Inheritance Flashcards
1
Q
Define gene
A
A sequence of DNA that codes for a polypeptide and which occupies a specific locus on a chromosome.
2
Q
Define allele
A
a variant nucleotide sequence for a particular gene at a given locus which codes for an altered phenotype.
3
Q
What is a locus?
A
a site on a chromosome
4
Q
Describe the chromosomes in a diploid individual
A
- it has one of each chromosome from each parent, therefore one copy of each gene from each parent.
- if the genes are the same from both parents the individual is homozygous, but if they’re different they are heterozygous.
5
Q
What is the genotype of an individual?
A
All the alleles they contain.
6
Q
What is the phenotype off an individual?
A
The characteristics displayed by the individual- their appearance but also features that can’t be seen like their blood group.
The genotype and the environment determine the characteristics of an individual so the phenotype is best thought of as an expression of the genotype in a specific environment.
7
Q
When an allele is always expressed if present it is described as…
A
dominant
8
Q
When an allele is only expressed when it is homozygous it is described as…
A
recessive
9
Q
Define monohybrid inheritance.
A
The inheritance of a single gene, such as that controlling plant height or seed colour.
10
Q
Gregor Mendel conducted his experiments using garden peas, why was this a good plant choice?
A
- they’re easy to grow
- the flowers can self and cross fertilise
- they make flowers and fruit in the same year
- they make large numbers of seed from each cross- this makes their numbers statistically significant
11
Q
Gregor Mendel conducted his experiments using garden peas and observed the characteristic of height and round/wrinkled seeds to study, why was this a good choice?
A
These characteristics are…
- controlled by single genes
- controlled by genes on different chromosomes
- clear-cut and easy to tell apart
12
Q
A diagram of genetic cross shows…
A
- the generation- F1 or F2
- the genotypes of parents and their offspring
- the phenotypes of parents and their offspring
- alleles present in the gametes
- the symbols for the alleles are defined
13
Q
What do F1 and F2 stand for?
A
F1- first filial generation- the offspring of the parents of the cross
F2- second filial generation- the offspring of the members of the F1 generation
14
Q
What does filial mean?
A
relating to a son or daughter
15
Q
Define test back/back cross.
A
A cross between an individual with the phenotype of the dominant characteristic, but unknown genotype, with an individual that is homozygous recessive for the gene in question.
16
Q
Why is a test cross done?
A
If an organism has a dominant characteristic it could be homozygous dominant or heterozygous. The test cross shows if a dominant characteristic is determined by one or two of the alleles.
17
Q
What is co-dominance?
A
When genes are co-dominant both alleles in a heterozygote are expressed individually.
18
Q
Give two examples of Co-dominance.
A
Example 1:
Chickens with white feathers and those with black can produce speckled offspring as neither allele is dominant.
Example 2:
In the human ABO blood group system the I gene has three alleles IA, IB and IO. Two homozygous parents, one with the genotype IAIA and one with the genotype IBIB can have offspring with the genotype IAIB on their red blood cells so that they are blood group AB. This means they have both A and B antigens on their red blood cells, which is an example of co-dominance.
19
Q
What is incomplete dominance?
A
For some genes the phenotype of the heterozygous is an intermediate between the two parental phenotypes, rather than them both being expressed individually.
20
Q
Give an example of incomplete dominance.
A
When white-flowered carnations are crossed with red flowered carnations their F1 generation has pink flowers. This is incomplete dominance.
21
Q
When completing a punnet square when incomplete dominance is applied what symbols are used?
A
Neither allele is dominant so they are not given an upper case and lower case letter, instead they have a symbol as follows…
C^R for red
C^W for white
(in terms of carnations)
22
Q
What is mendel’s first law of inheritance?
A
The law of segregation- the characteristics of an organism are determined by factors (alleles), which occur in pairs. Only one pair is present in each gamete.
23
Q
Define independent assortment.
A
Either of a pair of homogenous chromosomes moves to either pole at anaphase I of meiosis, independently of the chromosomes of other homologous pairs.
24
Q
What is dihybrid inheritance?
A
The simultaneous inheritance of two unlinked genes i.e. genes on different chromosomes.
