Genetics and Populations Flashcards
What is a gene?
A gene is a small section of DNA (a sequence of bases) that codes for a specific polypeptide or functional RNA.
What are alleles?
Alleles are alternate versions of a gene that are found at the same locus in a pair of chromosomes.
What are homologous chromosomes?
Homologous chromosomes are chromosomes which are similar in size and shape and they carry genes controlling the same features at the same loci on each.
What are dominant alleles?
Versions of a gene which are always expressed when present. They stop recessive alleles from presenting. They are shown in genetic diagrams with a capital letter.
What are recessive alleles?
Versions of a gene which are only expressed when there is no dominant allele of the gene present (must have 2 recessive alleles for that characteristic to show). Represented in diagrams with a lower case letter.
What is polygenic inheritance and what are some examples of polygenic traits?
Polygenic inheritance is when a characteristic is caused by the interaction of many different genes. Most characteristics are polygenic (such as height, natural hair colour, eye colour, intelligence, risk of disease, bipolar disorder).
What is a genotype?
The combination of alleles present (e.g. BB)
What is the phenotype?
The actual characteristic expressed caused by genetic constitution and the environment. E.g. brown hair
What does homozygous mean?
An individual has two identical alleles of a particular gene. They are either homozygous dominant or homozygous recessive. Two identical homozygous individuals that breed together are pure-breeding.
What does heterozygous mean?
An individual has two different alleles of a particular gene (not pure-breeding).
What is monohybrid inheritance?
The inheritance of a characteristic controlled by a single gene.
How do you format a punnet square?
Write out the parents genotypes and the possible gametes (define what each of the letters you are using means first). Place the gametes of one individual on the left of the punnet square and the gametes of the other on the top. Complete the cross, then write out the possible genotypes of the offering and the ratio of the phenotypes that would create.
What is the ratio you would expect in the F2 (second filial) generation from the monohybrid cross of a homozygous recessive individual and a homozygous dominant individual?
This cross produces only offspring which are heterozygous (the F1 generation). Breeding members of the F1 generation produces a 3:1 ratio of dominant:recessive traits.
What kind of individual would you use in a test cross (to work out the genotype of another individual)?
Homozygous recessive. If any of the offspring show the recessive trait, then the original individual must have been heterozygous. If none on the offspring show the recessive trait, the individual is homozygous dominant.
What was Mendel’s law of segregation?
In diploid organisms, characteristics are determined by alleles that occur in pairs. Only one of each pair can be present in a single gamete.
What is dihybrid inheritance?
The inheritance of two distinct traits, controlled by two separate genes, located on different chromosomes.
What was Mendel’s law of independent assortment?
Each member of a pair of alleles may combine randomly with either of another pair.
How do you do a punnet square for a dihybrid cross?
The same as for monohybrid, but the square will be 4x4 instead of 2x2.
What is the expected phenotypic ratio for the F2 generation in a cross between a homozygous recessive individual and a homozygous dominant individual?
The F1 generation will all be heterozygous for both traits, and when crossed, they will produce a 9:3:3:1 ratio assuming no autosomal linkage or epistasis. 9 - both dominant traits 3- one dominant one recessive 3 - the other dominant and the other recessive 1 - both recessive
Why might the ratio of the offspring not match the expected ratio?
The expected ratios are just probabilities (the most likely outcome - not what will happen). If the ratios are drastically off, it is probably due to autosomal linkage or epistasis.
What are sex-linked characteristics?
Where the gene responsible for a characteristic is located on one of the sex chromosomes. This means that recessive sex-linked diseases carried on the x-chromosomes are more likely to be inherited by men.
What does hemizygous mean?
Biological males are hemizygous for characteristics on the sex chromosomes (they only have one allele for each characteristic).
What is codominance?
If multiple alleles in a heterozygous organism are expressed ‘equally and independently’ in the phenotype, then they are codominant.
What are some examples of codominance?
The alleles for flower colour in snapdragon flowers are codominant. The alleles for white and for red coats are codominant in cattle, producing a roan. Blood group alleles I^A and I^B are codominant, producing the AB blood group when both are present.
What are the autosomal chromosomes?
All of the chromosomes which are not the sex chromosomes (numbers 1-22)
How do pedigree charts indicate an autosomal dominant condition?
If both parents are affected and an offspring is unaffected, the trait must be dominant (both parents are heterozygous). All affected individuals must have at least one affected parent. If both parents are unaffected, all offspring must be unaffected (homozygous recessive).
