William Bateson and Reginald Punnett used the sweet pea (Lathyrus odoratus) in genetics studies in the early 20^th century. Pure-breeding plants that produced purple flowers and long pollen grains were crossed with pure-breeding plants that produced red flowers and round pollen grains. The resulting offspring all produced purple flowers and long pollen grains. Two of the F₁ generation plants were crossed. The table shows the ratio of phenotypes in the F₂ generation. 

What is an explanation for these experimental results?

A. Purple flowers and long pollen grains are dominant and the alleles have assorted independently.

B. The genes for flower colour and pollen shape are linked and all plants producing long pollen grains are recombinants.

C. The genes for flower colour and pollen shape are linked and all plants producing red flowers are recombinants.

D. Plants producing purple flowers and round pollen grains arose through crossing over.

D

This was a wordy question that turned out to be quite time consuming. Many candidates failed to realize that if the alleles had assorted independently the ratios would have been 9:3:3:1. The only way the results shown could have been obtained was through genetic linkage between purple and long and red and round. Purple and round are formed by recombination of alleles through crossing over.

When a cell divides by meiosis, chiasmata can be observed. Which are features of chiasmata?

I. They are points of attachment between chromatids of non-homologous chromosomes.

II. They occur during meiosis I.

III. They increase stability of bivalents.

A. I and II only

B. I and III only

C. II and III only

D. I, II and III

C

This question discriminated well. Many weaker candidates failed to recognise that item I was incorrect as it referred to points of attachment between non-homologous (as opposed to homologous) chromosomes. This would have been sufficient to eliminate all the incorrect responses.

Which cell is a polyploid zygote produced by fusion of one haploid and one diploid gamete?

A

This question proved difficult and few candidates, including those scoring well on the examination, selected the correct response. The great majority of students opted for response D, presumably by thinking haploid = 1, diploid = 2 and when fused you get a cell with 3 chromosomes. For answer D to be correct the original diploid number would need to have been 2 and, in that case, the 3 chromosomes in the cell would be identical in size. Answer A is the only possible response with the original cell having 2n = 4.

What can lead to reproductive isolation after just one generation?

A. Polyploidy

B. Increased mutation rate

C. Changed allele frequencies

D. Independent assortment of chromosomes

A

The graph shows the relationship between mass at birth and the percentage that die shortly after birth for Scottish Blackface lambs.

[Source: Dwyer, C.M., Conington, J., Corbiere, F., Holmoy, I.H., Muri, K., Nowak, R., Rooke, J., Vipond, J. and Gautier,
J.-M., 2016. Invited review: Improving neonatal survival in small ruminants: science into practice. Animal, 10(3),
pp. 449–459.]

What type of selection for the lambs is shown in the graph?

A. Disruptive selection, as there is a drop in mortality at intermediate birth masses

B. Stabilizing selection, as lambs with low or high birth mass are less likely to survive

C. Directional selection, as lambs with a high birth mass have high mortality

D. There is no evidence in the graph of selection, as survival frequency is not shown

B

Two thirds of candidates chose the correct answer but the discrimination index was lower than expected. Perhaps some candidates were tiring by this stage of the exam and therefore did not think carefully enough. The shape of the graph looks superficially like disruptive selection, but the y-axis shows mortality, not survival.

Which process could cause non-disjunction if it occurred during meiosis?

A. Sister chromatids do not align in metaphase I.

B. Homologous chromosomes do not separate in anaphase I.

C. Sister chromatids do not align in metaphase II.

D. Homologous chromosomes do not separate in anaphase II.

B

The image shows variation in height of adult humans.

What can explain the variation?

A.  One pair of alleles and age

B.  Polygenic inheritance and nutrition

C.  Nutrition and age

D.  Autosomal inheritance only

B

What could account for this distribution of height in a population?