25
If one plant is dominant for two characteristics that are unlinked like RRYY and the other is homozygous recessive rryy then what gametes are produced by the parents?
What would be the genotype produced by these gametes?
If you then bred using the the F1 generation what gametes would be produced by this generation?
For RRYY the gamete is RY
For rryy the gamete is ry
Only one genotype would be produced from a cross between these two...
RrYy
The gametes of the F1 generation would be...
RY, Ry, rY, ry
26
How do you calculate progeny?
The total number is divided by the number of homozygous recessive individuals.
27
What phenotypic ratio would arise from a dihybrid cross of two individuals heterozygous for both genes?
9:3:3:1
28
What is Mendel's second law of inheritance?
Either one of a pair of contrasted characters may combine with either of another pair- each member of a pair of alleles may combine randomly with either of another pair on a different chromosome.
29
Describe a dihybrid test cross and what it does.
- The dihybrid test cross tests a genotype by crossing it with a with an individual that is recessive for both genes.
- The ratios of phenotypes in the progeny indicate the genotype of the parent.
30
What is meant when alleles are said to be linked?
The alleles of two genes that are on the same chromosome cannot segregate independently i.e. they cannot move to opposite poles of the cell at meiosis, they must move together being on the same physical structure.
31
What is meant by when a combination of characteristics is described as parental?
When certain traits occur together due to linkage.
32
What can allow for alleles that were on the same chromosome to be separated/swapped?
Crossing over
33
What impacts the chance of crossing over.
- Crossing over is a rare event and does not happen in most cells.
- The further apart two genes are on a chromosome the more opportunity there is for crossover to occur between them.
- This leads to more recombinant gametes and therefore more offspring with recombinant phenotypes.
34
If the number of progeny with different combinations of characteristics do not correspond to Mendelian ratios what is likely about the genes involved?
The genes are linked.
35
How can the crossover value be calculated?
(number of recombinants/number of progeny) x 100
36
Why are chi^2 tests used?
To test if the numbers of different phenotypes are close enough to predict the values to supports the genetic explanation of how they arose. If the numbers are not close enough the test tells us they have arisen for another reason.
37
What is the null hypothesis in statistical tests?
It is a statement that there is no difference between the observed and expected results of a cross. In genetics a null hypothesis states that the observed results are due to Mendelian inheritance and that any deviation from it is due to chance.
38
Mendelian inheritance is based on there being no difference in...
- the number of different types of gametes
- the probability of each gamete type fusing with another type of gamete
- the viability of the embryos, whatever their genotype
- that the genes are not linked
39
How do you calculate X^2?
X^2 = Σ((O-E)^2/E)
40
In the following formula...
X^2 = Σ((O-E)^2/E)
What does E stand for and what does O stand for?
O - observed value
| E - expected value based off the mendelian ratio
41
How doe you determine E in the following formula?
| X^2 = Σ((O-E)^2/E)
The total number of organisms split into the ratio 9:3:3:1.
42
What are degrees of freedom?
- This is a measure of the number of values that can vary independently.
- In the analysis of monohybrid and dihybrid crosses it is one less than the number of classes of data.
43
What is the significance level used in biology and explain what a value above and below this value would mean.
The significance level used is 5%.
- A value of this or greater tells us the biological reasoning is correct and any deviation from the exact prediction is not significant and is due to chance.
- If the value is below this is significant as the assumption on which the prediction was made is not correct so there must be a different explanation for the phenomenon.
44
What two pieces of information do you need to determine the probability deviation is due to chance?
Chi^2 and the degrees of freedom
45
How would you construct a conclusion from a Chi^2 test.
- compare your calculated X^2 value to the critical value
- state what probability it's equivalent to- is it greater of less than 5%
- state is the null hypothesis is accepted or rejected and at what significance level
- if you accept the null hypothesis state the inheritance is Mendelian and any deviation from the predicted ratio is due to chance, but if you reject it do not state this instead say there must some other explanation for the data.
46
What are monoecious plants?
They have separate male and female flowers on the same plant.
47
What are dioecious plants?
Plants that have separate male and female individuals.
48
What are hermaphrodites?
Angiosperms thats flowers make both pollen and ovules.
49
Name two animals that are considered hermaphrodites.