How do pedigree charts indicate an autosomal recessive condition?
If both parents are unaffected and the offspring is affected, the trait must be recessive (parents are heterozygous carriers). If both parents show a trait, all offspring must also exhibit the trait.
How do pedigree charts indicate an X-linked recessive condition?
If a female shows a trait, so too must all her sons as well as her father. An unaffected mother can have affected sons if she is a carrier (heterozygous). X-linked recessive traits tend to be more common in males.
How do pedigree charts indicate an X-linked dominant condition?
If a male shows a trait, so too must all daughters as well as his mother. An unaffected mother cannot have affected sons or an affected father. X-linked dominant traits tend to be more common in females.
If a pedigree chart indicates a particular kind of condition (e.g. X-linked recessive) is this a guarantee that that condition is X-linked recessive?
No, pedigree charts do not provide conclusive proof of a particular type of inheritance. Genetic testing is the only way to prove this.
What is autosomal linkage?
Alleles on the same chromosome are linked, and will not assort independently during metaphase I of meiosis. Linked alleles will stay together and the number of possible alleles decreases (recessive alleles stay together and dominant alleles stay together).
How do less frequent gametes form in linked genes?
Crossing over results in the formation of new combinations of alleles in linked genes. This is when homologous chromosomes wrap around each other and exchange genetic material at chiasmata (where they touch).
What is the expected ratio for the F2 generation of a homozygous dominant x homozygous recessive cross with autosomal linkage?
The only gametes are AB and ab, so the F1 phenotypes are all the dominant traits and all AaBb (heterozygous for both traits). The possible gametes are AB ab for both (assuming no crossing over), giving a ratio of 3 both dominant traits : 1 both recessive traits
What is epistasis?
The interaction of different gene loci so that one gene locus masks or suppresses the expression of another (can reduce phenotypic variation).
What is an epistatic gene?
A gene that interferes with or masks the expression of another gene (sometimes called inhibiting genes).
What is a hypostatic gene?
A gene whose expression is affected by an epistatic gene.
What is recessive epistasis?
The homozygous presence of a recessive allele may prevent the expression of another allele at a second locus.
What is dominant epistasis?
A dominant allele at one gene masks the expression of the alleles at a second gene locus.
What is complementary epistasis?
At least one dominant allele for both gene loci can result in them complementing one another.
What are the expected ratios for a cross between individuals heterozygous for two epistatically linked traits?
9:3:4 = recessive epistasis
12:3:1 = dominant epistasis
9:7 = complementary epistasis
What are examples of epistasis?
Fur colour in labradors (if they are homozygous recessive for one trait, this means they have no pigmentation, which masks another allele controlling if they have a black or brown coat). If you need two enzymes to get to a final product, but the first one is inhibited by epistasis, you won’t know if the second one would work or not.
What is Chi-squared testing?
Chi-squared tests compare expected and observed values in order to accept or reject a null hypothesis. The formula is sum of (O-E)^2/E = chi-squared. The degrees of freedom is the number of categories - 1 and the critical value is given for each probability value and degrees of freedom value.
What are some assumptions required to use the Hardy-Weinberg equation?
- there is a large population
- random mating occurs
- no selective advantage for any organism
- no mutations occur
- no immigration/emigration (populations are genetically isolated)
- no genetic drift (random changes in allele frequencies that occur over time)
What are the Hardy-Weinberg equations?
p + q = 1
p^2 + 2pq + q^2 = 1
p=frequency of the dominant allele
q = frequency of the recessive allele
p^2 = frequency of homozygous dominant
q^2 = frequency of homozygous recessive
2pq = frequency of the heterozygous
What is a population?
A group of organisms of the same species occupying a particular area at a particular time, that can potentially interbreed.
What is a gene pool?
All the alleles in a population
What is allele frequency?
The number of times an allele appears in a gene pool
Why are the assumptions made for the Hardy-Weinberg principle not realistic?
Mutations lead to new alleles and alter the relative frequency of others for the same gene. Natural selection can increase or decrease allele frequencies. Small populations are at greater risk of genetic drift, which involves random changes in allele frequency. Individuals migrating in or out of a population could change allele frequency. Individuals deliberately choosing mates that do or don’t have certain characteristics will alter allele frequencies over generations.
What is stabilising selection?
Stabilising selection preserves the average phenotype by selecting against extreme phenotypes. This tends to occur when environmental conditions are constant over long periods of time.
What is directional selection?
