chapter 20 p3

Question 1

Chi-squared test

Answer 1

• The observed results from a genetic cross will almost always differ to some extent from the expected results and this will be due to chance.
• If you toss a coin 10 times you would be unlikely to get five heads and five tails.
• The observed ratio of heads to tails will probably be quite different from the expected ratio.
• This does not mean there is anything wrong with the coin.
• If the same coin were tossed a thousand times you would see less relative difference between the expected and observed ratios.
• The number of observations made, therefore, determines how chance affects the results.
• It is important when making comparisons between observed and expected results that it is known whether any differences are due to chance or if there is a reason for the differences (they are significant).

Question 2

The chi-squared (x) test is a statistical test that

Answer 2

measures the size of the difference between the results you actually get (observe) and those you expected to get.
- It helps you determine whether differences in the expected and observed results are significant or not, by comparing the sizes of the differences and the numbers of observations.
- The chi-squared test is conventionally used to test the null hypothesis.

Question 3

The null hypothesis is

Answer 3

that there is no significant difference between what we expect and what we observe - in other words any differences we do see are due to chance.

Question 4

Calculated chi squared values are used to

Answer 4

find the probability of the difference being due to chance alone.

Question 5

Answer 5

Question 6

Large chi-squared values mean

Answer 6

there is a statistically significant difference between the observed and expected results and the probability that these differences are due to chance is low.
There must be a reason, other than chance, for the unexpected results.
The number of categories being compared in an investigation affects the size of the chi-squared value calculated.

Question 7

The degrees of freedom

Answer 7

the number of comparisons being made and is calculated as n-1, where n is the number of categories or possible outcomes (phenotypes in the case of phenotypic ratios) present in the analysis.
For example, if you were looking at yellow and green peas there would be two categories and therefore one degree of freedom.

Question 8

If the calculated x^2 value is less than the critical value found in a table at 5% significance (p=0.05)

Answer 8

we do not have sufficiently strong evidence to reject our null hypothesis. Therefore, we accept the null hypothesis - there is no significant difference between what we observed and what we expected.

Question 9

However if the calculated X^2 value is greater than the critical value

Answer 9

we reject the null hypothesis - some other factor, outside our original expectation, is likely to be causing a significant difference between expectation and observation.

Question 10

the critical value

Answer 10

The minimum x^2 value that gives a 5% probability
The critical value increases as the degrees of freedom increase.

Question 11

If X^2 is less than the critical value

Answer 11

there is no significant difference

Question 12

If X^2 is greater than or equal to the critical value

Answer 12

there is a significant difference

Question 13

Answer 13

Question 14

Corn and the chi-squared (x^2) test

Answer 14

• There will almost always be differences between expected and observed results because of the random nature of the processes involved.
• Statistical tests like the chi-squared test are performed to determine whether these differences are due to chance alone or caused by some other factor that may not have been considered.
• Maize plants have been used for many years to study genetic crosses.
• An ear of corn contains around 500 kernels, or seeds.
• The seeds are produced as the result of the cross-fertilisation of two maize plants.
• The colour of the seeds is controlled by one gene with a dominant (P, purple) allele and a recessive [p, yellow) allele.
• Another gene determines the texture of the seeds and there is again a dominant (R, round) allele and recessive (r, wrinkled) allele.
• A genetic study was carried out to determine if these two genes are linked.
• Two maize plants that were heterozygous for both colour and texture were cross-fertilised and an ear of corn produced as a result of this cross was analysed.
  The results are shown in Table 2.

Question 15

Epistasis:

Answer 15

• the interaction of genes at different loci.
• Gene regulation is a form of epistasis with regulatory genes controlling the activity of structural genes, for example, the lac operon.
• Gene interaction also occurs in biochemical pathways involving only structural genes.
• It was originally thought that all genes were expressed independently, and therefore their effects on the phenotype seen.
• Now it is known that many genes interact epistatically.
• It is the results of these interactions that we see in the phenotypes of living organisms.
• The characteristics of plants and animals that show continuous variation involve multiple genes and epistasis occurs frequently.

