* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Blueprint of life
Survey
Document related concepts
Gene expression programming wikipedia , lookup
Artificial gene synthesis wikipedia , lookup
Genome (book) wikipedia , lookup
Genetic engineering wikipedia , lookup
Genomic imprinting wikipedia , lookup
Gene expression profiling wikipedia , lookup
Heritability of IQ wikipedia , lookup
Genetically modified crops wikipedia , lookup
Biology and consumer behaviour wikipedia , lookup
Hardy–Weinberg principle wikipedia , lookup
History of genetic engineering wikipedia , lookup
Transgenerational epigenetic inheritance wikipedia , lookup
Microevolution wikipedia , lookup
Designer baby wikipedia , lookup
Hybrid (biology) wikipedia , lookup
Transcript
Gill Sans Bold Biology HSC Course Stage 6 Blueprint of life Part 2: Patterns of inheritance 0 20 I er b to T S c O EN g in D M t a r EN o p or AM c n 2 Gill Sans Bold Contents Introduction ................................................................................2 Gregor Mendel and his work ......................................................4 The principles of heredity .........................................................15 Monohybrid crosses ...........................................................................15 Mr Punnett and his squares...............................................................19 Investigating human variation ..................................................22 Pedigrees.................................................................................25 Constructing a pedigree.....................................................................25 Using a pedigree to investigate a trait ...............................................28 Using a pedigree to make predictions ...............................................30 Hybridisation ............................................................................33 Suggested answers .................................................................35 Exercises – Part 2....................................................................41 Part 2: Patterns of inheritance 1 Introduction The idea that offspring inherit features from their parents has been known for a long time. It was believed that the features inherited from each parent blended together. The Austrian monk, Gregor Mendel, decided to breed pea plants and study one clearly defined characteristic at a time. He kept a careful statistical record of results and his conclusions became the basis of the laws used to explain how offspring inherit features from their parents. Once fundamental patterns of inheritance were understood the ability to predict the types of offspring likely to be produced became possible. This study of inheritance gave birth to the science of genetics and was a boost to agriculture and breeding. Today it is important to medical practitioners investigating the likelihood of people inheriting diseases. This section outlines the experiments carried out by Mendel, now regarded as the founder of genetics, and explains how his experimental techniques led to an understanding of the patterns in inheritance. This will enable you to describe the basis of Mendel’s experiments and teaches you skills that you can use to solve problems involving inheritance. In this part you will have opportunities to learn to: • outline the experiments carried out by Gregor Mendel • describe the aspects of the experimental techniques used by Mendel that led to his success • describe outcomes of monohybrid crosses involving simple dominance using Mendel’s explanations • distinguish between homozygous and heterozygous genotypes in monohybrid crosses • distinguish between the terms allele and gene, using examples • explain the relationship between dominant and recessive alleles and phenotype using examples • outline the reasons why the importance of Mendel’s work was not recognised until some time after it was published. In this part you will have the opportunity to: 2 • perform an investigation to construct pedigrees or family trees, trace the inheritance of selected characteristics and discuss their current use • solve problems involving monohybrid crosses using Punnett squares or other appropriate techniques • process information from secondary sources describe an example of hybridisation within Blueprint of life a species and explain the purpose of this hybridisation. Gill Sans Bold Extract from Biology Stage 6 Syllabus © Board of Studies NSW, originally issued 1999. The most up-to-date version can be found on the Board's website at http://www.boardofstudies.nsw.edu.au/syllabus_hsc/syllabus2000_lista.html This version October 2002. Part 2: Patterns of inheritance 3 Gregor Mendel and his work It requires indeed some courage to undertake a labour of such far-reaching extent; this appears, however, to be the only right way by which we can finally reach the solution of a question the importance of which cannot be overestimated in connection with the history of the evolution