Changes in the phenotypes of a population by favouring phenotypes that vary in one direction from the mean of the population (selects for one extreme phenotype). This tends to occur when there is environmental change.
What is disruptive selection?
Favours individuals with extreme phenotypes rather than those which lie around the mean. It tends to occur when an environmental factor takes two or more distinct forms and can bring about evolutionary change (e.g. squirrels with long tails can balance better in trees while squirrels with shorter tails are better at avoiding predators - those in the middle don’t really get either benefit).
What is genetic diversity?
The total number of different alleles in a population.
What is a population?
A group of organisms of the same species occupying a particular place at a particular time, that can potentially interbreed.
What is a species?
Species refers to a group of organisms that can interbreed to produce fertile offspring. They consist of one or more populations.
Describe the process of natural selection.
Within a population, there is large genetic diversity. Random mutation of alleles may result in new alleles of genes, creating more variation in a population. As organisms reproduce, they create more offspring than can survive, there is competition for the limited resources available. There is a struggle for survival and individuals that are better adapted are able to outcompete the others for resources (survival of the fittest). These individuals are more likely to grow and have better chances of breeding successfully. Those that can’t adapt start to die out. Only individuals which reproduce successfully pass on their alleles to the next generation. Over many generations, organisms with the advantageous allele outnumber those without it (shift in allele frequency). This is evolution.
How do antibiotic resistant bacteria evolve?
By random mutation, some bacteria have an allele for resistance. Use of antibiotics is the selection pressure, so non-resistant bacteria die. Resistant bacteria are more likely to survive and reproduce then pass on their alleles. The frequency of the resistance allele increases in the population, so antibiotic resistance increases. This is an example of directional selection.
What is variation?
Variation is the differences between individuals of the same species and of different species.
What are sources of variation in a population?
Random mutation can result in new alleles of a gene. Many mutations are harmful but some alleles may result in increased reproductive success. Meiosis (crossing over and independent segregation) produces genetic variation, which allows natural selection. Random mating in a population, and random fertilisation of gametes produces further genetic variation.
What is the difference between phenotypic variation and genetic variation?
Genetic variation is the differences in allele combinations between individuals, while phenotypic variation is caused by both genetic and environmental factors.
What is the difference between continuous and discontinuous variation?
Continuous involves a range of phenotypes with no distinct categories, which are controlled by many genes and are affected by the environment (e.g. height). Discontinuous variation has a limited number of phenotypes and has distinct categories. Usually only controlled by a few genes and is unaffected by the environment.
What is genetic drift?
Genetic drift describes random changes in allele frequencies within a population. It can happen in any population but is most significant in small, geographically separated populations where variant/mutant alleles become progressively more common in offspring
What is genetic bottlenecking?
A sharp reduction in the size of a population due to environmental affects or human activities. This leads to a big decrease in genetic variation as the new population formed from free individuals will have similar allele combinations.
What is the founder effect?
Reduced genetic diversity which occurs when a population is descended from a small number of colonising ancestors. Allelic frequency is representative of the original founders.
What can genetic drift lead to?
Reduction in genetic variation, reduction in the ability of the population to survive in a new environment. May contribute to the extinction of a population or species or the creation of a new population or species.
What is speciation?
The separate evolution of two separate populations of the same species, to form two separate species.
What is allopatric speciation?
Two populations of the same species are geographically isolated from each other (e.g. by a river or mountains). This leads to reproductive isolation since the two populations are physically separated so there is no gene flow between them. There are different selection pressures in different habitats. Mutations cause differences that lead to further reproductive isolation and different alleles are selected for in each population, leading to a change in allele frequencies. Eventually the two populations can’t interbreed to produce fertile offspring, so they have become two different species.
What is sympatric speciation?
Sympatric speciation occurs in a population which all occupy the same habitat with no geographical isolation (they occupy different niches of the same habitat). Mutations cause differences, leading to reproductive isolation, different alleles are selected for and passed on in each population. There is a change in allele frequency and eventually the two populations can’t interbreed to produce fertile offspring, so they are two different species. This is usually an example of disruptive selection.
What are the causes of reproductive isolation in a population where there is no geographical isolation?
Seasonal changes (such as different flowering times or mating seasons), mechanical changes such as changes to the size, shape or function of the genitalia can prevent successful mating. Behavioural changes, such as changes to courtship behaviours may prevent organisms from breeding even when they could do successfully.
What is ecology?
The study of interrelationships between organisms and their environment.
