Variation, discrete variation, continuous variation, polygenic, singlegene

Transcription

1 Variation Variation, discrete variation, continuous variation, polygenic, singlegene Learning Outcomes You will be able to: Give some examples of variation within a species. Describe how sexual reproduction maintains variation. Explain the difference between discrete and continuous variation. Explain the difference between polygenic and single gene traits and give examples of each. Variation Although members of the same species are very similar to one another they are not identical. This is due to variation. Variation is all the differences which exist between members of the same species. Sexual reproduction increases variation within a species as it allows genes from separate parents to combine. There are two kinds of variation; continuous and discrete. Continuous variation is variation where there is a wide range between two extremes within a population. Most features of an individual show continuous variation for example: height weight heart rate leaf length A line graph or a frequency graph is used to display the large range of phenotypes values (phenotypes) shown in continuous variation. Formatted: Indent: Left: 1.11 cm, Right: 0.1 cm, Space After: 6 pt, Line spacing: Multiple 1.15 li Formatted: Indent: First line: 0.48 cm, Line spacing: Multiple 1.15 li, Bulleted + Level: 1 + Aligned at: 0.63 cm + Tab after: 1.27 cm + Indent at: 1.27 cm Formatted: Indent: First line: 0.48 cm, Space After: 0 pt, Line spacing: Multiple 1.15 li, Outline numbered + Level: 1 + Numbering Style: Bullet + Aligned at: 0.63 cm + Tab after: 1.27 cm + Indent at: 1.27 cm Formatted: Indent: First line: 0.48 cm, Space After: 0 pt, Outline numbered + Level: 1 + Numbering Style: Bullet + Aligned at: 0.63 cm + Tab after: 1.27 cm + Indent at: 1.27 cm Formatted: Line spacing: Multiple 1.15 li 1 P a g e

2 Features showing continuous variation are said to be polygenic as they are caused by many genes working together with the environment. Most characteristics are inherited by polygenic inheritance. Discrete variation is variation that has distinct groups. Some features showing discrete variation are: 1. tongue rolling 2. finger prints 3. eye colour 4. blood groups A bar graph is used to display the few phenotypes categories (phenotypes) possible with discrete variation. Formatted: Indent: Left: 0.8 cm, Space After: 6 pt, Line spacing: Multiple 1.15 li Formatted: Indent: Left: 1.26 cm, First line: 0.35 cm, Space After: 0 pt, Line spacing: Multiple 1.15 li, Outline numbered + Level: 1 + Numbering Style: Bullet + Aligned at: 0.63 cm + Tab after: 1.27 cm + Indent at: 1.27 cm Formatted: Indent: Left: 1.26 cm, Line spacing: Multiple 1.15 li Formatted: Line spacing: Multiple 1.15 li 2 P a g e Discrete variation is under the control of different alleles (forms) of a single gene. Single gene inheritance results in few discrete categories.

3 Learning Activity 1 1. Collect the worksheet Class Variation Data Sheet and complete. 2. Write a clear definition of continuous variation. 2. Write a clear definition of continuous variation. 3. State how continuous variation arises 4.3. Write a clear definition of discrete variation Make a table to show which of the characteristics you have investigated show continuous variation and which show discrete variation Use graph paper to draw a bar graph to present the class data on tongue rolling, showing how many people in the class can and can t roll their tongues. Stick the graph in your jotter Present the class data on height range in the class as a frequency graph using the grid on your data sheet. Formatted: Space After: 0 pt, Line spacing: Multiple 1.15 li, Numbered + Level: 1 + Numbering Style: 1, 2, 3, + Start at: 1 + Alignment: Left + Aligned at: 0.63 cm + Indent at: 1.27 cm Formatted: Font: Comic Sans MS, 12 pt, Font color: Black Formatted: Space After: 0 pt, Line spacing: Multiple 1.15 li, Numbered + Level: 1 + Numbering Style: 1, 2, 3, + Start at: 1 + Alignment: Left + Aligned at: 0.63 cm + Indent at: 1.27 cm Formatted: Font: Comic Sans MS, 12 pt, Bold Polygenic Vs. Single-Gene Discrete variation is controlled by the alleles of a single-gene while continuous variation is controlled by the alleles of two or more genes by polygenic inheritance. Use the information on page 1-2 to answer the following questions in sentences. 1. Name the 2 types of variation shown in organisms. 2. Describe what is meant by single gene inheritance, with reference to: the type of variation displayed the number of genes involved the way in which the data can be graphed (with example) 4 examples of characteristics controlled in this way. 3. Describe what is meant by polygenic inheritance, with reference to: Formatted: Space After: 0 pt, Bulleted + Level: 4 + Aligned at: 1.25 cm + Indent at: 1.89 cm the type of variation displayed 3 P a g e

