes need to be switched off and the proto-oncogenes need to be switched on for a cancerous cell to form?
Within a cell
What do we need a mutation in both of for a cancer to form?
Tumor suppressor genes
Proto-oncogenes
Are very specific conditions needed for a normal cell to turn into a cancer cell?
Yes
What happens once a cancer cell has formed?
Uncontrolled growth
What do carcinogens make more likely to occur?
Mutations in tumor suppressor genes and proto-oncogenes
What makes carcinogens different from mutagens?
They specifically effect tumor suppressor genes and proto-oncogenes
What are mutations individual to?
The individual person
How do we attempt to treat an individual with cancer effectively?
Samples are taken and DNA is sequenced so that treatments can be targeted for that individual to be treated effectively
What type of condition is sickle-cell anaemia?
Genetic
What does sickle-cell anaemia lead to the formation of?
Sickle shaped red blood cells
Why does sickle-cell anaemia lead to the formation of sickle-shaped red blood cells?
Due to the structure of the haemoglobin within them
What mutation causes sickle-cell anaemia?
A mutation of the gene which codes for one of the polypeptides in the haemoglobin molecule
When do the red blood cells change shape for someone with sickle-cell anaemia?
Wherever there’s low O2 levels (low O2 partial pressures in a tissue)
What shape are normal red blood cells?
Biconcave disc shaped
Explain why sickle-cell anaemia is problematic
Capillaries in tissues are narrow, and regular red blood cells can only just fit
When they have a sickle shaped they’re less flexible and so don’t fit as easily in the capillaries
They therefore get stuck, and this affects circulation, including to major organs
Why don’t sickle shaped red blood cells fit easily in capillaries?
Less flexible than with the biconcave disc shape and the capillaries are very narrow anyway
When are red blood cells sickle shaped for anyone with sickle cell anaemia?
In any tissue where O2 levels are low
What type of protein is haemoglobin?
Quaternary
How many polypeptides is haemoglobin made up of? Why?
4
It’s a quaternary protein
How many genes is one polypeptide coded by?
1
What is the cause of sickle-cell anaemia?
Single base (point) mutation in the gene which codes for beta-haemoglobin (a polypeptide)
The gene for which polypeptide in haemoglobin undergoes a mutation for someone with sickle-cell anaemia?
Beta-haemoglobin
What does the mutation in beta haemoglobin’s gene lead to?
A change in the codon in mRNA and leads to a change in the amino acid in the polypeptide
What leads to a change in the amino acid in the polypeptide of beta haemoglobin?
A mutation that changes the codon in mRNA and leads to a change in the amino acid in the polypeptide
Which amino acid changes to what for someone with sickle-cell anaemia?
Valine (val) replaces glutamine (glu)
Give an example of how a single letter change in a DNA sequence can have a big effect
When a single base mutation occurs in the gene which codes for beta-haemoglobin in haemoglobin and the amino acid changes from glutamine to valine
What happens to haemoglobin because of the mutation in the beta haemoglobin polypeptide?
The haemoglobin molecule straightens out in low O2 levels instead of being globular
What is haemoglobin doing to change shape for someone with sickle-cell anaemi?
The haemoglobin molecule straightens out instead of being globular
Explain why exactly the red blood cell changes shape for someone with sickle cell anaemia
As (val) has replaced (glu), the molecule is less soluble in water (as valine is non-polar) and this is why it stretches out, making the red blood cells elongated and rigid
Why is haemoglobin less soluble in water for someone with sickle cell anaemia?
As valine is non polar
Which amino acid is non-polar and what does this do to someone with sickle cell anaemia?
Valine
It makes the molecule less soluble in water
Describe red blood cells after they’re stretched out (sickle cell anaemia)
Elongated and rigid
What causes red blood cells to be stretched out (sickle cell anaemia)?
As (val) has replaced (glu), the molecule is less soluble in water (as valine is non-polar), so the molecule stretches out
How is the mutation for sickle cell anaemia obtained?
It’s inherited
How is the mutation for sickle cell anaemia inherited?
It leads to the formation of a recessive allele
What is the probability of having sickle-cell anaemia with 2 heterozygous parents
1/4
Can someone be affected by sickle cell anaemia with a heterozygous genotype?
They can be slightly affected, with some haemoglobin being impacted
Which part is the world is having sickle cell anaemia quite common? Why was this organically quite strange to understand?
