7.2 Populations (A Level only)

In bird species, females are the heteromorphic sex (ZW) and males are homomorphic (ZZ). The genus Meleagris includes the commonly known turkey. Their feathers can be either brown or bronze in colour. Figure 1 shows the inheritance of feather colour in a group of turkeys. 

 Figure 1

Use Figure 1 to provide evidence that feather colour is sex-linked. 

Use Figure 1 to explain the evidence that the brown phenotype is recessive. 

Turkey 4 was found to be heterozygous. It was further crossed with turkey 3 producing more offspring. Use a genetic diagram to predict the likely ratio of phenotypes expected in these offspring.

The rate of Feather colour production in one species of turkey is controlled by a pair of codominant alleles which are not sex-linked. The allele R^F codes for fast feather production while the allele R^S codes for slow feather production. Heterozygous chickens produce feathers at a normal rate.

On a farm, 4% of the chickens had fast feather production. Use the Hardy-Weinberg equation to calculate the percentage of this population that you would expect to produce feathers at a normal rate. 

State what is meant by the Hardy-Weinberg principle. 

Rhagoletis pomonella is otherwise known as the apple maggot fly. Initially, these maggots would lay their eggs on hawthorn fruit. In the 19th century a distinct group of maggots emerged that lay their eggs on apples only. Apples and hawthorn fruit are closely related. 

The inheritance of eye colour in the apple maggot flies was investigated. Two flies with red eyes were crossed. Of the offspring, 304 had red eyes and 96 had black eyes.

Using suitable symbols, state and explain the genotypes of the parents. 

Apple maggot flies can be either white or cream coloured. Scientists studying a population of apple maggot flies found that 43% were cream coloured, which is the recessive phenotype. Use the Hardy-Weinberg equation to calculate the percentage of apple maggot flies that are heterozygous.

The Hardy-Weinberg equation makes many assumptions about a given population.

Suggest three reasons why the frequency of an allele, or phenotype, calculated using the Hardy-Weinberg equation may be inaccurate. 

The Galapagos Islands are a group of small islands 600 miles off the coast of South America. They are the site of Charles Darwin's famous study of finches. The islands are home to many endemic species of animals and plants which means they are not found anywhere else. 

Scientists studying the islands found the prevalence of one species of finch, Geospiza fortis. They discovered a gene, called ALX1, which codes for facial features such as beak shape. This particular species of finch may have blunt or pointed beaks. The scientists found that the blunt beak was better suited for cracking open tough seed coverings. They discovered that the blunt beak was recessive and investigated the frequency of this gene variant within a population. Figure 1 shows the scientists results.

Figure 1

85% of finches possessed blunt beaks in 2010. Use Figure 1 to calculate the frequency of the heterozygous phenotype in 2010.

On one island there were 500 finches in the total population. In this population, 300 hadlong beaks with the genotype B^LB^L, 150 had medium length beaks with the genotype B^LB^S, with the remainder having B^SB^S. 

Use these figures to calculate the actual frequency of the B^S allele. 

Use your answer to part b) and the Hardy-Weinberg equation to calculate the number of finches that would be expected to have the phenotype B^LB^S.

Scientists use statistical tests to determine the significance of the data. 

State which statistical test scientists should use for this data. 

Malaria is caused by a parasite that attacks red blood cells, producing repeated bouts of serious illness and often causing death. The allele for normal haemoglobin in red cells is Hb^A. In the West African country of Burkina Faso, 20% of people are heterozygous for a different allele, Hb^C, which has no effect on their health. People homozygous for Hb^C suffer a very mild anaemia. Figure 1 shows how the Hb^C allele affects the chance of getting malaria.

Figure 1

The H^bC allele is increasing in frequency in parts of Africa, such as Burkina Faso. Suggest an explanation for this.

Scientists investigated the frequency of homozygous Hb^A individuals in different parts of the world. Table 1 shows their results in Europe and Asia. 

Table 1

Using Table 1, calculate the frequency of heterozygous genotype in Asia. 

Figure 2 shows the prevalence of malaria and the frequency of the Hb^C allele. Those individuals homozygous with Hb^C Hb^C develop mild anaemia. Scientists concluded that the Hb^C allele is more prevalent in areas with higher risk of mortality from malaria. 

