SB29. StudyPacks KS4 SCIENCE STUDY. Genetics. Inherited Diseases

Transcription

1 StudyPacks STUDY. KS4 SCIENCE Genetics Inherited Diseases This Study Pack aims to cover:. Genetic disorders 2. How to draw and analyse Genetic diagrams to show genetic disorders. SB29 Study Packs are prepared by Qualified Teachers and Specialists and are a complete range of comprehensive compiled resources based on the UK National Curriculum covering the Primary and Secondary Frameworks including SATs and GCSE examinations. Student Name

2 Contents Inherited diseases notes 2-6 Practice Questions 7 Exam questions 8-7 Mark scheme 8-23

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5 Sex determination Most body cells contain chromosomes in matched pairs. The number of pairs of chromosomes varies between species. Human body cells have 23 pairs of chromosomes in the nucleus. One of these pairs controls the inheritance of gender - whether offspring are male or female, in males, the two sex chromosomes are different. They are XY in females, the two sex chromosomes are the same. They are XX. A set of human chromosomes can be separated from its cell, spread out on a microscope slide and magnified many thousands of times. When stained and photographed, they look like this: Chromosomes from a male Chromosomes from a female Gametes Gametes are sex cells. The male gametes are the sperm, and the female gametes are the eggs. Gametes contain half the number of chromosomes as body cells do. They contain half the genetic information that body cells do. Alleles Some characteristics, such as eye colour and the shape of the earlobe, are controlled by a single gene. These genes may have different forms. Different forms of the same gene are called alleles (pronounced al-eels). The gene for eye colour has an allele for blue eye colour and an allele for brown eye colour. Alleles are dominant or recessive, meaning: the characteristic controlled by a dominant allele develops if the allele is present on one or both chromosomes in a pair 4

6 the characteristic controlled by a recessive allele develops only if the allele is present on both chromosomes in a pair. For example, the allele for brown eyes is dominant, while the allele for blue eyes is recessive. An individual who inherits one or two alleles for brown eyes will have brown eyes. An individual will only have blue eyes if they inherit two copies of the allele for blue eyes. Individuals A and B have brown eyes - only individual C has blue eyes Higher tier An individual is homozygous for a certain gene if they have two identical alleles. They are heterozygous for a certain gene if they two different alleles. The genotype is the genetic makeup of an individual. For example, it is the particular combination of alleles in each of the three examples above. The phenotype is the characteristics expressed by an individual. For example, it is the actual eye colour for the three examples above Inherited disorders Some disorders are inherited, such as: red-green colour blindness sickle cell anaemia cystic fibrosis. Inherited disorders are caused by faulty genes, which are mostly (but not always) recessive alleles. For example, cystic fibrosis is an inherited disorder that affects the cell membranes, causing the production of thick and sticky mucus. It is caused by a recessive allele, which means that it must be inherited from both parents. Genetic testing Scientists are now able to test adults and unborn babies for alleles that can cause genetic disorders. However, the scientific information produced raises many issues that science cannot address. For example, should a couple with a one in four risk of having a child with cystic fibrosis take the gamble, or decide not to have any children at all? If a woman becomes pregnant with a child that is going to have cystic fibrosis, should she have the child, or choose to have an abortion? These are questions about values that science cannot answer. Different people will have different views 5

7 Genetic diagrams- Higher tier Genetic diagrams are used to show the possible outcomes of a particular cross. A dominant allele is shown by a capital letter, and a recessive allele by a lower case letter. Cystic fibrosis Cystic fibrosis is an inherited disorder caused by a recessive allele. This genetic diagram shows the possible outcomes when both parents are heterozygous for the faulty allele. There is a one in four chance of the offspring being homozygous for the faulty allele, and so having cystic fibrosis. Mother and father both have one F and one f allele This genetic diagram shows the possible outcomes when only one parent carries the faulty allele. There is no chance of the offspring being homozygous for the faulty allele and therefore having cystic fibrosis. Mother has two F alleles, father has one F and one f allele. Gender determination Female gametes (eggs) contain X chromosomes, while male gametes (sperm) contain either X chromosomes or Y chromosomes. This genetic diagram shows that equal numbers of male and female offspring should be produced. Females are XX and males are XY 6

