Topic 7: Genetics, populations, evolution and ecosystems

Define genotype.

All of the alleles that an organism carries on its chromosomes.

Define phenotype.

Observable characteristics of an organism which the result of the genotype as well as environmental factors.

What are most organisms?

Diploid and therefore carry two alleles for each gene that they possess on each chromosome.

What is a dominant allele?

Only a single allele is required for the characteristic to be expressed in the phenotype. Whether the organism only has one dominant allele (heterozygous dominant) or has two dominate alleles (homozygous dominant) the trait will be expressed.

What is a recessive allele?

The characteristic is only expressed if there is no dominant allele present. It therefore means that there must be two recessive alleles (homozygous recessive) present in order for the phenotype to be expressed.

What are codominant alleles?

Both alleles are expressed equally and contribute to the phenotype, with there either being a blend of the two characteristic or both characteristics being seen together.

What is monohybrid inheritance?

Monohybrid inheritance is when a phenotype or trait is controlled by a single gene. For instance, cystic fibrosis where the individuals with doubly recessive phenotype are affected.

What is the ratio of phenotypes in monohybrid inheritance?

3:1

What is dihybrid inheritance?

When two characteristics are studied and phenotype is determined by two different genes that are present on two different chromosomes at the same time.

What is an example of codominance?

Blood groups.

What is sex linkage?

The expression of an allele dependent on the gender of the individual as the gene is located on a sex chromosome. Humans have 22 pairs of autosomal chromosomes that are not involved in sex determination, and one pair of sex chromosomes, called either X or Y. Most sex linked traits are located on the X chromosomes with there being no equivalent locus on the Y chromosome. Therefore females will carry two alleles of sex linked gene, but males will only carry a single allele.

Give an example of a sex linked disease.

Haemophilia - disease in which the blood clots slowly and there may be slow and persistent internal bleeding, especially around the joints.

• Caused by a change to the DNA sequence resulting in a faulty protein being created.

• As a male can only obtain the Y chromosome from his father, the X chromosome must come from his mother. As this disease is not found on the Y chromosome it means that this disease is always inherited from the mother in males.

• If the mother doesn’t suffer from the disease but is heterozygous with the alleles then she is a carrier.

What is autosomal linkage?

In an autosomal linkage two or more genes are on the same autosomal chromosome. When this happens there are only 2 possible allele combinations of alleles.

What is epistasis?

The interaction of different loci on the gene, one gene locus affects the other gene locus. One gene loci can either mask or suppress the expression of another gene locus.

• Recessive epistasis - when the presence of a recessive allele prevents the expression of another allele at a second locus. Ratio of 9:3:4.

• Dominant epistasis - when a dominant allele at one locus completely masks the alleles at a second locus. Ratio of 12:3:1.

What is the Chi squared test?

A statistical test which can be used to establish whether the difference between observed and expected results is small enough to occur purely due to chance. It can be used to test the null hypothesis.

What is a null hypothesis?

One in which the results of a scientific investigation will produce no statistical significance.

What are the criteria for the chi squared test?

• The sample size must be sufficiently large enough, that is over 20.

• Used only for data that falls into discrete categories.

• Only raw counts and not percentages, rates, etc can be used.

How is the critical value used?

it is compared to the value obtained.

Value obtained ≥ critical value: accept null hypothesis

Value obtained ≤ critical value: reject null hypothesis

What can the Hardy Weinberg equation be used for?

Estimate the frequency of alleles in a population and to see whether a change in allele frequency is occurring in a population over time.

What is assumed in the Hardy Weinberg expression?

• no mutations occur to create new alleles

• there is no movement of alleles into or out of the population by migration

• the population is large

• there is no selection, so every allele has an equal chance of being passed to the next generation

• mating is random

What is the formula for the Hardy Weinberg equation?

p + q = 1.0 AND p² + 2pq + q² = 1

p = the frequency of the dominant allele

q = the frequency of the recessive allele

p2 = frequency of AA (homozygous dominant) 2

pq = frequency of Aa (heterozygous)

q2 = frequency of aa (homozygous recessive)

What factors cause variations in the alleles of genes for members of the same species?

1. Random fertilisation - the gametes that are carrying different alleles will join together randomly.

2. Meiosis - meiosis is the nuclear division that creates gametes and means that the alleles will be assorted in the gametes at random.

3. Mutation - the mutation of an allele can go on to lead to the creation of another new allele which can then be passed to the next generation

How can environmental influences influence phenotype?

The majority of phenotypic traits are influenced by environmental factors. Ex. two plants that possess the same alleles for the flower colour. However an environmental factor of one plant growing in a soil that lacks a certain mineral may mean the pigment in one will not develop so will have a slightly different flower colour.

What is disruptive selection?

The opposite of stabilising selection and in this case both extremes of the normal distribution are favoured over the mean. An example of this is where certain large mammals are adapted to surviving long period without food due to increase body fat, whilst small mammals of the same species survive due to their decreased need for food. As a result intermediate sized animals are selected against.

