Biology Unit 4 Chapter 8 - Inheritance and Selection

Several years later, scientists repeated their study on this population. They found that the frequency of the recessive allele had decreased. A statistical test showed that the difference between the two frequencies of the t allele was significant at the P = 0.05 level. Use the terms probability and chance to help explain what this means. (2)

There is a probability of 0.05; That difference in results are due to chance

(Refer to Jan 2011) The Rhesus blood group is genetically controlled. The gene for the Rhesus blood group has two alleles. The allele for Rhesus positive, R, is dominant to that for Rhesus negative, r. The diagram shows the inheritance of the Rhesus blood group in one family. Explain one piece of evidence from the diagram which shows that the allele for Rhesus positive is dominant. (2)

Two Rhesus positives produce Rhesus negative children; if Rhesus positive was recessive, all children of 3 and 4 would be Rhesus recessive

Changes in ecosystems can lead to speciation. In Southern California 10 000 years ago a number of interconnecting lakes contained a single species of pupfish. Increasing temperatures caused evaporation and the formation of separate, smaller lakes and streams. This led to the formation of a number of different species of pupfish. Explain how these different species evolved. (5)

Variation due to mutation; Different selection pressures; Selection for advantageous allele; Selected organisms survive and reproduce; Leads to change in allele frequency

Changes in ecosystems can lead to speciation. A high concentration of copper in soil is toxic to most plants. In some areas where the soil is polluted with copper, populations of grasses are found to be growing. These populations of grass belong to a species also found growing on unpolluted soils. It has been suggested that a new species of grass may evolve on soil that has been polluted with copper. Explain how this new species might evolve. (5)

Variation; Mutation; Some plants have allele to survive in high concentration of copper; Selected organisms survive and reproduce; Increase in frequency of allele

What is meant by geographical isolation? (1)

When a physical barrier prevents two populations from breeding with each other

(Refer to June 2010 paper) The table shows the frequencies of some alleles in the population of cats in three cities. White cats are deaf. Would the Hardy-Weinberg principle hold true for white cats? Explain your answer.

White cats unlikely to survive; Will not pass on allele for deafness to next generation

The actual percentage of heterozygotes is likely to be lower in future generations than this answer. Explain why. (1)

Affected individuals usually do not reproduce

Give all the genotypes which would result in a brown-shelled snail. (1)

CBCB, CBCP and CBCY

Explain what is meant by a recessive allele. (1)

Only expressed in the phenotype when homozygous alleles are present

Use evidence from the diagram to explain that hairlessness is caused by a gene on the X chromosome. (1)

Only males are hairless

Explain one piece of evidence from the diagram which proves that the allele for Tay-Sachs disease is not on the X chromosome. (2)

11 is affected, 3 is not; If on X 11 would not receive the recessive allele on X chromosome from father

(Refer to June 2012 paper) Tay-Sachs disease is a human inherited disorder. Sufferers of this disease often die during childhood. The allele for Tay-Sachs disease t, is recessive to allele T, present in unaffected individuals. The diagram shows the inheritance of Tay-Sachs in one family. Explain one piece of evidence from the diagram which proves that the allele for Tay-Sachs disease is recessive. (2)

3 and 4 are both unaffected and have affected offspring; So both 3 and 4 are carriers

Explain one piece of evidence from the diagram which shows that the gene is not on the X chromosome. (2)

3 is Rhesus positive; 3 could not receive Rhesus positive allele from mother if it were on the X chromosome

Hair length in cats is determined by a single gene with two alleles. The allele for long hair (h) is recessive. The allele for short hair (H) is dominant. Use the information in the table and the Hardy-Weinberg equation to estimate the percentage of cats in London that are heterozygous for hair length. Show your working. (2)

44.22;

Sixteen percent of the population of Europe is Rhesus negative. Use the Hardy-Weinberg equation to calculate the percentage of this population that yo would expect to be heterozygous for the Rhesus gene. Show your working. (3)

48(%); q2 = 16%; 2pq = heterozygotes

Sea otters were close to extinction at the start of the 20th century. Following a ban on hunting sea otters, the sizes of their populations began to increase. Scientists studied the frequencies of two alleles of a gene in one population of sea otters. The dominant allele, T, codes for an enzyme. The other allele, t, is recessive and does not produce a functional enzyme. In a population of sea otters, the allele frequency for the recessive allele, t, was found to be 0.2. Use the Hardy-Weinberg equation to calculate the percentage of homozygous recessive sea otters in this population. Show your working. (2)

4; 0.2 x 0.2

(Refer to Jan 2012 paper) A single gene controls the presence of hair on the skin of cattle. The gene is carried on the X chromosome. Its dominant allele causes hair to be present on the skin and its recessive allele causes hairlessness. The diagram shows the pattern of inheritance of these alleles in a group of cattle. Use evidence from the diagram to explain that hairlessness is caused by a recessive allele. (2)

Animals 2 and 5 have hair but offspring do not; So 2 and 5 parents must be carriers