of organic forms. From Mendel’s famous paper, Experiments in Plant Hybridisation, published in 1865. In the mid 1800s, an Austrian monk and keen botanist called Gregor Mendel observed that the offspring of certain plants had physical characteristics similar to those of its parents and ancestors. He wondered why related organisms had a tendency to resemble one another and if it could be explained. Observing traits Mendel reasoned that close observations of inheritance might provide the answers. So in 1857 he decided to conduct breeding experiments with the pea plant (Pisum sativum) because it had a short reproductive cycle. He selected plants and caused them to reproduce. This is called performing a cross (X), or crossing the plants. Then he compared features of the parent plants with those of the new plants produced. 4 Blueprint of life Gill Sans Bold Mendel studied seven separate characteristic features of pea plants. These features or characteristics are referred to as character traits (pronounced trays with a silent ‘t’ near the end). The seven traits Mendel examined were plant height, flower colour and position, seed colour, seed shape and pod colour and shape. He noted that each trait comes in two different forms or factors. For example, the seed pod colour is either yellow or green. This means that the trait for pod colour has a yellow factor or green factor. Examinations of the seed shape factor led to an interesting observation. Mendel wrote: the hybrid character resembles that of one of the parental forms so closely that the other either escapes observation completely or cannot be detected with certainty. A hybrid refers to the offspring that result from a cross between parents with different factors. When Mendel used plants with round peas and ones with wrinkled peas as the parents, only plants with round peas were observed. 1 Which plants were crossed? _____________________________________________________ _____________________________________________________ 2 What feature do the offspring (or progeny) have? _____________________________________________________ 3 What is the hybrid character that Mendel refers to in the extract above? _____________________________________________________ Check your answers. Dominant and recessive traits Mendel stated that those characters that are transmitted unchanged are termed dominant. The feature not visible in the appearance of the offspring was said to have ‘hidden’ or ‘stored’ hereditary information. In Mendel’s experiment, when the round pea hybrids were crossed, the wrinkled factor reappeared unchanged in some of their offspring. Mendel wrote: the expression ‘recessive’ has been chosen because the characters thereby designated withdraw or entirely disappear in the hybrids, but nevertheless reappear unchanged in their progeny. Each of the seven traits that Mendel studied had a dominant factor and a recessive factor. The feature exhibited by all of the offspring in the first generation is the dominant factor and the recessive factor is the feature that has been hidden but is shown, or expressed again in the second generation. Part 2: Patterns of inheritance 5 Mendel’s results are summarised in the following table. Expt Trait examined First generation Second generation 1 seed shape all round 5474 round, 1850 wrinkled 2 seed colour all yellow 6022 yellow, 2001 green 3 flower colour all violet-red flowers 705 violet-red, 224 white 4 form of seed pods all inflated pods 882 inflated, 299 constricted 5 colour of seed pods all green pods 428 green pods, 152 yellow 6 position of flower all axial 651 flowers axial, 207 flowers terminal 7 length of stem all long stem 787 long stem, 277 short stem Use information from the previous table to complete these tasks. 1 Has the recessive factor been destroyed in the first crossing? How did Mendel explain its reappearance? _____________________________________________________ _____________________________________________________ _____________________________________________________ 2 Complete the table below. The first row has been done for you. Expt Trait examined Dominant factor Recessive factor Ratio in second generation 1 seed shape round wrinkled 5474 round = wrinkled 1850 2 seed colour 3 flower colour 4 form of seed pods 5 colour of seed pods 6 position of flower 6 Blueprint of life Gill Sans Bold 7 length of stem 3 Are the ratios for the second generation similar? ______________ 4 What is the average ratio of dominant to recessive factors in the second generation of all the traits examined? _____________________________________________________ 5 In terms of scientific method, why did Mendel use such a large number of pea plants for his investigations? _____________________________________________________ _____________________________________________________ Check your answers before reading on. Mendel’s conclusions From his investigations with pea plants, Mendel made some conclusions about how characteristics are inherited. • Each characteristic is