8 Worksheet inherited conditions in humans 7

9 Exam questions Q. Complete each sentence by choosing the correct terms from the box ADH DNA XX XY YY dominant female male recessive strong weak A gene is made up of a substance called.... Genes are found on chromosomes and most human cells contain... pairs of chromosomes. In females the two sex chromosomes are..., but in males the two sex chromosomes are.... Alleles are alternative forms of a gene. Two healthy parents can sometimes have a child with a genetic disorder such as cystic fibrosis. This is because cystic fibrosis is caused by a... allele. The two parents are healthy because they also have the... allele. (Total 6 marks) Q2. In humans, sex chromosomes control whether a person is male or female. (a) Use letters X and Y to complete the Punnett square for sex inheritance. Eggs or Sperm or...xx (3) zb) A couple already have three boys. What is the probability that their next child will be a girl? 8

10 () (Total 4 marks) Q3. Sometimes babies are born with extra fingers or toes as shown in the photograph. This condition is called polydactyly. CNRI / SCIENCE PHOTO LIBRARY The diagram shows the inheritance of polydactyly in a family. (a) Polydactyly is caused by a dominant allele, D. The recessive allele of the gene is represented by d. Use one genetic diagram to show the inheritance of the polydactyly gene by R and S. 9

11 (4) (b) In this question you will be assessed on using good English, organising information clearly and using specialist terms where appropriate. Embryos can be screened for genetic disorders. Many people would favour the use of embryo screening for cystic fibrosis but not for polydactyly. Compare the issues involved in the use of embryo screening for cystic fibrosis and for polydactyly. You should use your knowledge and understanding of the process and the two conditions. (6) (Total 0 marks) 0

12 The passage below is an extract from an article in The Independent newspaper. This Mouthwash could mean a lot to your Children A SIMPLE TEST SCREENS WOULD-BE PARENTS FOR THE DEFECTIVE GENE THAT CAUSES CYSTIC FIBROSIS It is not every week that a university professor tells you to spit in front of him. But Bob Williamson, professor of biochemistry and molecular genetics asked me to do just that. In future, some people may choose their marriage partners on the basis of the contents of their spit. The professor handed me a sample bottle containing sterile salt solution. I rinsed my mouth out and spat back into the tube. Incredible though it may sound, the mouthwash now contained enough cells from the inner lining of my cheek for an assistant to dissect my DNA and check whether I carry any of the mutations responsible for cystic fibrosis. Professor Williamson s team can locate genetic markers, distinctive segments of DNA, that are inherited along with the mutant genes in people affected with cistic fibrosis. About people who bought The Independent this morning unwittingly carry a cystic fibrosis gene. The statistics indicate that 23 of the staff of this newspaper are unknowing carriers. Carriers are normal healthy individuals who do not have the disease. (a) Describe, as fully as you can, where genes are located inside cheek cells (2) (b) The gene for cystic fibrosis has two forms called alleles. Only the recessive allele causes cystic fibrosis. Explain how two healthy carriers of the cystic fibrosis allele could produce a child with the disease. Use the symbol A for the normal allele of the gene and a for the allele which produces the disease. You may use a diagram if you wish (4)

13 (d) In the test used to identify the genetic markers, DNA is extracted from the cheek cells. The DNA molecules are then made to produce hundreds of millions of copies of themselves. (i) Explain, as fully as you can, how the structure of DNA molecules allows them to replicate themselves. ii) Explain how DNA controls the structure of proteins. (6) (3) (Total 5 marks) Q5. The diagram shows a family tree in which some individuals have an inherited disorder, which may cause serious long-term health problems. 2

14 (a) What proportion of the children of A and B have the disorder?... () (b) Explain the evidence from the diagram which shows that the allele for the disorder is dominant. Use the appropriate letters to identify individuals in your answer. You may use genetic diagrams in your explanation. There is space for you to draw a genetic diagram at the top of the facing page (3) (c) (i) What is meant by embryo screening? () (ii) A doctor suggests that couple C and D should have their embryos screened but that couple G and H do not need this procedure. Explain the reasons for the doctor s suggestions. (3) (Total 8 marks) 3