[Source: Graph adapted from Six Minutes http://sixminutes.dlugan.com/good-public-speaker-average/]

A. Gene linkage

B. Dominant alleles

C. Independent assortment

D. Multiple genes

D

A dihybrid cross was carried out between two plants to determine whether the genes for seed shape and colour are linked. If the genes are unlinked, the expected ratio of 9:3:3:1 should occur. A chi-squared test was carried out on the observed results of the cross. The critical value for chi squared at the 5 % level of significance in this test was 7.82. The calculated value for chi squared was 6.25. What can be concluded from this data?

A. The results prove that the genes are linked.

B. The results prove that the genes are unlinked.

C. There is significant evidence that the genes are linked.

D. There is significant evidence that the genes are unlinked.

D

Although slightly less than half of candidates answered correctly and the discrimination index was respectable rather than good, this was a relatively successful question on statistical hypothesis testing, which is typically a difficult area both for question setters and candidates. Performance seems to be improving in this part of the program.

Andalusian fowl have varied colours and types of feathers. The allele for black feathers is codominant with the allele for white, producing blue feathers in the heterozygote. The texture of feathers is controlled by another gene, with silky feathers recessive to normal. Blue silky birds are crossed with black silky birds. What is the expected proportion of blue silky offspring?

A. 0%

B. 25%

C. 50%

D. 100%

C

The genetic determination of dogs’ coats can be quite complex, with many different genes acting at the same time.

• The dominant allele E gives brown tones. The recessive allele e results in red tones.
• The colour intensity is due to another gene. The dominant allele B gives a dark colour, whereas the recessive allele b results in a light colour.

What would be the genotype of a light brown dog produced from a cross between a dark brown dog and a light red dog?

A. EEbb

B. EeBb

C. eeBb

D. Eebb

D

The graph shows variations in beak size for the bird Geospiza fortis on an island in the Galápagos archipelago.

What evidence from the graph indicates that disruptive selection is occurring?

A. An intermediate beak size is less common.

B. Median beak size is the most common.

C. Smaller beaks are favoured.

D. Larger beaks are favoured.

A

Fossil records show that black bears increased in size during the Ice Age and decreased in size with warmer temperatures. What type of selection do these changes in size represent?

A. Allopatric
B. Directional
C. Disruptive
D. Stabilizing

B

What is always passed to the next generation as a result of sexual reproduction?

A. Homologous chromosomes from the mother

B. A chromatid from every chromosome of the father

C. A haploid set of chromosomes from the mother

D. All alleles from each parent

C

What process occurs in both mitosis and meiosis?

A. Formation of chiasmata

B. Reduction division

C. Separation of chromatids

D. Exchange of alleles between non-sister chromatids

C

Which event happens in meiosis II but not in meiosis I?

A. Spindle microtubules attach to centromeres.

B. Crossing over occurs.

C. Sister chromatids move to opposite poles.

D. Chromosomes become shorter and thicker by coiling.

C

Many commercially produced bananas are triploid instead of diploid. The nucleus of a triploid cell has three sets of chromosomes. What is the effect of triploidy?

A. Seeds are larger.

B. Chromosomes cannot pair in meiosis.

C. Sexual reproduction is more rapid.

D. Mitosis cannot occur.

B

In fruit flies (Drosophila melanogaster), grey bodies (b⁺) are dominant to black bodies (b) and normal wings (vg⁺) are dominant to vestigial wings (vg). Homozygous vestigial winged, black bodied flies were crossed with individuals that were heterozygous for both traits. 2300 individuals were counted and the phenotypes observed were recorded as shown.

965 normal wings, grey bodies
944 vestigial wings, black bodies
206 vestigial wings, grey bodies
185 normal wings, black bodies

Which statement is valid?

A. The predicted phenotypic ratio was 9:3:3: 1.

B. There is independent assortment of wings but not body colour.

C. The expected number of vestigial winged, grey bodied flies was 575.

D. The traits are on different chromosomes.

C

This proved to be the third most difficult question, with all of the four choices attracting some candidates. Perhaps the 1:1:1:1 ratio from a dihybrid test cross was unfamiliar. It was not a quick question to answer and was close to the end of the exam, but the number of questions left unanswered by candidates (blanks) did not rise with the last few questions, suggesting that lack of time was not the problem.