What is speciation?

The process by which new species arise after a population becomes separated and cannot interbreed.

What is allopatric speciation?

allopatric speciation is caused by a physical barrier. As the two groups become separated and reproductively isolated the result is that the gene flow is reduced. Each group experiences a different selection pressure as the environment they live in is different. Over time, the frequency of alleles changes through natural selection and the two parts of the population can no longer interbreed and become separate species.

Give an example of allopatric speciation?

An example is the camel and the llama which both came from a common ancestor that live on the landmasses that are now Africa and South America before continental drift separated these two continents.

What is sympatric speciation?

Where new species evolve from a single ancestral species when inhabiting the same geographic region, for example as a result of a chromosomal error during cell division which leads to reproductive isolation. Other reasons include mutations that mean courtship behaviour isn’t recognised and anatomical changes that mean the penis cannot enter the vagina in mammals.

What is an ecosystem?

An ecosystem includes all the organisms living in a particular area known as the community as well as all the non-living elements of that particular environment.

What factors determine the size of a population?

Initially a population may grow quickly due to there being no limiting factors, however there are many abiotic and biotic factors that will affect the size and rate of population growth.

1. Temperature - each species has a different optimum temperature that it is best able to survive at, the further away from this the fewer individuals that are able survive.

2. Light - this is a basic necessity of light, with the rate of photosynthesis increasing as light intensity increases.

3. pH - this can have an impact on the action of enzymes with each enzyme having an optimum pH that it can work at. Where the appropriate pH exists there is a larger proportion of organisms.

4. Water and humidity - in instances where water is scarce only small populations of adapted species will exist. Humidity affects transportation in plants and therefore only those that are adapted to environments where transpiration is high will survive.

What is carrying capacity?

The size of population an ecosystem can support.

What is intraspecific competition and what do they compete for? Give an example.

Competition between members of the same species. Shelter, minerals, light, food, water, mates. An example is male robins which maintain a large territory to support their families, however in winter when food is short they move into other males territories to obtain enough food to survive.

What is interspecific competition? And give an example.

Members of different species compete with one another for the same resources. This occurs most commonly when different species occupy the same niche. An example is red and grey squirrels in the UK.

What is predation?

When one species (the prey) is caught and eaten by another species (the predator).

How do predator-prey relationships occur?

1. When the prey is eaten by the predator the population of the prey falls.

2. This results in the predator population growing, however means that more prey is consumed.

3. Therefore the population of prey reduces and there is increased competition for the the prey between the predators.

4. The lack of food for the predators means that the population falls meaning that less prey is eaten.

5. This allows the population of the prey to recover and therefore the cycle occurs over in a oscillating manner.

How can the size of a population be estimated?

Using randomly placed quadrats, or quadrats along a belt transect, for slow moving or non-motile organisms

How can the abundance of different species be measured?

• Percentage cover - suitable for plants or algae whose individuals numbers are difficult to count.

• Frequency - expressed as a decimal or percentage, and is the number of times an organism appears in the sampling area.

How can the population of fast moving or hidden animals be measured?

Mark-release-recapture method.

1. Known number of species is captured and marked in a way that doesn’t reduce their chance of survival.

2. These marked organisms are then released again into the same area they were caught.

3. After a suitable length of time another known number of organisms are captured, with the number of these that are marked being recored.

What is the equation for estimated population size.

estimated pop size = # in first sample * # in second sample/ # of marked individuals recaptured

What assumptions does the mark recapture method rely upon?

• The proportion of marked to unmarked individuals in the second sample is the same as the proportion of marked to unmarked individuals in the population as a whole.

• The marked individuals released from the first sample distribute evenly amongst the remainder of the population and have sufficient time to do so.

• There is a definite boundary to the population so there is no immigration into or emigration out of the population.

• There are few, if any, births and deaths within the population.

• The method of making is not toxic and does not reduce the chance of survival for the population.

• The marks or labels do not rub off during the investigation

What is succession?

Succession is the change of one community of organisms into the other.

When does primary succession occur?

When area previously devoid of life is colonised by communities of organisms. The area is first colonised by the pioneer species such as lichens which are adapted to survive in such harsh conditions. As organisms die, they are decomposed by microorganisms adding humus, this leads to formation of soil which makes the environment more suitable for more complex organisms. Over time, the soil becomes richer in minerals enabling larger plants such as shrubs to survive. Eventually, a climax community is established which is the final stage of succession, a self-sustaining and stable community of organisms.

What is secondary sucession?

Occurs in a previously colonised area in which an existing community has been cleared. This type of succession can occur after events such as forest fires. As a soil layer is already present, succession begins at a later stage.

What is conservation?

The human management of the Earth’s resources and typically involves the managing of succession. For example the controlled burning of land is done in order to stop the formation of a climax community.