15 Q6. Cystic fibrosis is an inherited disease which causes the tubes in the lungs to be blocked with sticky mucus. Two parents who do not have the disease can still produce children who do have the disease. (a) Explain how children can inherit this disease from parents who do not have it (use a genetic diagram in your answer if you want to) (4) (b) Mucus contains protein. The information for the production of this protein is stored in a gene. Explain how a change in a gene causes a different protein to be produced (3) (Total 7 marks) Q7. Cystic fibrosis is an inherited disorder that can seriously affect health. (a) Which one of these is affected by cystic fibrosis? Draw a ring around your answer. blood cell membranes kidneys nervous system () (b) The diagram shows the inheritance of cystic fibrosis in a family. The allele that produces cystic fibrosis is recessive. 4

16 (i) Explain why Alice inherited cystic fibrosis. (2) (ii) Explain why Ted did not inherit cystic fibrosis. (2) (c) Bob and Carol know that there is a risk that their next baby will have cystic fibrosis. Embryos can be screened for the allele that produces cystic fibrosis. Many people support the screening of embryos, but others do not. (i) Suggest one reason why many people support the screening of embryos for the cystic fibrosis allele. () (ii) Suggest one reason why many people are against the screening of embryos for the cystic fibrosis allele. () (Total 7 marks) Q8. Huntington s chorea is a disease found in 5 out of every people. (a) Describe, as fully as you can, how the disease is likely to affect these people

17 (3) (b) Some members of the Smith family suffer from Huntington s chorea. The family tree gives details. (i) Are Mary and John Smith s children likely to have Huntington s chorea? (ii) Explain your answer as fully as you can. Use a diagram if it will help. Suppose Mary and John both carry one gene for Huntington s chorea. How likely are their children to have Huntington s chorea? Explain your answer as fully as you can. Use a diagram if it will help. (4)... (2) (Total 9 marks) 6

18 Q9. The black pigment in human skin and eyes is called melanin. Production of melanin is controlled by a single pair of genes. A person who is homozygous for a recessive allele of the gene has no melanin and is said to be albino. (a) A man is albino. His wife is heterozygous for the melanin-producing allele. (i) The fertilised egg cell produced by the couple divides to form two cells. Name the process of cell division involved. () (ii) How many albino genes would there be in each of these two cells? Explain you answer. (3) (b) (i) Albino people are more likely than people with melanin to suffer mutations that cause cancer in their skin. Suggest why albino people have an increased chance of mutation in their skin cells. () (ii) Sometimes, mutation in skin cells leads to cancers in other organs, such as the liver. Explain how. (2) (Total 7 marks) 7

19 Answers to Exam questions M. in the correct order DNA 23 XX XY recessive dominant [6] M2. (a) (X) (XX) XX Y XY XY mark for Y in sperm box mark if XX box correct mark if both XY boxes correct (b) : or 50% or ½ or 0.5 or in 2 or out of 2 or 50 : 50 do not accept 50/50 accept equal (probability) 3 [4] M3. (a) (genotype / gametes from P / father) D and d (*) (genotype / gametes from Q / mother) d and d / accept d(*) offspring genotypes correctly derived from correct gametes(*) offspring phenotypes R and S identified (*) eg may be in punnett square allow own upper and lower case symbols or allow any symbol correctly used with key (b) Marks awarded for this answer will be determined by the Quality of Written Communication (QWC) as well as the standard of the scientific response. No relevant content. There is a brief description of the issues involved in screening for at least one condition. 0 marks Level ( 2 marks) 8

20 There is some description of issues involved in screening for both conditions but there is a lack of both pros and cons for the two conditions. There is a clear, balanced and detailed description of the issues involved in screening for both conditions, giving pros and cons for each condition. Level 2 (3 4 marks) Level 3 (5 6 marks) examples of biology points made in the response For cystic fibrosis pros: reduce number of people with cystic fibrosis (in population) reduce health-care costs allows decision / emotional argument, eg allows people to make choices about termination cons: possible damage / risk to embryo / fetus / baby possible harm / risk to mother (may) have to make ethical / moral / religious decisions for polydactyly: cures disfigurement but condition not life threatening so risks to foetus / mother unjustified M4. (a) on chromosomes/dna within the nucleus each for mark2 (b) parental genotypes correct i.e. Aa Aa; gamete genotypes correct i.e. A or a A or a/correct lines; F genotypes correct i.e. AA Aa Aa aa; aa recognised as child with cystic fibrosis each for mark (d) (i) molecule has two long strands/double helix; idea of held together by (weak) bonds; each strand has 4 different types of base; ) which pair with specific bases in opposite strand; ) when strands separate; ) OWTTE each strand acts as a complementary template; ) makes 2 identical strands ) each for mark (ii) order of bases acts as a code; which controls the order; in which amino acids are assembled into protein; read in triplet each for mark [0] [5] 9