4 the number of genes involved the way in which the data can be graphed (with example) 4 examples of characteristics controlled in this way. 4. Copy and complete the following table: 5. Insert and complete the Polygenic Inheritance in Maize diagram from your pack into your notes. Formatted: List Paragraph, Left, Indent: First line: 0 cm, Line spacing: single Describe, with reference to maize kernel colour, the: range of phenotypes resulting from this crop relationship between number of alleles and number of phenotypes. Type of inheritance Single gene Polygenic Number of genes Number of values Type of variation 4 P a g e

5 Inheritance Inheritance, phenotype, true-breeding, genotype, dominant, recessive, alleles, heterozygous, homozygous, homologous pair Learning Outcomes You will be able to: Identify examples of dominant and recessive traits. Explain what is meant by the term phenotype and give examples. Explain what is meant by the term genotype and give examples. Explain what is meant by the terms homozygous and heterozygous Alleles Populations of any organism show a great deal of variation. In the human population there are many different types of hair or eye colour, blood types etc. The information for these characteristics is found on chromosomes. For each chromosome that has come from the mother, 5 P a g e

6 there is a chromosome of equal size and shape that has come from the father. This pair of chromosomes is called a homologous pair. Information that is carried from parent to offspring is carried on the chromosome. Each characteristic is controlled by a section of the chromosome called a gene. Whenever a gene exists in more than one form, these different forms are called alleles e.g. the gene for height in pea plants has two alleles, tall and dwarf. Other examples include: Characteristic (gene) Pea flower colour Human colour vision Alleles Red : White Normal : Colour-blind When the two alleles are identical we say the plant or animal is homozygous or true-breeding (the prefix homo- means the same). When the two alleles are non-identical we say the plant (or animal) is heterozygous (the prefix hetero- means different). A homozygous organism has two identical alleles of a gene. A heterozygous organism has two different alleles of a gene. Gneotype & Phenotype The combination of alleles that an organism receives for a characteristic is known as its genotype. The organism s physical appearance of a characteristic e.g. tall, short, blue-eyed, brown-eyed, etc., is called its phenotype. The genotype of an organism is the alleles it carries for a particular characteristic. 6 P a g The e phenotype of an organism is its physical appearance for the characteristic.

7 Genotypes are usually written as letter symbols (e.g. Tall = T, dwarf = t). For example, a pea plant with two alleles for tallness has the genotype TT and is certain to be tall. A plant with two alleles for dwarfness has the genotype tt and is certain to be dwarf. A heterozygous pea plant would have one of each allele, hence the genotype is Tt. A mix of alleles does not mean a mix of characteristics. The genotype Tt is tall + dwarf, which does not produce a medium sized plant!!! The phenotype of this plant depends on which of these two alleles is dominant, and which is not dominant (recessive). In this case, the allele for tall plants, T is dominant over the recessive allele, t for dwarf plants; hence this plant Tt will be tall. A dominant allele, if present, will always show up in the appearance of an organism. A recessive allele will only shows up in the appearance of an organism if it has inherited two of them (one from mother, one from father). Learning Activity 1 1. Copy and complete the table below. Genetic term Definition Example Different forms of the gene for a 7 P a g e

