1
Q
What is genetic variation caused by?
A
- Independent assortment of chromosomes, and therefore alleles, during meiosis
- Crossing over between chromatids of homologous chromosomes during meiosis
- Random mating between organisms within a species
- Random fertilisation of gametes
- Mutation
- The first four of these processes reshuffle existing alleles in the population
- Offspring have combinations of alleles which differs from those of their parents and from each other
- This genetic variation produces phenotypic variation
2
Q
What does a mutation do?
A
- Mutation does not reshuffle alleles that are already present
- Mutation can produce completely new alleles
- E.G a mistake occur in DNA replication so that a new base sequence occurs in a gene
- Such a change in gene, which is quite unpredictable is called a gene mutation
3
Q
What is a gene mutation like?
A
- The new allele is often recessive and so does not show up in the population until some generations after the mutation occurred
- When by chance two descendants of organisms in which the mutation happened mate and produce offspring
4
Q
When is a mutation passed on?
A
- Mutations that occur in somatic cells often have no effects at all on the organism
- Somatic mutations can be passed on to the offspring by sexual reproduction
- Mutations in cells in the ovaries or testes of an animal, or in the ovaries or anthers of a plant may be inherited by the offspring
5
Q
How is a mutation passed on?
A
- If a cell containing a mutation divides to form gametes, then the gametes may also contain the mutated gene
- If such a gamete is one of the two which fuse to form a zygote, then the mutated gene will also be in the zygote
- The single cell then divides repeatedly to form a new organism, in which all the cells will contain the mutated gene
6
Q
What is genetic variation?
A
- Genetic variation, whether caused by the reshuffling of alleles during meiosis and sexual reproduction or by the introduction of new alleles by mutation, can be passed on by parents to their offspring, giving differences in phenotype
- Genetic variation provides the raw material on which natural selection can act
7
Q
What does variation within a population mean?
A
- Some individuals have features that give them an advantage over other members of that population
- Variation in phenotype is also caused by the environment in which organisms live
- E.G some organisms might be larger than others because they had access to better quality food while they were growing
- Variation caused by the environment is NOT passed on by parents to their offspring
8
Q
What are examples of quantitate and qualitative phenotypic differences?
A
- Qualitative: blood groups
- Quantitative: height and mass
9
Q
What is discontinuous variation?
A
-Qualitative differences fall into clearly distinguishable categories, with no intermediates, for example you have one of four possible ABO blood groups A, B, AB or O
10
Q
What is continuous variation?
A
- Quantitative differences between your individual heights or masses may be small and different to distinguish
- When the heights of a large number of people are measures, there are no distinguishable height classes, instead there is a range of heights between two extremes
11
Q
What are the similarities of continuous and discontinuous aviation?
A
- Both qualitative and quantitative difference in phenotype may be inherited
- Both may involve several different genes
12
Q
What happens in discontinues (qualitative) variation?
A
- Different alleles at a single gene locus have large effects not he phenotype
- Different genes have quite different effects on the phenotype
13
Q
What happens in continuous (quantitative) variation?
A
- Different alleles at a single gene locus have small effects on the phenotype
- Different genes have the same, often additive, effect on the phenotype
- A large number of genes may have combined effect on a particular phenotypic trait; these genes are known as polygenes
14
Q
What are examples of discontinuous variation?
A
- Inheritance of single cell anaemia and haemophilia
- Flower colour in snapdragon and stem colour of tomato plants and feather colour of chickens
15
Q
What does gene interaction and dominance do?
A
Reduce variation
16
Q
What are the effects of the inheritance of continuous variation?
A
- The small effects of the different alleles of one gene on the phenotype
- The additive effect of different genes on the same phenotypic character (hypothetical is organisms height)
17
Q
How can height be determined?
A
- Height controlled by two unlinked (on different chromosomes) genes A/a and B/b and a and b add xcm and A and B add 2x cm
- Effect of genes additive, aabb 4x cm and AABB potentially 8xcm so other genotypes will fall between two extremes
18
Q
What do these hypothetical results rely on?
A
- Assuming that two unlinked genes, each with two alleles, contribute to the height of the organism
- The number of discrete height classes increases as more genes are involved and the differences between these classes get less
- Even if two or more of the genes are liked on the same chrome, potentially reducing the number of classes of offspring an increasing the difference between them, crossing over in meiosis will restore the variation
- The differences between different classes will be further smoothed out by environmental effects
- NOTES?