In areas where malaria is common
Malaria is an infection, sickle cell is a genetic disease
Why is it advantageous to have sickle cell in areas where malaria is common?
Individuals have a lower parasite count with sickle-cell since the sickle cell makes the malaria parasite less likely to reproduce
What is the ideal scenario in terms in sickle cell anaemia in areas where malaria is common and why? What is the problem with the alternatives?
Being heterozygous (Aa) for sickle cell since it gives resistance to malaria without suffering from sickle cell anaemia
Homozygous dominant would make the individual more likely to catch malaria
Homozygous recessive would mean that, although the individual has protection from malaria, they have sickle-cell disease
Give an example of when a mutation can give a selective advantage in an environment and is an example of natural selection
Being heterozygous for sickle cell anaemia in areas where malaria is common
Chromosome mutations
Changes in the structure or number of chromosomes in cells. Happens on many genes on that section of chromosome.
Explain how a mutation occurs and leads to a change in structure of a chromosome (chromosome mutation)
During prophase I of meiosis, homologous chromosomes pair and exchange material at chiasmata
Mutation arises when a chromosome does not rejoin accurately at the corresponding position on its homologous partner
Why would a chromosome not rejoin accurately at the corresponding position on its homologous partner? (Chromosomal mutations)
Could be due to…
-deletion of part of a chromosome
-duplication of part of a chromosome
-inversion of part of a chromosome
What type of chromosomal mutations only involve mutations on part of the chromosome?
Changes in structure
What are mutations that cause changes in chromosome shapes as a result of?
Mistakes in crossing over during prophase I of meiosis
What happens when mistakes occur in crossing over during prophase I of meiosis (chromosomal mutations)?
The homologous chromosomes, and therefore the gametes they are in, end up with some different genes
What happens as a result of changes in chromosomal structure (chromosomal mutations)?
Each gamete may still fuse with another and produce a new individual, but further meiosis will be impossible as the mutant chromosomes will not be able to make homologous pairs at meiosis
When are whole chromosomes involved and when are only parts of a chromosome involved in terms of chromosomal mutations?
Whole chromosomes —> changes in numbers of chromosomes
Parts of chromosomes —> changes in structure of chromosomes
When are changes in chromosome number most likely to occur? Specifically?
During meiosis
When homologous chromosomes separate at anaphase I or when chromatids separate at anaphase II
What is it called when a faulty spindle results in the chromosomes not being shared equally between the daughter cells?
Non-disjunction
What does a faulty cell division due to a mutation lead to?
One of the daughter cells receives two copies of a chromosome while the other gets none
What is Down’s syndrome caused by?
Chromosomal non-disjunction
Explain how Down’s syndrome comes about
It occurs due to the failure of one homologous pair of chromosomes to separate during the first meiotic division (chromosomes not pulled apart during anaphase I)
There is a third copy of chromosome 21 on the Karotype
Which chromosome is there a third copy of for someone with Down’s syndrome?
Chromosome 21
Aneuploid cells
Cells with an extra copy or copies of one chromosome
Cells with an extra copy or copies of one chromosome
Aneuploid cells
What happens when non-disjunction happens during oogenesis (Down’s syndrome)?
A secondary oocyte has either no chromosome 21 (which can’t produce a viable embryo) or has 2 copies instead of 1 (which is trisomy 21 once the secondary oocyte with 2 copies has fused with a viable sperm = person with Down’s syndrome)
What is it called when a secondary oocyte with 2 copies of a chromosome fuses with a viable sperm?
Trisomy
What’s wrong with a secondary oocyte with no chromosome 21?
Can’t produce a viable embryo
Monopsony
Refers to a missing chromosome from the typical diploid set
Trisomy
Refers to an extra chromosome
Example of trisomy
Down’s syndrome
Give and explain an example of monosomy
In terms of sex gametes, you could get XO (only 1 X chromosome)
Can you get XXY and XYY?
Yes
What gender would the genotype XO (monosomy) be and why?
Still be female since there’s no Y chromosome
Are chromosomal mutations more common in sex chromosomes or autosomes? Why?
More common in sex chromosomes since they have less affect on the general health of the person
What is the cause of trisomy and monosomy?