- the garden snail
- the earthworm
This is because it is hard for them to find a mate because it's difficult to communicate through soil.
50
What factors can impact if an individual animal is male or female?
- Temperature- lizard and crocodile eggs hatch as male when the temperature is above 32℃ and female when below that temperature- tho opposite applies to sea turtles.
- Sequential hermaphroditism- the common limpet makes stacks of individuals, those at the top are males and as more males join the stack, those below them become females.
- The male sledge worm can become hermaphrodite if females are not present.
- Hierarchy- when the dominant female dies the dominant male changes sex and takes her place.
- Ploidy level- bee and grass hopper eggs that are not fertilised are haploid and develop as males.
- Chromosome structure- In mammals females have two X chromosomes, whereas males have an X and a Y chromosome. The reverse occurs in other species.
51
What is the arrangement of homologous pairs in decreasing size order called?
Karyotype.
52
The 22 pairs of chromosome that have identical genes are known as?
Autosomes.
53
When the sex chromosomes are different sizes, what are they called?
Heterosomes.
54
What is PAR1 and PAR2.
- The pseudoautosomal regions on the human X and Y chromosomes.
- The two regions on these chromosomes that are homologous so can pair with each other at meiosis.
- They reflect the evolutionary loss of genes from one sex chromosome leaving the larger X and smaller Y that exist now.
55
What is meant by the homogametic sex?
- The gametes are identical with respect to the sex chromosome.
- In mammals all the female secondary oocyte contain an X chromosome hence they are known as the homogametic sex.
56
What is meant by the heterogametic sex?
- Gametes are of different types with respect to the sex chromosomes.
- In male mammals at meiosis I an X chromosome passes into one secondary spermatocyte and a Y into another, hence half a male's sperm contain X chromosomes and half contain Y chromosomes.
57
Why is there an equal chance of a foetus being male or female?
- An oocyte may be fertilised by either an X or Y carrying sperm with an equal probability.
58
What is Haemophilia and why is it known as a sex linked condition?
- Haemophilia is a potentially lethal condition occurring when an individual cannot produce enough of one of the 13 blood clotting proteins.
- It causes blood to clot slowly or not at all.
- It is an X-linked gene that codes for the blood clotting protein.
- The recessive gene results in haemophilia, hence there are only two possible genotypes for males and three for females (as they can be a carrier due to having two X chromosomes, compared to men that only have one), meaning men are more likely to suffer from this condition.
- Due to men only needing one mutant allele and so it being more common for them, the condition is described as sex linked.
59
Define sex linkage.
A gene is carried by a sex chromosomes so that a characteristic it encodes is seen predominately in one sex.
60
Define a carrier.
A phenotypically normal female with one normal, dominant allele and one mutant, recessive allele.
61
Who do sons get their X chromosome from?
Their mother as their father provides the Y. Whereas in daughters they get an X form each.
62
What is DMD?
- DMD- Duchenne muscular dystrophy.
- caused by an X-linked recessive allele of the dystrophin gene.
- The gene codes for the protein dystrophin, which is a component of a glycoprotein that stabilises the cell membranes of muscle fibres.
- Symptoms include the loss of muscle mass and progressive muscle weakness.
63
Why are pedigree diagrams useful?
- The inheritance of a medical condition marked onto a family tree allows geneticists to infer an explanation of how it is inherited.
- The pedigree diagram can indicate whether the condition is sex linked or a result of a dominant or recessive allele.
64
What is a mutation and what are the two features of mutations?
A mutation is a change in the amount, arrangement or structure in the hereditary material of an organism- either DNA or RNA.
They are considered...
- spontaneous- may happen without apparent cause
- random- appear to happen with an equal probability anywhere in the genome of diploid organisms
65
Where do mutations occur?
They can occur in all cells, but are only inherited if they occur within the gametes.
66
Are mutations good or bad?
Most are harmful, however there are very rarely good ones that provide a selective advantage.
67
What organisms show a greater rate of mutation?
Mutations can occur during DNA replication, prior to cell division so in general organisms with short life cycles and frequent meiosis show a greater rate of mutation than others.
68
What can mutation rates be increased by?
- Ionising radiation like gamma rays, X-rays and ultra-violet light.