controlled by a pair of factors in the parents which are passed on in the gametes (sex cells that form offspring). • These factors separate when gametes are formed. The gametes then contain only one factor from the pair. • At fertilisation, gametes unite at random therefore ratios of offspring are predictable. • One factor (the dominant factor) may mask the effect of the other (the recessive factor) when both are present. • The factors are unchanged from generation to generation, even when their effect is not expressed (does not produce an observable feature). Mendel’s success Mendel succeeded in finding a pattern to explain and predict the inheritance of many features. From what you have read about his experiments, why do you think he was successful in finding the pattern? _________________________________________________________ _________________________________________________________ _________________________________________________________ _________________________________________________________ Here are some features of his experimental techniques that contributed to his success. Part 2: Patterns • of inheritance Mendel studied 7 character traits that had a clearly distinguishable pair of factors. • Mendel carried out an enormous number of experiments using a large number of plants for each. • Mendel took care to keep the plants of one experiment isolated from other experiments. He personally transferred the pollen from one plant to the other using hand pollination techniques. He also collected the seeds and sowed them himself. • Mendel rigorously collected data and kept a thorough record of results obtained. He used mathematical and statistical analysis on this data to interpret the results of his breeding experiments. Explaining Mendel’s observations Mendel explained his observations by denoting the dominant factor with a capital letter (eg. A) and the recessive factor with a small letter (eg. a). Each individual has a pair of factors. Therefore, the possible factors that individuals can have for this feature are: • AA pure dominant form • aa pure recessive form • Aa hybrid form. Mendel used letters such as these to explain how the 3:1 ratio was obtained in each of his experiments. Remember that offspring inherit one factor for the feature from each parent during crossbreeding. Looking at an example Consider the example of round seeds (R) and wrinkled seeds (r). Complete the following table for this feature. Form Factors pure dominant pure recessive hybrid Check your answers. What will happen if a pure dominant (round-seeded) parent is crossed with a pure recessive (wrinkle-seeded) parent? The pure dominant parent can only produce gametes that contain the dominant (R) factor. The diagram to the right shows how this is often represented. RR R parent R gametes The pure recessive parent can only produce gametes with the recessive (r) factor. 8 Because each offspring must inherit a factor from each parent, you can see that the progeny in Blueprint of life the first generation can only ever be Rr. Gill Sans Bold Here is a diagram to represent this cross. wrinkled (rr) round (RR) X RR Rr rr Rr Rr Rr All round but carry the ‘hidden’ factor for wrinkled peas (Rr) So all the offspring of a cross between pure dominant round-seeded plants (RR) and pure recessive wrinkle-seeded plants (rr) are round-seeded hybrids (Rr). Now look at what happens when you cross the hybrid forms to produce the second generation. round (Rr) round (Rr) X Rr RR Rr Rr Rr rr round : wrinkled = 3 : 1 Did you notice how each hybrid parent can contribute a dominant (R) factor or a recessive (r) factor? To determine the possible combinations of these factors, Mendel used a mathematical device called binomial expansion. Here is an explanation. There are four possible ways the offspring can inherit a factor from each parent. Part 2: Patterns of inheritance 9 Using the first factor in each pair: Rr X Rr RR Using the second factor in each pair: Rr X Rr rr Using the inside factors in each pair: Rr X Rr Rr Using the outside factors in each pair: Rr X Rr Rr There is an equal chance of each of these four possibilities. Therefore, if you observe a large enough number of crosses (and Mendel used thousands), there should be an equal proportion of each. Hence: • one quarter will be RR, pure round-seeded • one quarter will be rr, pure wrinkle-seeded • two quarters (or a half) will be Rr, the hybrid round-seeded form. The ratio predicted by this explanation is – round-seeded plants : wrinkle-seeded plants = 3 : 1 This was the ratio that Mendel observed. Another example The diagram below represents the observed results of Mendel’s experiments to study the inheritance of plant stem length in pea plants. 0 Blueprint of life Gill Sans Bold Parents (one pure tall, one pure short) X First generation (all tall) X Second generation (787 tall and 277 short) Using the letter T for the dominant factor and t for the recessive factor, answer the questions that follow. 