What are abiotic factors?
Non-living components of the environment (e.g. temperature, rainfall, availability of nutrients, light intensity).
What are biotic factors?
Living components of the environment (e.g. competition, predation).
What is a species?
A group of organisms that are similar in terms of shape, physiology, genetics and behaviours, which can interbreed to produce fertile offspring.
What is an ecosystem?
Dynamic systems made up of a community and all the interacting biotic and abiotic factors, in which energy flows and nutrients are cycled.
What is a community?
Populations of different species living and interacting in a particular place at a particular time.
What is a habitat?
A place where an organism typically lives, characterised by physical conditions and other organisms present.
What is a microhabitat?
Smaller units within a habitat with their own microclimate.
What is a niche?
A term used to describe how an organism fits into its environment, which includes all the abiotic and biotic conditions to which it is adapted and how it responds to them.
What is the difference between a natural and an artificial ecosystem?
Natural ecosystems are not managed or altered by human activity. Artificial ecosystems are managed or altered by human activity (e.g. fish farms or replanting woodlands).
Why do different species occupy different niches?
Different species have adapted to occupy different niches because this reduces interspecific competition.
What is a limiting factor in population size?
When the rate of a process (e.g. population growth) is affected by a number of factors, the limiting factor is the one whose magnitude limits the rate of the while process.
What is the carrying capacity?
The maximum population size that can be maintained over a period of time in a particular habitat.
What affects the size of a population?
Food supply, predation, disease, climate change, abiotic factors such as pH, water availability, CO2, O2
Describe the standard growth curve of a population size?
The population size starts low and increases slowly (lag phase). The reproduction rate is low as is the growth of the population size. Organisms are acclimatising to new conditions, so there are few of them. Then there is a rapid increase in population (growing conditions are favourable with plentiful resources, rate of reproduction is greater than mortality rate (log/exponential phase)). Then the stationary phase is reached, there the rate of growth equals the rate of death - this is the carrying capacity of the environment. This is where there is a stable population with minor fluctuations from the changing environment. The death/decline phase follows, where the death rate is greater than the rate of regeneration, this may be due to a lack of nutrients/resources and intraspecific competition.
What is the difference between a fundamental and realised niche?
Fundamental niches are the areas an organism could be found, the realised niche is where they are actually found. The realised niche is often smaller than the fundamental niche due to interspecific competition.
What is intra and interspecific competition?
Interspecific competition is when individuals of different species compete for resources. Intraspecific competition is when individuals of the same species compete for resources.
What is Gause’s competitive exclusion principle?
Where populations of two species occupy the same niche, one will normally have an advantage over the other. If conditions remain the same, this will lead to interspecific competition which will remove one species.
Why is it difficult to prove if competition is the reason for variations in population?
There are many other factors that influence populations, such as abiotic factors. A causal link has to be established to show that competition is the cause of an observed correlation. There is often a time lag in many cases of competition. Data on population sizes is not always reliable.
What are common characteristics of predators and prey animals?
Camouflaging is common in both and both may have poisons. Predators often have enhanced senses of smell and high intelligence. They have lots of speed and power with sharp teeth and claws. Prey are fast and often travel in packs. They use distraction or mimicry techniques to confuse predators and sometimes have protective layers for defence (such as porcupines).
What are predator-prey cycles?
They show how numbers of predators and prey vary over time. If there is an increase in prey numbers, there are few predators and limited intraspecific competition for resources. There is then an increase in predator numbers as there is more food available. There is a decrease in prey numbers due to increased predation, but also competition for the resources available. There is a decrease in predator numbers as the pry population has fallen, so there is more competition and some predators migrate from their habitat to find food. Prey numbers will then increase again as there are fewer predators, and with reduced prey numbers, there is less competition.
What is mark, release, recapture?
A technique used to estimate the size of a population in which a large sample of animals is caught, marked and released back into their habitat. Make sure the marking does not harm the animals or affect their survival chances. Allow to,e for the marked animals to randomly distribute. After time has passed, collect and count a second sample, and count the number of individuals already marked.
How can mark, release, recapture be used to estimate population size?
Estimated population size = total number of individuals in 1st sample x total number of individuals in second sample / number of marked individuals recaptured
What assumptions are made when using the MRR method to estimate population size?
The marked individuals form the first sample distribute evenly within the whole population and have enough time to do this. The proportion of marked to unmarked individuals in the second sample is the same as in the population as a whole. The population has a definite boundary so that there is no immigration/emigration. There are few if any births/deaths within the population. The marking method is not toxic to individuals and it does not marked them more conspicuous to predators. The mark is not rubbed off during the investigation.