What is polyploidy?

A. Having an extra set of chromosomes

B. Having an extra sex chromosome

C. Having an extra autosome

D. Having two or more nuclei

A

The cheetah (Acinonyx jubatus) is a large cat found in Africa. It has been discovered that organs could be transferred between any two individuals without rejection of the organ.

What is the probable reason for this?

A. Cheetahs have poor reproductive success.

B. Cheetahs have high heterozygosity.

C. Cheetahs have a large gene pool.

D. Cheetahs have a small gene pool.

D

This question discriminated well.

Natural selection can operate in different ways. What is the effect of disruptive selection?

A. It eliminates individuals with intermediate forms of a characteristic.

B. It eliminates individuals at random regardless of their characteristics.

C. It favours individuals with intermediate forms of a characteristic.

D. It favours individuals at one extreme of the range of variation in a characteristic.

A

How do the concepts of gradualism and punctuated equilibrium differ?

A. The timing of evolution

B. The mechanism causing evolution

C. The sequence of evolutionary events

D. The reality of evolution

A

Which statement is valid regarding chromatids?

A. Sister chromatids separate during meiosis I.

B. Chiasmata form between non-sister chromatids.

C. Crossing over is the exchange of DNA between sister chromatids only.

D. Non-sister chromatids have the same combination of alleles.

B

Which gametes can result from the following crossover?

D

In a plant, dark leaves are dominant to pale leaves and yellow seeds are dominant to white seeds.

A heterozygous dark-leaved plant with yellow seeds was crossed with a pale-leaved plant with white seeds. A large number of offspring were produced. They were either dark-leaved with yellow seeds or pale-leaved with white seeds in equal number.

What is the most likely cause of this pattern?

A. Crossing over has occurred.

B. The two genes are linked.

C. The traits are polygenic.

D. The genes are codominant.

B

An individual is heterozygous for two linked genes .

To investigate the frequency of crossing over, a test cross is carried out between the individual and another that is homozygous recessive for both genes. What are the possible recombinants in the offspring of this cross?

C

Which example shows disruptive selection?

A. Giraffe necks have become longer over time.

B. Medium-sized beaks in hummingbirds have decreased in frequency over time.

C. The peppered moth became less common in polluted environments.

D. Human babies with a very high or a very low birth mass have a higher mortality rate. 

B

Some of the ratios that Morgan investigated in genetic crosses did not correspond with expected Mendelian ratios.  What was the cause?

A. The genetic crosses used insects rather than plants.
B. The results were counted more reliably than Mendel’s.
C. The genes in the genetic crosses were linked.
D. Drosophila has more genes than plants.

C

What forms when two different chromatids of the same homologous pair cross over?

A. Daughter centromere

B. Chiasma

C. Chromosome mutation

D. Telomere

B

A hunter tends to kill the bigger individuals of a population for their meat or for large ornamental trophies. Therefore, the population tends to have more individuals who are smaller. What is this an example of?

A. Directional selection

B. Disruptive selection

C. Natural selection

D. Stabilizing selection

A

Three-spined stickleback fish (Gasterosteus aculeatus) vary in the number of armour plates. The graph shows the frequency of individuals with low, partial or complete plating in a three-spined stickleback population living in Kennedy Lake, Vancouver Island, Canada.

[Source: Reprinted from Current Biology, 24, Marchinko, K.B., Matthews, B., Arnegard, M.E., Rogers, S.M. and Schluter, D., Maintenance of a Genetic Polymorphism with Disruptive Natural Selection in Stickleback.
2014.  pp.1289–1292 with permission from Elsevier.]

Which type of natural selection could result in this pattern of variation in the population?

A. Disruptive

B. Directional

C. Stabilizing

D. Convergent

A