19
Q
How might environment stunt growth?
A
- Less food, less nutritious food than another with the same genetic contribution
- Plant may be in lower light intensity pr in soil with fewer nutrients than another with the same genetic potential height
20
Q
What is another example of environmental effects?
A
- Development of dark tips to ear, nose and paws, and tail in Himalayan colouring or rabbits or Siamese and Burmese cats
- Allele allow formation of dark pigment only at low temperature (extremities coldest part)
- Area other placed of fur, new fur will be black
21
Q
What is the experiment for genetic variation?
A
- Crossed two varieties of maize which differed cob length both of the parental varieties were pure bred lines
- Cob lengths of the plants used as parents and the first and second generations of offspring resulting form the cross were measured to nearest cm
- Pure bred so were homozygous at a large number of loci and first generation genetically different from the parents but were genetically the same as one another
1. The phenotypic variation that you can see within the two parental varieties and also within the first generation of offspring shows the effect of the environment
2. The second generation of doddering shows a much wider variation in cob length and this is both genetic and environmental
22
Q
What is genetic and environmental variation?
A
- Just as genetic variation provides the raw material on which natural selection can act, so in selective breeding it is important to know how much of the phenotypic variation is genetic and how much is environmental in origin
- There is no point in selecting parents for a breeding programme on the basis of environmental variation
23
Q
What happens to the population if it is left unchecked by environmental factors?
A
Number in a population may increase exponentially
24
Q
What is an example of when the population can increase exponentially?
A
- Rabbits, fed on abundance of vegetation
- Few predators
- Numbers so great affected availability of grazing for sheep
25
What are the different types of environmental factors?
1. Biotic: caused by other living organisms such as through preditation, competition for food or infection by pathogens
2. Abiotic: caused by non-living components of the environment e.g. water supply, nutrient levels in the soil
26
What might an increase in rabbit population cause?
1 .Eat increasing amount of vegetation until food in short supply
2. Larger population of rabbits may allow populations of predators such as foxes, stoats and weasels to increase
3. Overcrowding occur increasing ease disease e.g. myxomatosis may spread which is transmitted by fleas so closer together virus pass more easily
- These environmental factors reduce the rate of growth of the rabbit population
- Of all rabbits born many die lack of food, killed by predator or myxomatosis
27
How does the population of rabbits fluctuate?
1. Only a small proportion of young grow into adulthood and reproduce so population growth is slow
2. Population size decreases if environmental factor pressure increase
3. Population only grow when only when numbers of rabbits falls a lot
4. Over period of time population oscillate about a mean value
28
What are some describable characteristics that rabbits may have?
- Number of young produced much greater than number survive to adulthood so many die before reaching reproductive age
1. Coat colour; e.g. if white not brown stand out so predator easier to spot so chances of reproducing of white is low so allele for white coat remain rare in population
29
What is fitness?
Capacity of an organism to survive and transmit its genotype to its offspring
30
What is an example of a selection pressure?
Predation by foxes
31
What does a selection pressure do?
Increases the chances of some alleles being passed on to the next generation and decreases the chances of others
32
What is natural selection?
- Effects of such selection pressures on the frequency of alleles in a population
- Raises the frequency of alleles conferring an advantage and reduces the frequency of alleles conferring a disadvantage
33
What is directional selection?
-If a new environmental factor or a new alleles appears then alleles frequencies may also change
34
What is disruptive selection?
- Occur when conditions favour both extremes of a population
| - This type of selection maintains different phenotypes (polymorphisms) in a population
35
What is a new environmental factor?
- Ice age so snow, white rabbits better camouflage so more likely to survive and reproduce passing on their alleles for white fur
- Frequency of alleles for white fur increases
36
What is a new allele?
1. Since random events most mutations that occur produce features that are harmful (organisms less well adapted)
2. Other mutations may be neutral, conferring neither an advantage or disadvantage on the organism within which they occur
3. Occasionally mutations produce useful features
37
Why does evolution occur?
-Because natural selection gives some alleles a better chance of survival than others
38
Why does evolution occur?
- Because natural selection gives some alleles a better chance of survival than others
- Over many generations populations may gradually change becoming better adapted to their environments
e. g. antibiotic resistance and peppered moth
39
What is antibiotic resistance?
1. One with allele that gives resistance to antibiotic e.g. staphylococcus bacteria produce enzyme which inactivate penicillin
2. As only one single loop of DNA, they have only one copt of each gene, so the mutant will have an immediate effect on the phenotype of any bacterium possessing it
3. Bacteria with this resistance curve and reproduce
40
How does antibiotic resistance spread?