8 genotype characteristic Physical appearance of a characteristic Dominant Recessive True-breeding Homozygous Organism that is homozygous and can only pass on one type of allele for a particular characteristic. heterozygous 2. In mice, the allele for black coat (B) is dominant to the allele for white coat (b). Using the information above, complete the following table. Genotype Description Phenotype BB homozygous Bb Black coat White coat What is the genotype for a mouse which is homozygous for the dominant allele in coat colour? 8 P a g e

9 Why will the phenotype of this mouse be black in colour? Why is a mouse with the genotype described as heterozygous? Why is the phenotype of this mouse also black coat colour? What is the only genotype which gives a mouse white coat colour? Would this mouse be described as homozygous or heterozygous? Monohybrid Cross Inheritance, phenotype, true-breeding, genotype, dominant, recessive, alleles, heterozygous, homozygous, monohybrid cross, punnet square Learning Outcomes You will be able to: Explain what is meant by the term Monohybrid Cross Use a monohybrid cross to follow the inheritance of a single characteristic. Use punnet squares to predict the inheritance of genes. Monohybrid Cross 9 P a g e A monohybrid cross is one in which the inheritance of a single characteristic is being investigated. The

10 first recorded genetics experiments were carried out by an Austrian monk called Gregor Mendel. He transferred pollen from the anthers of one pea plant to the stigma of another pea plant. He then planted the seeds that were produced and recorded the appearance of the new plants when they matured. A monohybrid cross is a breeding experiment which follows the inheritance of one characteristic. Mendel s Cross Mendel obtained two homozygous varieties of pea plant. One had yellow seeds and one had green seeds. Plants from these two seed varieties were called the parent generation (P) since they were used as parents for the breeding experiment. Experiment 1 Seed colour He carefully took pollen from one variety and used it to pollinate the other variety. The seeds produced were harvested and grown so that the seed colour of the offspring could be observed. The offspring of this cross were the first generation (F1) The surprising observation was made that the entire F1 generation had yellow seeds! The explanation for this is that the allele for yellow seed colour (Y) is dominant to that of the recessive green seed colour (y). So, although all F1 plants contain a recessive allele for green seeds, this 10 P a g e

11 characteristic does not show up in their phenotype. Learning Activity 1 1. Which characteristic was dominant? 2. Which characteristic was recessive? 3. Why did green seeds not show up in the F 1 generation? Mendel was interested in what had happened to the recessive characteristic in his cross, and in an attempt to work this out he tried crossing the F1 plants with each other. He pollinated the F1 plants with their own pollen and observed the seed colour of their offspring, the F2 generation. First generation (F 1 ): Gametes: Pollen: Y or y Ovules: Y or y We now know that chromosomes, and hence alleles, separate from each other during the process of gamete production. As fertilisation is a random process, any of the pollen gametes can fertilise any of the ovules. We show this by using a grid called a Punnett square. The square shows that four fertilisations are possible and each has an equal chance of happening. 11 P a g e

12 Male gametes (pollen) National 5 Biology Multicellular Organisms Variation & Inheritance Female gametes (ovules) Y y Y YY Yy y Yy yy 12 P a g e

13 Experiment 2 Plant height Parents P 1 Homozygous Tall plants Homozygous Dwarf plants First Filial generation F 1 All Tall plants Parents P 2 Tall plants (from F1) Tall plants (from F1) Second Filial generation F 2 3 Tall plants : 1 Dwarf plant Breeding tall plants with dwarf plants produced only tall plants, therefore in the first generation (F1), the tallness seems to mask the dwarfness. Because of this, the factor for tallness is said to be dominant and the factor for dwarfness is said to be recessive. Dominant factors are written with a capital letter, the recessive with a small letter (e.g. Tall = T, dwarf = t). However when pollen from one of these F1 plants landed on the stigma of another F1 plant, the seeds and subsequent plants produced, showed that the dwarfness reappears in the second (F2) generation. It only appears in one plant out of every four plants i.e. a 3:1 ratio in favour of tall. Learning Activity 2 1. If 432 seeds were sown from the F1 generation, how many dwarf plants would you expect to see? 2. How many tall plants would you expect to see? 13 P a g e