21 M5. (a) in 4 / /4 / : 3 / 25% / 0.25 do not accept 3: / :4 / 2:6 (b) either from C and D accept synonyms for dominant / recessive eg Normal / faulty accept genetic diagram if clearly referring to correct individuals or genotypes on family tree allow gene for allele any three from: C and D have disorder I/J don t have disorder ignore C & D are carriers C and D have dominant and recessive alleles recessive alleles from C and D passed to I/J or I/J have two recessive alleles or from A and B NB if allele was recessive then all offspring of C and D would have the disorder = 3 marks assume response refers to A + B unless contradicted A is homozygous recessive / rr, and B is heterozygous / Rr can be shown in words or symbols allow any symbol offspring can be rr or Rr described allow without key 3 (c) (i) (embryos) checked for inherited / genetic disorders / conditions accept diseases for disorders (ii) any three from: C/D have disorder / have dominant allele accept disease / condition accept gene for allele ignore reference to carriers chance of embryo / foetus / child having disorder or may pass on alleles for disorder to their offspring C/D might want to decide on termination or prepare for child with disorder G and H don.t have disorder / both homozygous recessive / have no dominant alleles (for this disorder) so offspring (of G and H) cannot / don.t have disorder 3 [8] M6. (a) caused by a recessive* gene / allele (allow non / not dominant) both parents heterozygous / carry the gene / allele for mark each offspring needs two recessive genes to have / inherit disease 20

22 for 2 marks or Nn Nn NN Nn Nn nn for mark each nn identified as having the disease* for 2 marks 4 (b) any reference to DNA gains mark but different genes means difference in DNA gains 2 marks idea of different codes / instructions for making proteins or different (order of) amino acids (in proteins) for mark 3 [7] M7. (a) cell membranes (b) (i) two recessive / cystic fibrosis / faulty / diseased / the allele(s) / genes two can be implied by second marking point ignore chromosomes from Bob and Carol / both parents / the parents if no other marks awarded Carol is a carrier gains mark (ii) (inherited) dominant / normal allele / gene from Carol / mother ignore references to recessive allele / gene from father / Bob if no other marks awarded he has just / only one recessive allele gains mark (c) (i) reduce number of people with cystic fibrosis (in population) or reduce health-care costs or expensive to have baby with cystic fibrosis accept to allow decision / emotional argument qualified eg allows abortion or allows people to make choices about termination or help to prepare financially / emotionally etc 2

23 (ii) any one from: possible damage / risk to embryo / fetus / baby allow possible harm / risk to mother screening / it is expensive (may) have to make ethical / moral / religious decisions ignore not natural / playing God / unethical / immoral / religious unqualified right to life M8. (a) idea: mental/brain deterioration involuntary muscular movement/fidgety starts in 40/50 s/middleage for mark each 3 [7] (b) ideas that: mother homozygous * father heterozygous * (*these terms not essential) ) these marks ) can be gained allele dominant (H. chorea) ) on diagram* children normal or heterozygous chance 50/50 Diagram gains max. 3 marks for mark each 4 (c) 3 in 4 chance (or 3:) (of Huntington s chorea) gains mark or equivalent in words gains mark [9] 2 M9. (a) (i) mitosis for mark (ii) fertilised egg cell has albino gene from father splits to produce identical cells / produced by mitosis each for mark 3 (b) (i) less protection from UV light / UV radiation for mark (ii) ideas of uncontrolled multiplication of mutated cells reject fast / rapid cell division cells invade of other parts / cells transported in blood each for mark 2 [7] 22

24 Worksheet Answers a A genetic disorder which causes extra fingers or toes. b The faulty allele is dominant, so only one parent needs to have the allele and pass it on for the offspring to be affected. c A Pp only as produced a child that was unaffected. B Pp because mother must pass on a recessive allele to produce two unaffected children. C-E could be PP or Pp as each parent has the genotype Pp. 2 a Carriers have a normal dominant allele, so their bodyworks normally. b CF (cystic fibrosis) recessive must inherit one from each parent to get the disease but if parents had the disease themselves, they would almost certainly be infertile so parents must be carriers. 3 Genetic diagram based on Figure 3 in Student Book, showing how the cc (cystic fibrosis) arises. 23