- Mutagenic chemicals like the polycyclic hydrocarbon in cigarette smoke, methanal and mustard gas.
69
How does ionisation radiation ike gamma rays, X-rays and ultra-violet light cause mutations?
- They are mutagenic as they are of the wavelength that DNA absorbs.
- Radiation joins adjacent pyrimidine bases in a DNA strand so that at replication DNA polymerase may insert incorrect nucleotides
70
How do mutagenic chemicals cause mutations?
Some chemicals such as acridine are mutagenic as they have flat molecules which can slide between base pairs in the double helix, this prevents DNA polymerase inserting the correct nucleotide at replication.
71
Name the four ways mutations can occur.
- gene/point mutation- DNA is not copied accurately in S phase before cell division- can involve one or more bases.
- chromosome mutation- chromosomes get damaged and break- they can repair themselves but may not do so correctly, so may alter their structure
- aneuploidy- a whole chromosome may be lost or added- occurs during a phenomenon called non-disjunction
- polyploidy- the number of chromosomes may double if the cell fails to divide following the first nuclear division after fertilisation.
72
What is non-disjunction?
A faulty cell division in meiosis following which one of the two daughter cells receives two copies of a chromosome and the other receives non.
When chromosomes fail to separate to the poles of deciding cells at anaphase I or when chromatids fail to separate anaphase II.
73
What is a gene/ point mutation and how does it occur?
DNA is not copied accurately in S phase before cell division causing changes in the base sequence- can involve one or more bases.
This can occur through five different means...
- addition- a base is added and if this occurs in three different places an extra amino acid id produced
- subtraction- a base is deleted and if this occurs in three different places one fewer amino acid is produced
- duplication- the same base is incorporated twice
- substitution- a different base is incorporated
- inversion- adjacent bases on the same DNA strand exchange position
74
In what was can a point mutation effect the polypeptide produced at translation?
- The new codon produced may code for the same amino acid or a similar one so that there is no change to the polypeptide- this is a silent mutation
- If an amino acid with a similar chemical nature is substituted the effect may be small.
- If the mutation is at a significant site in the protein molecule it may make a significant difference to the activity of the protein.
75
How does sickle cell anaemia occur?
- a substitution point mutation in the gene producing the B polypeptide of haemoglobin results in sickle cell anaemia
- The DNA triplet CTC codes for glutamate, but when T is replaced with A valine is coded for
- glutamate is large and hydrophilic, whereas valine is small and hydrophobic.
- when the oxygen tension is low, the cell membrane collapses on the precipitated haemoglobin and the red blood cell becomes sickle shaped - the cells are fragile and may break easily in the capillaries
76
What is the symbol for normal haemoglobin and what is the symbol for the mutant haemoglobin resulting in sickle cell anaemia?
HbA- normal haemoglobin
HbS- mutant haemoglobin
The gene responsible is written as Hb^s
77
What would the phenotype of the following be...
- Hb^s Hb^s
- Hb^s Hb^A
- Hb^A Hb^A
- Hb^s Hb^s
sickle cell anaemia- 100%of haemoglobin is - HbS
- Hb^s Hb^A
they are co-dominant so 50% of the DNA is HbA and 50% is HbS
- Hb^A Hb^A
100% is HbA
78
What are chromosome mutations?
Changes in the structure or number of chromosomes in cells- most likely to occur during meiosis
79
When can chromosome mutations arise?
- During prophase I of meiosis homologous chromosomes pair and exchange genetic material at the chiasmata.
- Mutations arise when a chromosome does not rejoin accurately at the corresponding position on its homologous partner.
- The chromosomes therefore end up with some different genes.
- the gametes from these mutations can still fuse to form a new individual, but further meiosis is impossible.
(The above is referring to changes in the structure.)
80
What causes non-disjunction?
A faulty spindle.
81
How does Down's syndrome occur?
- Chromosome 21 is effected which leads to Down's syndrome.
- If non-disjunction occurs during oogenesis a secondary oocyte has either two copies of chromosome 21 or non, instead of one.
- The one with no copies is not viable, but the other is and goes on to fuse with a normal sperm producing a viable embryo.
- The viable embryo now contains three copies of chromosome 21, so that individual has 47 chromosomes.