Alleles for antibiotic resistance often occur on plasmids and plasmids can be transferred between bacteria of same species allowing another species of bacteria to become resistance
41
What is an example of hewn changing environmental factors produced change in allele frequencies? (peppered moth)
- Biston betalarna moth pale wings with dark markings
- Numbers of black ones increase in some areas and numbered of speckled ones increase
- Normal speckled colouring produced by recessive allele of gene and black colour by dominant allele C
- In industrial areas frequency of allele C increased and non industrial c remained
42
Why did the peppered moth change it appearance?
1. Unpolluted air, tree branches covered with grey, brown and green lichen so speckled moths better camouflaged from predation by birds (selection pressure)
2. No lichen, darker bark on trees so dark moths better camouflaged
43
Why was there a mutation in the moths?
- The mutations of the c allele to the C allele was not caused by pollution (may happen frequently but strong selection pressure in favour meant it stayed)
- Changes in environmental factors only affect the likelihood of an allele surviving in a population: they do not affect the likelihood of such an allele arising by mutation
44
Why does sickle cell anaemia exist?
- Homozygous, selective disadvantage so die before reproductive age
- Place where common also with malaria and heterozygous are. less likely to suffer from serious attack of malaria than people homozygous (haemophilia free)
45
What are the selection pressures acting on the alleles?
1. Selection against people who are homozygous for the sickle cell allele HbSHbS is very strong because they become seriously anaemic
2. Selection against people who are homozygous HbAHbA is very string because they are more likely to die from malaria
46
How does haemophilia range?
- Places where malaria is common heterozygous have strong selective advantage
- Where no malaria, people with HbsHbs almost removed HbS allele from population
- The examples of naturals selection given above show the effect of non-random process on the allele frequencies of a population of organisms
- These allele frequencies may also change due to random process of genetic drift
47
What is genetic drift?
Genetic drift is a change in allele frequency that occurs by chances because only some of the organisms of each generation reproduce
48
When is genetic drift noticeable? What is founder effect?
1. When a small number are separated from rest of large population form a small sample of original population so unlikely to have same allele frequencies as large population
2. Further genetic drift in small population alters the allele frequencies more and evolution of this population may be different from larger parent population
3. This process in a recently isolated small population is called the founder effect
49
What is the Hardy-Weinberg principle?
1. When a phenotypic trait controlled by two alleles of single gene A/a, population made up of AA, Aa and aa
2. Calculates proportion of each of these genotypes in large randomly mating population
50
What is the first Hardy Weinberg principle? What does p and q represent?
1. p is frequency of dominant allele in population
2. q is frequency of recessive allele
p + q = 1
51
What is equation 2 of the Hardy Weinberg what does it mean?
1. The chance of an offspring inheriting a dominant allele from both parents p x p = p^2
2. Chance of an offspring inheriting a recessive allele from both parents q x q = q^2
3. Chance of offspring inheriting a dominant allele from the father and a recessive allele from the mother p x q = pq
4. Chance of offspring inheriting a dominant allele from mother and a recessive allele from father is p x q = pq
p^2 + 2pq + q^2 = 1
52
When does Hardy-Weinberg calculations not apply?
1. When population is small
2. There is significant selective pressure against one of the genotypes
3. Migration of individuals carrying one of the two alleles into or out of the population
4. Non-random mating
53
What happens when ratios of different genotypes in a population have been determined and their predicted ratios in the next generation can be compared with observed values?
- Any differences can be tested for significance by chi squared test
- If differences are significant and migration and non-random mating can be discounted the there is evidence that directional selection is occurring in population
54
What are desired features of cattle?
- Docility
- Fast growth rates
- High milk yield
55
What are the issues with selective breeding? What are background genes?
ADD EXAM NOTES
1. Animals large and take time to reach maturity
2. Gestation period long and number of offspring is small
3. Bull cannot be assessed for milk production as this is a sex limited trait (non same as sex linked) need to do progeny testing
4. Have to consider whole genotype not just genes affecting desired trait (within each organisms genotype are all alleles of genes that adapt to its particular environment) - BACKGROUND GENES
56
Why are background genes an issue?
- If Parents not from same part of world so inherit appropriate alleles from only one parent so may not be well adapted to environment
- Same happens for cultivated plant and related wild species
57
How has there been crop improvement in relation to disease and selective breeding?