Question 16

epistasis example p1

Answer 16

Question 17

epistasis example p2

Answer 17

Question 18

Dominant and recessive epistasis:

Answer 18

• An epistatic gene may influence the activity of other genes as result of the presence of dominant or recessive alleles.
• In the example previously, if the presence of two recessive alleles at a gene locus led to the lack of an enzyme then it would be called recessive epistasis.
• Dominant epistasis occurs if a dominant allele results in a gene having an effect on another gene.
• This would happen if an epistatic gene (not present in this pathway) coded for an enzyme that modified one of the precursor molecules in the pathway.
• The next enzyme in the pathway would then lack a suitable substrate molecule and so the pigment would again not be produced.
• All of the genes in the sequence would be effectively ‘masked’.

Question 19

Labrador colours:

Answer 19

• The colour of Labrador dogs is produced as a result of the pigment melanin being deposited in the skin and fur.
• One gene codes for the production of the pigment and has the alleles B (dominant, black pigment produced) and b (recessive, brown pigment produced).
• A second gene codes for where the pigment is deposited and, again, has two alleles E (dominant, pigment deposited in the skin and fur) and e (recessive, pigment deposited in the skin only).
• The colour of a Labrador varies depending on which alleles are present at each locus.
• The genes are not expressed independently and so this is an example of epistasis.
• The gene at the E locus is epistatic to the hypostatic gene at the B locus.
• The different phenotypes and genotypes are shown in Table 3.
• There are only three registered colours, black, brown (chocolate), and yellow (golden).
• The yellow coat is an example of recessive epistasis and it ranges from deep gold to pale blond.

Question 20

Evolution

Answer 20

the change in inherited characteristics of a group of organisms over time, occurs due to changes in the frequency of different alleles within a population.

Question 21

Population genetics:

Answer 21

• Population genetics investigates how allele frequencies within populations change over time.

Question 22

gene pool.

Answer 22

• The sum total of all the genes in a population at any given time is known as the gene pool.
• The gene pool of a population includes millions of genes, but you will look at the variation in the different alleles of a single gene within the gene pool.

Question 23

allele frequency.

Answer 23

• The relative frequency of a particular allele in a population is the allele frequency.
• The frequency with which an allele occurs in a population is not linked to whether it codes for a dominant or a recessive characteristic, and it is not fixed.
• It can change over time in response to changing conditions.
• Evolution involves a long-term change in the allele frequencies of a population, for example, alleles for antibiotic resistance have increased in many bacteria populations over time.
• Biologists have developed ways of determining allele frequencies and use them in models to determine whether evolution is taking place.

Question 24

Calculating allele frequency:

Answer 24

Imagine a population of 100 diploid organisms that can all breed successfully.
You are going to look at a gene that has two possible alleles, A and a.
The frequency of allele A in the population is represented by the letter p.
The frequency of allele a in the population is represented by q.
If every individual in your population of 100 is a heterozygote (Aa), then the frequency of each allele is 100/200 or 0.5 (50%) so p + q = 1
In a diploid breeding population with two potential alleles, the frequency of the dominant allele plus the frequency of the recessive allele will always equal 1.
This simple formula is very important when using the Hardy-Weinberg principle.

Question 25

The Hardy-Weinberg principle:

Answer 25

- The Hardy-Weinberg principle models the mathematical relationship between the frequencies of alleles and genotypes in a theoretical population that is stable and not evolving. - The Hardy-Weinberg principle states: in a stable population with no disturbing factors, the allele frequencies will remain constant from one generation to the next and there will be no evolution. - A completely stable population is not common in the real world, but this is still a useful tool. - The Hardy-Weinberg principle provides a simple model of a theoretical stable population that allows us to measure and study evolutionary changes when they occur.