14 We can represent this monohybrid cross using the following method. T=tall and t=dwarf P 1 TT tt Gametes T T t t F 1 All Tt P 2 Tt Tt Gametes T t T t F 2 TT, Tt, Tt, tt 3. Complete the Punnett square below, to show the random fertilisation of gametes. 14 P a g e

15 Learning Activity 3 1. Collect and complete the Single Gene Inheritance worksheet and paste into your workbook. 2. Collect and complete the Mouse Genetics worksheet and paste into your workbook. Challenge Task 1. Mendel crossed homozygous plants with round seeds with homozygous plants bearing wrinkled seeds. All the progeny (offspring) were round seeded plants. Fill in the blanks below. Parents P 1 Phenotype Round seed Wrinkled seed Genotype Gametes Offspring F 1 Phenotype All round seed Genotype 2. If the F 1 from the above cross were self-pollinated, what would be the possible genotypes in the F 2 generation? (show all the working) 3. What is the ration of round to wrinkled seeds? : 15 P a g e

16 4. In mice, one gene is responsible for coat colour. Use the symbols B to represent black fur and b to represent brown fur. Fill in the genotypes of the following genetic cross. Parents (both P 1 Phenotype Black fur Brown fur Genotype Gametes Offspring F 1 Phenotype All black fur Genotypes (a) Which allele in this cross was dominant, and give a reason for your answer. (b) In the same cross, what would be the expected ratio of coat colours in the F 2? (show all the working) (c) If a mouse from the F 1 generation had been crossed with a mouse with brown fur, what would be the expected ratio of phenotypes in the F 2 generation? (show working) 16 P a g e

17 Research Task Use any resources you find, including to find out about the life and work of Gregor Mendel. Formatted: Font: Comic Sans MS, 12 pt Make notes to include: When he lived. Where he carried out his research on pea plants. Why his work wasn t recognised as being important when it was published. Why his work was recognised as important when the structure and functionrole of DNA chromosomes was worked out in the next century. All sources of your information. 17 P a g e

18 Forensic Genetics Inheritance, phenotype, true-breeding, genotype, dominant, recessive, alleles, heterozygous, homozygous, monohybrid cross, punnet square Learning Outcomes You will be able to: Work out the genotype and phenotype of an unknown Scottish animal using a simulation. Scientists can collect genetic information (DNA) from the bone marrow of skeletons of unknown animals. By comparing the DNA to the genes of similar organisms the genes on the DNA can be deduced. Once the genotype is known, the phenotype can be worked out so scientists can work out the appearance of the unknown animal. In this activity your group will work out the genotype of a previously unknown Scottish animal using a simulation and information from the table. Then use all the information you have to: work out the phenotype of your animal make a model of this animal give your animal a suitable name and describe its habitat and diet. 18 P a g e

19 Learning Activity 1 1. Collect the experiment card Forensic Genetics and the worksheet Unknown Animal Phenotype/Genotype. 2. Follow the instructions to complete the simulation to obtain the genotype and phenotype of your sample. i.3. Stick in your results table into your workbook Use the information and all resources available to you to make a model of your animal Give your animal a suitable name and make a display information sign. Formatted: Justified, Space After: 0 pt, Line spacing: Multiple 1.15 li, Numbered + Level: 1 + Numbering Style: 1, 2, 3, + Start at: 1 + Alignment: Left + Aligned at: 0 cm + Indent at: 0.63 cm 19 P a g e