- Resistance to fungal disease such as head blight important as can result in loss of yield as a result of these infections
1. Introduction of allele for residence: takes many generations and when comes from wheat grown in different parts of work
2. Wheat Genetic improvement Network in 2003 UK bring together research workers and commercial plant breeders
3. Support development of new varieties by screening seed collections for plants with traits of ideas resistance, climate resilience, efficient use of nitrogen fertilisers
4. Any plant with suitable trait grown in large numbers and passed to commercial breeders
58
How is wheat grown today?
Shorter stems:
- Easier to harvest and higher yields (as more energy to making seeds that growing tall)
- Plants less susceptible to being knocked fast by heavy rains
- Produce less raw which has little value and costs money to dispose of
59
How can crops be improved with DELLA?
1. Dwarf varieties of wheat carry mutant alleles of two reduced height (Rho) genes
2. These code for DELLA proteins that reduce the effect of gibberellins on growth
3. Mutant alleles cause dwarfism by producing more of, or more active forms of these transcription factors
- A mutant allele of another gene has dwarfing effect because plant cells do not have receptors for gibberellins and so cannot respond to the hormone
60
How can rice be used in selective breeding?
1. International Rice Research Institute holds rice gene bank and coordinates with Global Rice Science Partnership research to improve ability of rice farmers to feed growing population
2. Yield of rice reduced by bacterial diseases such as bacterial blight and fungal disease various 'spots' and 'smuts' and blast
3. Researchers use selective breeding to produce varieties of rice that show some resistance to all of these diseases
61
What is the effect of inbreeding and out breeding on maize?
- Maize, sturdy tall grass with broad strap-shaped leaves and grows in long hot summers so cobs can ripen
1. If maize plants are inbred (crossed with other plants with same genotypes) the plants in each generation become smaller and weaker
2. Inbreeding depression occurs as in maize homozygous plants less vigorous than heterozygous plants
3. Outbreeding (crossing with other less closely related plants) produced heterozygous plants that a re healthier, grow taller and produce higher yields
62
What are the problems with outbreeding? How is it overcome
1. If outbreeding at random, maize field full of variation between individual plants
2. So to make able to harvest and sell crop easily, plants should be uniform so same height and ripen at same time
3. Need heterozygosity and uniformity so buy maize seed from companies specialise in using inbreeding to produce homozygous maize plants and then cross them and so produce F1 plants same genotype
4. Many different homozygous maize varieties and different crosses between them can produce large number of different hybrids suited for different purposes
5. Every year thousands of new maize hybrids are trialled, searching for varieties with characteristics sick as high yields, resistance to pests and disease and good growth in nutrient poor soils or where water is in short supply
63
What is the general theory of evolution?
Organisms have changed over time
64
What were the observations and deductions of Darwin and Wallace?
1. Organisms produce more offspring than are needed to replace the parents
2. Natural populations tend to remain stable in size over long periods
D1: There is competition for survival (a 'struggle for existence')
3. There is variation among the individuals of a given species
D2: The best adapted variants will be selected for by the natural conditions operating at the time, in other words natural selection will occur. The 'best' variants have a selective advantage; 'survival of the fittest' occurs
-Now can think of natural selection as selecting particular allele or groups of alleles
65
What is speciation?
ADD
66
How might you determine if same species?
1. Only morphological (structural) features as physiological and biochemical ones and some behavioural ones are more time-consuming to investigate
2. Sometime detailed DNA sequences to asses how similar
3. Can be difficult to define is sufficiently similar or different to define two organisms as belonging to same or different species
4. Mostly if interbreed successfully
67
What is reproductive isolation?
ADD
68
What is prezygotic (before zygote formed) isolating mechanism?
1. Individuals not recognising one another as potential mates or not responding to mating behaviour
2. Animals being physically unable to mate
3. Incompatibility of pollen and stigma in plants
4. Inability of a male game to fuse with a female game
69
What do polyzygotic isolating mechanisms include?
1. Failure of cell division in the zygote
2. Non-viable offspring (offspring that soon die)
3. Viable but sterile offspring
70
What do postzygotic isolating mechanisms include?
1. Failure of cell division in the zygote
2. Non-viable offspring (offspring that soon die)
3. Viable but sterile offspring
71
What is hard about reproductive isolation?
- Investigating how reproductive isolation arises as takes time
- Evidence is mostly circumstantial looking at patterns
72
What is geographical isolation?