82
What is translocation Down's?
- It is when a fragment of one chromosome attaches to another.
- This is a form of downs when the individual has 46 chromosomes.
- During meiosis a fragment of chromosome 21 attaches to chromosome 14.
- When the abnormal gamete fuses it forms two normal copies of 21 and one additional one attached to chromosome 14.
83
Define polyploidy.
Having more than two complete sets of chromosomes.
84
What are cells with complete sets of chromosomes described as?
euploid
85
What are cells that have too few/many chromosomes described as?
aneuploid
86
How may polyploidy arise?
- A defect in the spindle at meiosis may result in non-disjunction, which while the triploid zygote may survive it will not be able to produce homologous pairs at meiosis so will be infertile.
- If two diploid gametes fuse a tetraploid is produced.
- endomitosis- the replication of chromosomes that is not followed by cytokinesis- if it occurs in an early embryo four sets of chromosomes are incorporated into the new envelope and successive rounds of mitosis occur to produce tetraploid cells- in rare situations infertile triploids undergo endomitosis making fertile hexaploids.
87
What beneficial characteristics in flowering plants is due to polyploidy?
- vigour
| - disease resistance
88
Why is polyploidy more common in plants than animals?
- they can reproduce asexually
| - they're hermaphrodite and so do not use chromosomes to determine sex
89
Define carcinogen.
An agent that causes cancer.
90
What genes regulate mitosis and prevent cells dividing too quickly?
tumour suppressor gene
91
What happens if a mutation impacts the tumour suppressor gene?
they could undergo continual repeated mitosis which characterises cancer- if the cell escapes the attack of the immune system it could result in a tumour.
92
Name the two types of tumours.
benign- not as harmful
| malignant- results in secondary tumours/metastases
93
What is the common tumour suppressor gene that often mutates causing cancer?
TP53 which codes for the p53 protein.
94
Compare normal p53 to a mutant p53 protein.
Normal:
- activates repair of damaged DNA
- prevents the cell from entering the S phase, holding it in G1 while damaged DNA is repaired.
- initiates apoptosis if damaged DNA cannot be repaired.
Mutant:
- no DNA repaired
- cells with damaged DNA enter S phase and DNA is replicated
- mutant cells survive and undergo mitosis
95
Define oncogene
A proto-oncogene with a mutation that results in cancer.
96
What is a porto-oncogene and what does it mutation result in?
- a protein that contributes to cell division
| - mutations may switch the gene on permanently so that an excessive amount of the protein is made
97
How do proto-oncogenes become oncogenes?
- a mutation causes chromosomes to rearrange and places the proto-oncogene next to a DNA sequence that permanently activates it
- there is an extra copy of the proto-oncogene resulting in too much of its product being made causing excessive mitosis.
98
Define epigenetics
The control of gene expression by modifying DNA or histones, but not affecting the DNA nucleotide sequence.
99
Variation in members of a species is typically due to?
- differences in the DNA nucleotide sequence
| - physiological effects of the environment
100
How does methylation of cytosine impact it?
- cytosine can have a methyl or hydroxymethyl group added
- it is still read as cytosine and pairs with guanine
- if regions of DNA are heavily methylated they are less likely to be transcribed
101
What happens if you alter histone proteins?
- it impacts how they interact with DNA
- it changes the arrangement of the nucleosomes- when unmodified they pack more tightly so the DNA is less accessible to enzymes so transcription is reduced
- when modified the coiling is more relaxed transcription is increased
102
How can histones be modified?
By attaching an/a...
- acetyl group to the amino acid lysine
- methyl group to lysine and arginine
- a phosphate group to serine and threonine
103
What are the consequences of epigenetic changes?
- Genome imprinting
| - X inactivation
104
Explain genome imprinting.
- if genes are inactivated in gametes, the inactivation may be transferred to the next generation
- genome imprinting is when the gene is potentially permanently switched off by DNA methylation on the chromosome derived from one parent
- if this switching is damaged then a medical condition may ensue
105
Explain X inactivation.
- epigenetic changes can switch off a whole chromosome.
- cells of female mammals uses only one X chromosome, the other is inactivated and becomes a mass of densely staining chromatin called the Barr body