20 Pedigree Charts and Genetic Counselling phenotype, true-breeding, genotype, dominant, recessive, alleles, heterozygous, homozygous, pedigree chart, family tree, genetic counselling Learning Outcomes You will be able to: Understand the use of pedigree charts to investigate the inheritance of a characteristic. Describe the importance of genetic counselling. Family relationships can be drawn in a diagram called a family tree or pedigree chart. The medical teams caring for families affected by genetic conditions draw family trees using the standard symbols shown here. Formatted: Indent: Left: 0.5 cm, Right: cm, Space After: 12 pt, Line spacing: Multiple 1.15 li Males are represented as squares. Females are represented as circles. A couple in a relationship are linked by a horizontal line. When someone is affected by a genetic condition the shape is shaded. Different generations are recorded on different levels on the diagram, with the oldest generation at the top. P Siblings are positioned alongside one another. F1 F1 A diagonal line drawn through an individual indicates that they have died. A diagonal line drawn through a relationship shows that it has ended. 20 P a g e

21 Learning Activity 1 Copy this family tree and the key for it into your workbook. Then answer the questions below. 1. How many sisters does Emma have? 2. How many uncles does Mark have? 3. How many of Mark s grandparents are still alive? Mark and one of his cousins are affected by cystic fibrosis (CF) a genetic condition. CF is inherited in a recessive pattern; this means that Emma and James must both be carriers of the allele for CF. This means that although they do not have CF they have one CF allele. 4. If James and Emma wanted to have another child, what is the chance of than having another child affected by CF? Use the symbols N for the normal allele and n for the CF allele to help you explain how you get your answer. 5. Who else in the family is definitely a carrier of the CF allele? Formatted: Font: Comic Sans MS, 12 pt, Not Bold 21 P a g e

22 6. How would a pedigree for a family affected by a dominantly inherited condition look different? 7. Draw Beth s family tree from the description given: Beth has two brothers and no sisters. Beth has one son with her partner David and their son is affected by CF. David is an only child. Both of Beth s and David s parents are alive. David has one daughter from a previous relationship. Learning Activity 2 Example 1: Inheritance of eye colour characteristic Below is a family tree showing the inheritance of eye colour. The dominant allele is B which gives an individual brown eyes. The recessive allele is b, which in homozygous individuals, produces blue eyes. If the person exhibits the recessive trait then we know that the genotype must have two recessive alleles i.e. homozygous recessive. If the person exhibits the dominant trait then they can be either homozygous dominant or heterozygous. Try to work out and write the genotype above each person s symbol then complete the following statements: The phenotype of person 2 is The phenotype of person 3 is 22 P a g e

23 The genotype of person 1 is The genotype of person 4 is Person 7 is likely to be homozygous dominant because The genotype of person 8 is The genotype of person 9 cannot be stated with certainly because Learning Activity 3 Example 2: Inheritance of tongue-rolling and hair-type characteristics In humans, the allele for tongue-rolling ability (R) is dominant to that for inability to roll the tongue (r). The allele for wavy hair (H) is dominant to the allele for straight hair (h). Study the following two family tree diagrams below. Family tree for tongue-rolling ability Family tree for hair type (a) From which parent did Wendy inherit her tongue-rolling ability? (b) From whom did Wendy receive the genetic information that gave her straight hair? 23 P a g e

24 (c) Using the format of the previous family tree, copy each family tree and write the genotype beside each symbol. Genetic Counselling In the UK genetic testing is provided by the NHS, only after patients have undergone genetic counselling. This is the provision of information and advice about inherited disorders given by a doctor or specialist genetic counsellor. It helps people to understand medical facts including identifying carriers (heterozygotes) understand the options for dealing with the disorder choose the course of action with which they feel most comfortable. Case Study on Inherited Diseases Choose one of the case studies below and complete thiscollect the information you need to write a letter independently. There are many websites you can use for information but you may wish to start with or The Chosen Child - Screening Genetic Content. Success Criteria Your letter is written in a suitable format You must provide information to help answer each question You have included a diagram to explain the inheritance of the condition 24 P a g e