ADD
| -E.G Galapagos
73
What does geographical isolation need?
- A barrier preventing two populations of same species from mixing e.g. stretch of water or forests cut down
- Therefore smaller group interbred and selection pressure different then mainland so different alleles selected for
- Overtime, morphological and physiological and behaviour features of island population so different from mainland that no longer interbreed and new species evolved
74
What is allopatric speciation?
ADD
| -Two population seated geographically
75
What is sympatric speciation?
ADD
| -New species without original populations being separated by a geographical barrier
76
When can a polypoid happen?
- A polypoid organisms is one with more than two complete sets of chromosomes in its cells
- When eroded foes wrong when games are being formed, so games ends ups with two sets of chromosomes instead of one, so if two of these gametes fuse, four sets of chromosomes so tetraploids
- Often sterile but can sometimes reproduce asexually as mitosis can happen as chromosomes not need to pair up
- Happens often in plants but not animals as most animals not reproduce asexually
77
Can a tetraploid produce games?
- Occasionally diploid gametes and if one fuse with normal form a triploid and sterile as cannot share three sets of chromosomes out evenly between the daughter cells so original diploid and tetraploid that was produce form it cannot interbreed successfully so considered to be different species
- A new species has arise in just one generation
78
What is an autopolypoid?
ADD
79
What is an allopolypoid? What is an example?
ADD
| -Cord Grass Spartina anglica
80
In autotetrapods or allotetraploids does meiosis happen more easily?
- In an allotetrapoloid
- As chromosomes from each species are not white identical so two chrosomes from one species apr up with each other, when two chromosomes from other species pair up
- The allopolypoid the chromosomes attempt to get in fours so more likely that meiosis can come to a success
- The allopolypod may be able to produce games and it is fertile
- Both cannot interbreed with individuals from its parent species and It is a new species
81
How can you reveal similarities between related species?
Molecular evidence from comparisons of the amino acid sequences of proteins and of the nucleotide sequences of mitochondrial DNA
82
Describe cytochrome c
- A protein in etc in oxidative phosphorylation in mitochondria
- Expected to have similar sequence of amino acids in different species as a poorly adapted version would result in death of organisms
83
What was the difference in cytochrome C from humans, mice and rats? What does it suggest?
1. All three molecules consist of 104 amino acids
2. The sequence of mouse end rat cytochromes c are identical
3. Nine amino acids in human cytochrome c are different from the mouse or rat sequence
4. Most of these substitutions in human cytochrome c are of amino acids with the same type of R group
- Suggest mice and rats closely related species sharing a recent common ancestor and humans are more distantly reacted sharing a common ancestor with mice and rats less recently
- Less closely related number of differences from the human sequence increases
84
What is comparing nucleotide sequences of mitochondrial DNA? How does it work?
-Differences in the nucleotide sequences of mitochondrial DNA (mtDNA) can be sued to study the robin and spread of our own species HomoSpaiens
-Human mitochondrial DNA is inherited through the female line
-Zygote contains the the mitochondria of the ovum but not the sperm
-Since the mitochondrial DNA is circular and cannot undergo any form of crossing over changes in the nucleotide sequence can only arise in mutation
-Mitochondrial DNA mutates faster than nuclear DNA,
acquiring one mutation every 25000 years and unlike nuclear DNA mitochondrial DNA is not protected by histone proteins and oxidative phosphorylation in the mitochondria can produce forms of oxygen that act as mutagens
-Different human populations show differences in mitochondrial DNA sequences and provide evidence for origin of H sapiens in Africa and for subsequent migrations of the species around the world
-Lead to all modern humans are descendant from one woman, assuming constant rate of mutation over time and that the greater number of differences in sequence of nucleotides the longer ago individuals shared a common ancestor
-Molecular clock calibrated by comparing nucleotide sequences of species whose date of specification can be estimated from fossil evidence
Z A Level Biology 2
flashcards
Decks in class (18)
# Cards
-
Biological Molecules15
-
DNA31
-
The Mitotic Cell Cycle39
-
Inherited change 291
-
Enzymes38
-
Meiosis38
-
Inherited Change15
-
Practical Skills30
-
Selection and Evolution84
-
Photosynthesis47
-
Genetic Technology128
-
Coordination64
-
Coordination 2106
-
Coordination 364
-
Practical skills 225
-
Homeostasis94
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Homeostasis 266
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AS Experiments19