Figure 2

Use the information in Figure 1 to evaluate their conclusion. 

In a village with a population of 500, there were 8 people who were homozygous for the Normal adult haemoglobin sickle–cell allele and 96 who were heterozygous. 

Calculate the frequency of the Hb^C allele in the village. 

Ecologists studied the shell colour of a population of banded snails (Cepaea nemoralis) at two different sites in a birch woodland. There are two phenotypes, pink and yellow which are determined by a single gene with two alleles. The pink allele (C^P) is dominant over the yellow allele (C^Y).

Explain what is meant by a population. 

Outline the method ecologists would use to collect sufficient data to determine whether the observable phenotype frequencies for the two shell colours were similar between the two sites in the birch woodland. 

A   test was carried out to determine if the observed phenotypic ratio of pink to yellow shell colour was expected (3:1). When the calculated  was looked up in a table it was found to correspond to a probability of more than 0.05.

What conclusion can you draw from this?

A population of snails sampled from one of the birch woodland sites contained 84% pink shelled snails. 

Use the Hardy-Weinberg equation to calculate the percentage of the snail population that you would expect to be heterozygous for pink shells. Show your working.

In a certain species of goat the coat colour is controlled by one gene, this is known as agouti fur. The coat colour may be brown or tan, black or white. The allele for brown, C^B, is dominant to the other two alleles. The allele for black, C^A, is dominant to the allele for white, C^W.

Explain what is meant by a recessive allele.

A cross between two brown-coated goats produced only brown-coated and black-coated goats. Use a genetic diagram to explain why.

Give three assumptions that must be made when using the Hardy-Weinberg equation.

In a hilly region in Portugal white-coated goats make up 25 % of the goat population. Assuming the conditions of the Hardy-Weinberg equilibrium apply, calculate the frequency of allele CW in this population. Show your working.

In Borneo, the blood type of 300 orangutans were sampled by ecologists. In this population, 81 orangutans had the genotype I^AI^O, 63 had the genotype I^BI^O, 153 had the genotype I^AI^B and 3 had the genotype I^OI^O. 

Using the given information, explain the term gene pool.

Explain what the Hardy-Weinberg principle predicts?

Calculate the actual frequency of the allele IO. 

The Hardy-Weinberg equation should not be used in your calculation.

In another population of 500 orangutans, the frequency of the I^O allele was 0.3. 

Determine what the Hardy-Weinberg principle will predict about the frequency of the I^O allele after another 5 generations?

In Asia, pyrethroids are sprayed insecticides used to kill mosquitoes, to try to control the spread of dengue. Dengue is a virus that is spread by Aedes aegypti mosquitoes. 

The A.aegypti mosquitoes have a gene called voltage-gated sodium channel (VGSC). Scientists studying pyrethroid resistance in mosquitoes discovered that some mosquitoes had an allele of this gene, V1016G minus, which gave them resistance to pyrethroids. The other allele, V1016G plus, did not give resistance.

Scientists investigated the frequency of the V1016G minus allele in a population of A.aegypti mosquitoes in Taiwan over a period of 10 years. Figure 1 below shows the scientists’ results.

Figure 1

Using the Hardy–Weinberg equation calculate the frequency of mosquitoes that are heterozygous for the V1016G minus gene in this population in 2014. Show your working.

Figure 2 shows the change in frequency of the V1016G plus in an area in which the pyrethroid insecticides were being used on a weekly basis. Describe and explain the shape of the curve.

Figure 2

Research has shown that the use of pyrethroid insecticides close to waterways has caused the numbers of rainbow trout to decrease. Ecologists studied the frequency of two alleles of a gene in a population of rainbow trout. The dominant allele M, codes for an enzyme. The other allele, m, is recessive and does not produce a functional enzyme.

In a population of rainbow trout, the allele frequency for the dominant allele, M, was found to be 0.6.

Calculate the percentage of homozygous dominant rainbow trout in this population. Show your working.

After 5 years the ecologists repeated their study of the rainbow trout population. They found that the frequency of the dominant allele had decreased.

A statistical test showed that the difference between the two frequencies of the M allele was significant at the p = 0.05 level. Explain what this means.