Question 26

Answer 26

Question 27

Answer 27

Question 28

Disturbing the equilibrium:

Answer 28

The Hardy-Weinberg principle assumes a theoretical breeding population of diploid organisms that is large and isolated, with random mating, no mutations, and no selection pressure of any type. In a natural environment these conditions virtually never occur. Species are continuously changing. In the peppered moths of the worked example, the light alleles were dominant historically but the allele frequencies changed dramatically after the Industrial Revolution, when the dark alleles gave individuals an advantage. Now the allele frequencies have changed again as cities and woodlands have become cleaner again. These changes in allele frequencies can be illustrated using the Hardy-Weinberg principle and upsetting the equilibrium may eventually result in evolution.

Question 29

Factors affecting evolution: There are a number of factors that lead to changes in the frequency of alleles within a population and so they affect the rate of evolution:

Answer 29

- Mutation is necessary for the existence of different alleles in the first place, and the formation of new alleles leads to genetic - Sexual selection leads to an increase in frequency of alleles which code for characteristics that improve mating success. - Gene flow is the movement of alleles between populations. Immigration and emigration result in changes of allele frequency within a population. - Genetic drift occurs in small populations This is the change in allele frequency due to the random nature of mutation The appearance of a new allele will have a greater impact is more likely to increase in number) in a smaller population than in a much larger population where there is a greater number of alleles present in the gene pool. - Natural selection leads to an increase in the number of individuals that have characteristics that improve their chances of survival. Reproduction rates of these individuals will increase as will the frequency of the alleles coding for the characteristics. This is how changes in the environment can lead to evolution

Question 30

The impact of small populations:

Answer 30

- The gene pool of a large population ensures lots of genetic diversity owing to the presence of many different genes and alleles. - Genetic diversity leads to variation within a population which is essential in the process of natural selection. - Selection pressures such as changes in the environment, the presence of new diseases, prey, competitors, or even human influences lead to evolution. - The population can adapt to change over time. - Small populations with limited genetic diversity cannot adapt to change as easily and are more likely to become extinct. - A new strain of pathogen could wipe out a whole population. - The size of a population can be affected by many factors. - Factors which limit or decrease the size of a population are called limiting factors

Question 31

two types of limiting factors:

Answer 31

Density-dependent factors Density-independent factors

Question 32

Density-dependent factors

Answer 32

are dependent on population size and include competition, predation, parasitism, and communicable disease.

Question 33

Density-independent factors

Answer 33

affect populations of all sizes in the same way including - climate change, natural disasters, seasonal change, and human activities (for example, deforestation).

Question 34

population bottlenecks

Answer 34

Large reductions in population size which last for at least one generation The gene pool, along with genetic diversity, is greatly reduced and the effects will be seen in future generations. It takes thousands of years for genetic diversity to develop in a population through the slow accumulation of mutations.

Question 35

Northern elephant seals

Answer 35

were almost hunted to extinction in the 19th century. There were probably only about 20 seals left by the time hunting stopped. They now have a population of about 30000 but show much less genetic diversity than southern elephant seals that did not experience a genetic bottleneck.

Question 36

Cheetahs are thought to have experienced an initial population bottleneck

Answer 36

about 10000 years ago with other bottlenecks happening more recently. The species now shows low genetic diversity. Cheetahs face the same threats as many other African animals such as habitat loss and poaching, but while the population sizes of other animals are increasing thanks to the efforts of conservationists, cheetahs are not recovering as quickly. They are, in fact, close to extinction. The reduced genetic diversity of cheetahs means that they share around 99% of their alleles with other members of the species, more than we share with members of our own family. Mammals usually share about 80% of their alleles with other members of a species. As a result they are showing problems of inbreeding including reduced fertility.

Question 37

Humans and chimpanzees split from a common ancestor

Answer 37

about six million years ago. A small group of chimpanzees are likely to show more genetic diversity than all the humans alive today. It is believed that humans have experienced at least one genetic bottleneck, reducing our genetic diversity, as we have evolved into our present form. A positive aspect of a genetic bottleneck is that a beneficial mutation will have a much greater impact and lead to the quicker development of a new species. This is thought to have a played a role in the evolution of early humans.