What is a quadrat?
A square metal frame which can be used to measure the abundance of a species, the species richness or biodiversity in an area.
How are random samples selected for where to place a quadrat?
If you want to randomly place your quadrat, arrange two tape measures at right angles to each other in the study area and use a random number generator to obtain a series of coordinates. Place the quadrats at the intersection of each pair of coordinates.
Why might a sample not be representative of the whole population?
Sampling bias (hence random sampling is used), or chance (larger sample sizes reduce the effect of random variations).
What is abundance?
The number of individuals of a species in a given area.
What is species richness?
The number of species in a given area
What is the frequency of a species calculated using quadrat sampling?
Likelihood of a particular species occurring in a quadrat. Useful for hard to count species but gives no information about density or detailed distribution of a species.
What is percentage cover (calculated using quadrats)?
Estimate of the area within a quadrat that a particular species covers. Useful for hard to count species, but is less useful if species occur in overlapping layers.
Why is it important that the counting method is standardised when using quadrats?
To ensure that organisms don’t get counted twice and to ensure that the results are representative of the whole population. Standardised counting methods include only counting organisms wholly within the quadrat or if some organisms are partially within, you could count ones which are on the bottom or left sides but not top or right.
How would you estimate the total population from quadrat data?
Use random sampling to decide where to place the quadrats within the sample area. Calculate the abundance of the species within each quadrat then work out the mean abunadance. Divide by the size of the quadrat then times by the area of the field of study.
How would you set up a transect line/belt transect?
Lay a tape measure along the environmental gradient to see the effects of an abiotic factor (e.g. light intensity) on the abundance of plants. Either place the quadrats along the whole length of the line (belt transect) or place the quadrats at regular intervals along the transect line (interrupted transect). Record what you need from each quadrat (e.g. abundance, percentage cover, species richness etc.). Repeat using several parallel transects to ensure the data is representative. This method is systematic sampling, not random.
What is Spearman’s rank?
Spearman’s rank correlation coefficient = 1 - (6 ΣD^2)/n(n^2 -1)
D= difference in rank for the two variables being investigated
n= number of observations (pairs of data)
What does the correlation coefficient tell you?
If the correlation coefficient is 0, the two variables have zero correlation. As the coefficient gets closer to one, it gets a stronger positive correlation, and as it gets closer to negative one, it gets a stronger negative correlation.
What is primary succession?
The development of an ecosystem in an area where no life previously existed. This could include colonisation of bare rock stemming from lava deposited from a volcano. Can also be the result of silt/mud deposits at river estuaries or retreating glaciers etc..
What is secondary succession?
The process of re-stabilisation following a disturbance / damage in an area where life had previously formed an ecosystem. This could result from land being cleared for agriculture or by disturbance by flood, fire or tornado. Where the soil remains intact, the area begins to return to its natural community. Unlike primary succession, secondary succession begins on sites which already bear soil so the soil contains a native seed bank.
How does primary succession occur?
Area is first colonised by a pioneer species, which change the conditions of the habitat. The environment is less hostile to other species, so new species colonise which will eventually outcompete the previous species. Biodiversity gradually increases until eventually a climax community is reached.
How does succession progress?
When organisms die, they are decomposed by saprobionts to add humus (fully decomposed dead material) to the soil. This makes the soil retain more water and adds mineral ions like phosphates to the soil. Plant roots bind to the soil to prevent it eroding plant growth. An increase in plant species can provide a wider variety of food sources to animal species, which increases animal biodiversity.
What are climax communities?
Long-term stable communities formed at the end of succession. Abiotic factors are mostly constant and populations will be stable around their carrying capacity.
How can succession be managed?
When a climax community is reached, the dominant plant and animals out compete species found in earlier stages of succession. To prevent this, humans can create artificial climax communities called pagioclimaxes. Burning heather and grazing sheep on moorland prevents it reaching its climax community by destroying young tree saplings.
What is conservation?
The method of maintaining ecosystems and the living organisms that occupy them (biodiversity) by human intervention.
Why is conservation important?
Economic reasons (provide medicines and useful raw materials), ethics (respect for other living organisms), cultural and aesthetic reasons.
How can conservation be achieved?
Management of succession, seed banks (to protect endangered plant species), captive breeding, relocation of species and legally protecting areas.