Explain what is meant by the term phenotype. (2)

Appearance due to genotype; and due to environment

Explain what is meant by codominant alleles. (1)

Both alleles are expressed in the phenotype

The shells of this snail may be unbanded or banded. The absence or presence of bands is controlled by a single gene with two alleles. The allele for unbanded, B, is dominant to the allele for banded, b. A population of snails contained 51 % unbanded snails. Use the Hardy-Weinberg equation to calculate the percentage of this population that you would expect to be heterozygous for this gene. Show your working. (3)

Correct answer of 42%; q2 = 0.49/49% OR q = 0.7/70%; Shows understanding that 2pq = heterozygotes / carriers / shows answer is derived from 2pq

Distinguish between stabilising selection and directional selection. (4)

Directional selection: Favours alleles/phenotype at one extreme of a population; Distribution curve stays the same but the mean shifts to the left or right Stabilising selection: Favours alleles/phenotypes around the mean of the population; Distribution curve becomes narrower and higher but the mean does not change

What type of natural selection appears to have occurred in this population of sea otters? Explain how this type of selection led to a decrease in the frequency of the recessive allele. (2)

Directional; The dominant allele confers advantage

The number of red-eyed females and red-eyed males in the offspring was counted. The observed ratio of red-eyed females to red-eyed males was similar to, but not the same as, the expected ratio. Suggest one reason why observed ratios are often not the same as expected ratios. (1)

Fertilisation is random

Large areas of land have to be used to grow the plants to make biofuels. Ecologists have suggested that changes in land use could lead to a decrease in biodiversity. Suggest how changes in land use could lead to a decrease in biodiversity. (2)

Fewer habitats; Fewer food sources

The Amazonian forest today contains a very high diversity of bird species. - Over the last 2 000 000 years, long periods of dry climate caused this forest to separate into a number of smaller forests. - Different plant communities developed in each of these smaller forests. - Each time the climate became wetter again, the smaller forests grew in size and merged to reform the Amazonian forest. Use the information provided to explain how a very high diversity of bird species has developed in the Amazonian forest. (5)

Geographical isolation; Mutation; Different selection pressures; Adapted organisms survive and breed; Increase in allele frequency

What is the evidence from the table that non-agouti and blotched are alleles of different genes? (1)

In Paris and London frequencies of these alleles add up to more than 1

In a species of snail, shell colour is controlled by a gene with three alleles. The shell may be brown, pink or yellow. The allele for brown, CB, is dominant to the other two alleles. The allele for pink, CP, is dominant to the allele for yellow, CY. Explain what is meant by a dominant allele. (1)

Is always expressed in the phenotype

The fruit fly is a useful organism for studying genetic crosses. Female fruit flies are approximately 2.5 mm long. Males are smaller and possess a distinct black patch on their bodies. Females lay up to 400 eggs which develop into adults in 7 to 14 days. Fruit flies will survive and breed in small flasks containing a simple nutrient medium consisting mainly of sugars. Use this information to explain two reasons why the fruit fly is a useful organism for studying genetic crosses. (2)

Large number of eggs therefore are representative; Markings make males and females easy to identify

Male fruit flies are more likely than female fruit flies to show a phenotype produced by a recessive allele carried on the X chromosome. Explain why. (2)

Males have one allele; Females could have dominant and recessive alleles

The population of trees in the forest evolved adaptations to the mountain environment. Use your knowledge of selection to explain how. (3)

Mutations took place; Some better adapted for survival in mountains; Allele frequencies change

A breeder crossed a black male cat with a black female cat on a number of occasions. The female cat produced 8 black kittens and 4 white kittens. Explain the evidence that the allele for white fur is recessive. (1)

Parents are heterozygous

The gene controlling coat colour has three alleles. The allele B gives black fur, the allele b gives chocolate fur and the allele bi gives cinnamon fur. ● Allele B is dominant to both allele b and bi. ● Allele b is dominant to allele bi. The breeder wanted to produce a population of cats that would all have chocolate fur. Is this possible? Explain your answer. (2)

Possible if parents homozygous; Don't know genotype of chocolate cat

Speciation is far less frequent in the reformed Amazonian forest. Suggest one reason for this. (1)

Same environmental factors

What is speciation? (1)

The evolution of new species from existing species

What does the Hardy-Weinberg principle predict about the frequency of the t allele after another 10 generations? (1)

The frequency remains the same

What does the Hardy-Weinberg principle predict? (3)

The proportion of alleles of a particular gene; Will stay constant from one generation to the next; Providing there is no mutation

In a human population, one in every 1000 children born had Tay-Sachs disease. Use the Hardy-Weinberg equation to calculate the percentage of this population you would expect to be heterozygous for this gene. Show your working. (3)

q2/p2/tt = 0.001 or 1 divided by 1000; p/q/T = 0.968 - 0.97; Understanding that heterozygous = 2pq; = 5.8 - 6.2%