25 You have included the sources of your information so that the recipient of the letter can find more information later. Case 1 Formatted: Line spacing: 1.5 lines Karen and Steven have a healthy 15 year old daughter, Naomi, who is a caring big sister to Nathan. Nathan has recently been diagnosed with cystic fibrosis (CF), an inherited disease. Naomi doesn t know much about the disease but wants to learn how it will affect her baby brother throughout his life. Since it is an inherited disease, she wonders why she doesn t have the condition and if she could still pass it on somehow if she ever has her own children. Her friends have been asking a lot of questions about Nathan that she doesn t quite know how to answer. Your task: Write a letter to Naomi to answer her questions about how cystic fibrosis will affect Nathan s life and potentially the lives of any children she may have in the future. Formatted: Line spacing: 1.5 lines, Bold Your letter must include: The genetic cause of cystic fibrosis (i.e. what change has occurred in the genes and the results of this change in the genetic code). The symptoms of the condition. The current treatment and prognosis (outcome of treatment) for a CF sufferer. Are there any new treatments being researched currently? How likely it is that Naomi could pass on the disease herself. ( ) Formatted: Indent: Left: 0.63 cm, Hanging: 0.63 cm, Space After: 0 pt, Bulleted + Level: 1 + Aligned at: 0.63 cm + Indent at: 1.27 cm 25 P a g e

26 Your task: Write a letter to Charlotte to answer her questions about how haemochromatosis could affect her life and potentially the lives of any children she may have in the future. Formatted: Line spacing: 1.5 lines Your letter must include: The genetic cause of haemochromatosis (i.e. what change has occurred in the genes and the results of this change in the genetic code). The symptoms of the condition. The current treatment and prognosis (outcome of treatment) for someone with the disease. Are there any new treatments being researched currently? How likely it is that Charlotte has the disease herself and that she could pass it to her children. 26 P a g e

27 Case 2 Formatted: Line spacing: 1.5 lines Charlotte has recently met her half-sister Elizabeth after many years apart. Elizabeth has been trying to find Charlotte to tell her about an inherited condition called haemochromatosis. Elizabeth has the condition as does their father (and Elizabeth s mother). Charlotte has never heard of the disease but is worried that she may have it after Elizabeth s news. She wants to find out more about how the disease could affect her if she has it and if she could pass it on to her children. 27 P a g e

28 Case 3 Rebecca and Martin have decided to get married. They are part of a community of Ashkenazi Jews, for whom the inherited disease Tay-Sachs disease is a real problem. Due to the high prevalence of this disease, all couples must be tested for the disease before marriage to find out if their future children could be affected. Rebecca and Martin are nervous about the outcome of the test and worry about what could happen if they are shown to carry the affected gene and choose to have children anyway. Your task: Formatted: Line spacing: 1.5 lines Write a letter to Rebecca and Martin to answer their questions about how Tay-Sachs disease would affect a child with the disease and the chances of their own children inheriting the condition. Your letter must include: The genetic cause of Tay-Sachs disease (i.e. what change has occurred in the genes and the results of this change in the genetic code). The symptoms of the condition. The current treatment and prognosis (outcome of treatment) for a Tay-Sachs sufferer. Are there any new treatments being researched currently? How likely it is that Rebecca and Martin could have a child affected by the Formatted: Don't add space between paragraphs of the same style, Bulleted + Level: 1 + Aligned at: 0.63 cm + Indent at: 1.27 cm 28 P a g e

29 Case 4 Philip is 15 years old and has a very active life. He enjoys sports and being outdoors and often goes on camping and fishing trips with his dad. Recently, life has started to change for Philip. His father has been behaving strangely and has become very clumsy. After tests at the hospital, they have been told that Philip s father has Huntington s disease. Philip doesn t know a lot about the disease or its symptoms. His dad has also suggested that Philip himself might want to be tested for the disease. The changes he is seeing in his dad are scaring him and he doesn t know how to help. Your task: Write a letter to Philip to answer his questions about how Huntington s disease will affect his father s life and potentially his own life and that of his future children. Formatted: Line spacing: 1.5 lines Your letter must include: The genetic cause of Huntington s disease (i.e. what change has occurred in the genes and the results of this change in the genetic code). The symptoms of the condition. The current treatment and prognosis (outcome of treatment) for a Huntington s sufferer. Are there any new treatments being researched currently? How likely it is that Philip could develop the condition himself or pass it on to his own children. 29 P a g e