Nondisjunction
Nondisjunction
A mutation that occurs during meiosis I
Homologous chromosomes do not separate, which produces gametes with extra and missing chromosomes
When does nondisjunction occur?
During meiosis I
How many chromosomes does nondisjunction affect in humans?
Just 1
Describe nondisjunction in meiosis I
Pair of homologous chromosomes fails to separate
Describe nondisjunction in meiosis II
Pair of sister chromatids fails to separate
A mutation that occurs during meiosis I. Homologous chromosomes do not separate, which produces gametes with extra and missing chromosomes.
Nondisjunction
What’s the difference when it’s during the second meiotic division that chromatids don’t separate compared to when the homogenous chromosomes during meiosis I fail to separate (non disjunction)?
There will be fewer affected gametes when it’s the chromatids that don’t separate
How many chromosomes does chromosomal Nondisjunction affect in some species?
Entire sets
Polyploidy
Have several sets of chromosomes
Policy
Number of chromosome sets that an individual has
List some types of polyploidy
Haploid (n)
Diploid (2n)
Triploid (3n)
Tetraploid (4n)
Example of haploids
Normal gametes
Example of diploid
Our body cells
Example of triploid
Endosperm in the seed
Example of tetraploid
Wheat
List and explain the different ways in which polyploidy can arise
Spindle defect - this leads to chromosomes failing to separate during anaphase I. The resulting gamete will have two copies of each chromosome instead of one. If this diploid gamete is fertilised by a normal haploid gamete, a triploid (3n) zygote results
If two diploid gametes fuse, a tetraploid (4n) zygote is produced
Endomitosis - when the chromosomes are replicated but cytokinesis does not occur, leading to tetraploid cells being formed
Endomitosis
When the chromosomes are replicated but cytokinesis does not occur, leading to tetraploid cells being formed
What is polyploidy common in?
Flowering plants
Explain how a spindle defect can lead to polyploidy
This leads to chromosomes failing to separate during anaphase I. The resulting gamete will have two copies of each chromosome instead of one. If this diploid gamete is fertilised by a normal haploid gamete, a triploid (3n) zygote results
What is polyploidy associated with?
Beneficial characteristics such as vigour and disease resistance
Examples of polyploidy plants
3n seedless water melons
4n potatoes
6n wheat bread
8n strawberries
Is polyploidy commoner in plants or animals?
Plants
Why is polyploidy much commoner in plants than in animals?
Because plants can reproduce asexually and are often hermaphrodite so do not use chromosomes to determine their sex
Hermaphrodite
Do not use chromosomes to determine their sex
How do proto-oncogenes become oncogenes?
Mutate to them
What would happen if tumour suppressor genes were switched off in all cells of the body? Explain
It leads to an increased risk of a tumour forming in various places
Risk of this being passed on to the next generation (since it’s in all cells, it involves gametes too)
Why does tumour suppressor genes being switched off in all cells of the body lead to the risk of it being passed onto the next generation?
Since its in all cells, it involves gametes too
What does conventional genetics show us?
That we inherit genes from our parents
What are other things that can affect the phenotype of an organism apart from the genes inherited from parents?
Environment and lifestyle
Can aspects in our environment and lifestyle that affect our phenotype be inherited?
According to epigenetics, yes
What could the lifestyle our parents had influence?
The genes that have been passed on to the next generation
What is epigenetics?
The study of heritable changes in gene expression without a change in DNA sequence
The study of heritable changes in gene expression without a change in DNA sequence
Epigenetics
What’s changes and what’s not with epigenetics
Change the phenotype without changing the genotype
What determines which genes are turned off and which are turned on?
A persons DNA sequence
But also there’s another level of genetic control
Epigenome
The control system that decides which genes are switched on/off in each cell
What is the same in each of the cells in our body?
The genome
What’s different in different cells of the body even though the genomes are the same?
Different epigenomes
Are all embryo cells the same?
Yes , they’re genetically identical
Why are embryo cells genetically identical?
Since they developed through mitosis
How does a cell “know” if it’s a liver cell or skin cell or another cell?
Not every gene is expressed in every cell
How are genes activated and deactivated in a cell?
DNA interacts with molecules in cells which can activate and deactivate genes
Give an example of genes being switched on in some cells and switched off in others
Pancreas cells secrete protease. This would be pointless in any other of the body cells, so the gene is switched off in all cells except for those in the pancreas.
