2020 Pure Biology TYS Answers

GCE O Level Biology 6093 Answer Scheme 2020 Paper 1

Paper 2 Section A Qn Answer Remarks 1(a) A: ovary B: oviduct C: cervix Incorrect answers for C: Uterus/ vagina (b) Cross placed at the fallopian tubes/ oviduct (c) The fertilised egg, also known as the zygote moves along the fallopian tube towards the uterus. It undergoes mitosis to form a ball of cells called an embryo. Once it reaches the uterus, the embryo implants itself into the uterine lining and develops into the fetus. Incorrect ans referred to implantation of the zygote and ref to meiosis. Extra details on fertilisation/ hormones and fetal development were not necessary. (d)(i) Its hair like structures sweep the developing embryo towards the uterus for implantation. (ii) Respiratory system Weaker responses referred to a specific organ eg. Trachea instead of a system. Incorrect ans confused cilia with villi and referred to digestive system. 2(a) D each chromosome is, duplicated / replication / doubled / copied into sister chromatids ; E chromosomes/ sisters chromatids have divided/ separated into 2 separate cells/ cytokinesis; Weak answers only stated what they could see in the graph. (b)(i) anaphase (ii) Before metaphase After telophase (iii) Haploid number 3 The figure shows 12 chromosomes formed from 6 sister chromatids that had separated during anaphase. Thus the diploid number is 6 and the haploid number is half of that which 3 chromosomes. (c) Crossing over of homologous chromosomes during prophase results in variation as it produces a new combination of alleles in the chromatids. The stages were not required, but some responses incorrectly assigned these processes to the wrong stage.

During independent assortment in metaphase I, depending on how the homologous chromosomes pair up, different combinations of chromosomes in the gametes can be produced. Such variations in gametes and the random fusion of male and female gametes during fertilisation leads to unique diploid DNA combinations leads to variation in the zygote. 3(a) Father x mother Genotype of IAIO IOIO parents Gametes IA IO IO IO Genotype of IAIO IAIO IOIO IOIO offspring (b) Codominance Occurs when both alleles are equally expressed in the heterozygous condition. For example a person of AB phenotype had both IA and IB alleles in his genotype. Multiple alleles Occurs when a gene for a particular trait exists in more than two alleles. For example, the gene for blood type is controlled by three different alleles, both IO , IA and IB. The alleles for the blood types must be stated and it should be clear in multiple alleles that they are alleles of the same gene or locus. (c) Discontinuous variation refers to traits/ characteristics that are controlled by one or a few genes and have few distinct types with no intermediates. These characteristics/ variation is also not affected by the environment and is only determined by inheritance of alleles. Weak answers only made reference to the feature being unaffected by the environment. (d) Gene is a segment of DNA, where the sequence of nucleotides codes for the production of a specific polypeptide that is responsible for a particular characteristic/ trait. Alleles are alternative forms of a gene that results in variations of a characteristic/ trait/ gene. They

occupy the same relative position in a pair of homologous chromosomes. 4(a) As the age of smoking decreases, the relative death rate increases. At the starting age of 10-14, the death rate is 2 times more than a non smoker. At the starting age of 25-29, the death rate decreases to 1.25 times that of a non smoker. There is a significant decrease in the death rate of 0.45 units between the starting age of 20-24 and 25- 29. Strong answers showed analysis of the data instead of just quoting data from the graph. (b) When the coronary arteries are occluded, The lumen of the coronary arteries are narrowed or completely blocked. This reduces blood flow to the heart muscles. The heart muscle cells receive less oxygen and glucose, resulting in reduced aerobic respiration. Less energy is released and may cause death of the heart muscle cells leading to muscle damage and a heart attack. Good answers focus on the effects of the cardiac/ heart muscle. Weaker responses focused on why the arteries became blocked, or the effects of artery blockage on the whole body. (c) Carbon monoxide in cigarette smoke binds to haemoglobin. This reduces the oxygen carrying capacity of the red blood cells. Less oxygen is transported to the fetus, resulting is reduced aerobic respiration and thus less energy released for growth of cells. This leads to low body mass in the baby. 5(a) 40/0.04 = 1000 (b) The insecticide is a toxic chemical that cannot be broken down and is not excreted from the bodies of organisms. The insecticide is stored in the cells of the organisms and accumulates. This is called bioaccumulation. The water plants take in the insecticides and stores it in its cells. The small fish eat the plants and accumulates the insecticide in its cells. The small fish is eaten by large fish which is eaten by the fish-eating birds.

The insecticide is passed down the food chain and become concentrated in the bodies of the final consumers by bioaccumulation. The concentration of insecticide which is toxic in the birds are the highest, thus resulting in the higher proportion deaths. (c) If all the fish-eating birds die, the population of large fishes will increase as the predator has been removed. This will lead to a drastic decrease in the numbers of small fishes as more of them will be eaten by the increased numbers of large fishes and the small fishes may not be able to reproduce quickly enough to replace the decrease in numbers. 6(a) The elastic layer allows the artery to maintain and withstand the high blood pressure by constricting and dilating the lumen. (ii) It helps to withstand the high blood pressure and prevent the vessel from bursting. Contraction and relaxation of the muscle fibres allow for the constriction and dilation of the lumen which controls the flow and pressure of blood and allows blood to be re directed such as in temperature regulation in the skin. Some incorrectly stated that with reduced size of the lumen, blood pressure in small arteries would be even higher than in large arteries and consequently answered in terms of the ability to withstand high pressure. (b) It contains semi lunar valves. Incorrect answers referred to tricuspid or bicuspid

Section B No. Answer Marks Comments 7 (a)(i) Scale Axes Plot Points Best fit line 4 Some responses did not draw a correct line of best fit. A common error was not having a balance of data points on either side of the line. (ii) As the year progressed from 2010 to 2016, the number of people worldwide who had HIV increased from 33.2 million to 36.7 million;; Between 2011 and 2016, there was an increase in 0.6 million people per year; The increase in number of people was proportional to time/year; 3 1 mark for correct trend 1 mark for correct figures quoted Stronger performing candidates compared or manipulated the data rather than simply quoting a series of data values, e.g. reference to the increase of 0.6 million per year. (b) Any 3 Keep to one sex partner; Practice abstinence / do not have sex; Use a condom when having sexual intercourse; Do not abuse drugs as drug addicts tend to share needles and needles can be contaminated with the virus; Do not share instruments that can break skin and get contaminated with blood; Mak