When is it possible to alter DNA with epigenetics?
After it is replicated
When DNA is altered after its been replicated by epigenetics, what does it not change and what does it change?
Does not change the base sequence of the DNA
Does interfere with the process of transcribing the gene during protein synthesis
What does altering DNA after replication not change?
The base sequence of the DNA
What does altering DNA after it’s been replicated interfere with?
The process of transcribing the gene during protein synthesis
What causes epigenetic changes?
Increasing evidence that the environment can cause these changes
What are epigenetic modifications caused by?
The epigenome
Two main types of epigenetic modification
DNA methylation
Histone modifications
What are both DNA methylation and histone modifications examples of?
Epigenetics modifications
What is DNA methylation a way of doing?
Switching off genes
What does DNA methylation do to genes?
Switches them off
What happens during DNA methylation?
Methyl group added
What does a gene still do when it’s not expressed?
It’s still used to make the protein and goes through transcription and translation etc, its just not expressed
Where are methyl groups added during DNA methylation?
To an organic base
What does adding a methyl group to an organic base do to it?
Interferes with its ability to form bonds with complementary bases
What does DNA methylation do in terms of expression of the gene?
Prevents it
Why does DNA methylation prevent the expression of genes?
Adding a methyl group to an organic base can interfere with its ability to form bonds with complementary bases and can therefore prevent the expression of the gene
What happens to the ability of a cell to transcribe the gene with a methylated base?
It’s impaired
What is impaired when a cell has a methylated base?
The ability of the cell to transcribe the gene
When is the ability of a cell to transcribe a gene impaired?
With a methylated base
What makes a gene less likely to be expressed?
More methyl groups
What does more methyl groups do to a gene?
Makes it less likely to be expressed
What is DNA within chromosomes wrapped around and what does this form?
Around histone proteins
Forms chromatin
When does histone modification occur?
Following translation
Give the ways in which histone modification occurs
By attaching…
An acetyl group (-CH3CO) to the amino acid lysine
A methyl group (-CH3) to lysine or arginine
A phosphate group (-PO4^2-) to serine or threonine
Acetyl group
-CH3CO
-CH3CO
Acetyl group
Phosphate group
-PO4^2-
PO4^2-
Phosphate group
What are histone proteins used for?
Organising the DNA in a chromosome
What are responsible for organising the DNA in a chromosome?
Histone proteins
What happens if histone winds tighter?
May inhibit gene expression
How is gene expression inhibited in terms of histone?
When it winds tighter
What happens when histone winds more loosely?
It can increase gene expression
When is gene expression increased by histone?
When it winds more loosely
What happens to histone when the different groups mentioned are added to it?
It’s wound less tightly
What happens to DNA when histone is wound less tightly?
DNA is less tightly coiled
What can e done when DNA is less tightly coiled?
RNA polymerase is able to access the gene and it can be transcribed
What happens when DNA is wrapped tightly around the histone?
It’s harder to read and therefore the gene is less likely to be expressed
When is the gene less likely to be expressed in terms of histone?
When DNA is wrapped tightly around the histone
How do we get different expression of the same gene in different parts of the same organism?
Different epigenetic modifications can occur in cells of the same tissue and in different tissues
Stem cell
An undifferentiated cell which can develop into any type of cell in the body
Undifferentiated cell which can develop into any type of cell in the body
Stem cells
What do the stem cells of an embryo do?
Progressively differentiate, switching off genes coding for enzymes that are not needed
Which cells of an embryo progressively differentiate and how?
Stem cells
By switching off genes coding for enzymes that are not needed
What are the only genes that differentiated cells express?
The genes that are necessary for their own activity
What type of cells only express the genes that are necessary for their own activity?
Differentiated cells
Examples of differentiated cells only expressing the genes that are necessary for their own activity
Skin cells produce melanin but retinal cells produce Rhodopsin
Where are retinal cells?
At the back of the eyes
Cells at the back of the eyes?
Retinal cells
Purpose of melanin
Gives in its colour and protects from UV rays
What gives skin its colour?
Melanin
What does rhodopsin do?
Responds to different light intensities
What is it that retinal cells produce that responds to different light intensities?
Rhodopsin
How many decimal places do we quote a chi^2 value to?
Match the number of decimal places used on the chi^2 table