1 Mendel used pure long-stemmed plants and crossed them with short-stemmed plants. What factors were inherited in the first generation of plants? _____________________________________________________ 2 What ratio of long-stemmed to short-stemmed plants was observed in the second generation? _____________________________________________________ 3 Use letters to represent the possible offspring in the second generation. _____________________________________________________ _____________________________________________________ _____________________________________________________ Part 2: Patterns of inheritance 11 _____________________________________________________ Check your answers. Recognition of Mendel’s work After eight years of collecting data from nearly 30 000 pea plants, Mendel discovered the natural laws of inheritance. Mendel’s findings on plant hybridisation were presented in two lectures before the Society for the Natural Sciences in 1865 in Brünn, Moravia (now called Brno in the Czech Republic). His paper, Versuche über Pfanzenhybriden, was published in the Society’s proceedings in 1866 and sent to 133 other associations of natural scientists and to the more important libraries in a number of different countries. Unfortunately, there was no response to his findings and its importance was not recognised until 34 years after the publication of his results. Mendel’s approach and the nature of his experimentation were simply too unconventional for his age. Nobody before him had ever attempted to use mathematical and statistical analysis as a means of interpreting the results of biological experiments. Biologists in those days were mostly people interested in natural history (observing and recording the features and behaviour of things in the world around them) and not necessarily familiar with mathematics. Mendel sent his reports to the most famous botanist in Europe at the time, Karl Wilhelm von Nageli of Switzerland. He hoped to gain his sponsorship (support of his work) but Von Nageli ignored Mendel’s work and sent it back to him. Mendel was able to get his paper published in a scientific journal several years later but no-one acknowledged it because he was an unsponsored amateur. Another reason for the absence of any response from the scientific fraternity of the day was the limited number of people who read the Brno Association’s records. Also Mendel was known as a relatively shy person and might not have presented his results with the necessary emphasis and stress to get others to notice them. Just before his death on January 6, 1884, he commented: my scientific labours have brought me a great deal of satisfaction, and I am convinced that before long the entire world will praise the result of these labours. And that is what happened. By the end of the nineteenth century, breeders and horticulturists were looking for a reliable set of rules to ensure the propagation of certain features in their crops and livestock. Then, in the spring of 1900, an amazing coincidence occurred. Three botanists, Hugo de Vries from Holland, Carl Correns from Germany and E. von Tschermak from Austria, independently reported the same conclusions as Mendel. This amounted to a rediscovery of Mendel’s inheritance research, verifying his work and giving rise to a new branch of biology called genetics. Each of these men gave full credit to Mendel for the discovery. Now complete Exercise 2.1. 2 Blueprint of life Gill Sans Bold The principles of heredity You have been studying Mendel’s experiments that examine the inheritance of a single trait involving one pair of factors. Breeding experiments of this kind are called monohybrid crosses. (Mono- refers to the one feature being studied and -hybrid refers to the two forms of the feature.) Monohybrid crosses Monohybrid crosses are the simplest way to study how features are inherited. There are only four kinds of monohybrid crosses involving simple dominance. They are: • crosses of two of the same pure breeding forms For example, AA AA or aa aa The offspring of these crosses will always be the same as their parents • crosses of a pure breeding dominant with a pure breeding recessive For example, AA aa You will recall that this type of cross results in all the offspring having the hybrid form of the feature. In this case, Aa. • crosses of a pure breeding dominant with the hybrid form For example, AA Aa. If you use the technique explained previously you will notice the following outcome: AA AA Aa Aa AA Aa Half of the offspring are pure breeding dominant and the other half are hybrids. However, they will all look alike. • crossing a pure breeding recessive with the hybrid form For example, aa Aa. Use the technique above to determine the possible outcomes of this cross. Show your working as a diagram. Check your Part 2: Patterns of inheritance answer. 13 Mendel’s explanations were based on the language of his day. It has limitations when trying to describe the complexities of today’s genetics. You need to learn how to use some new words so that you can go on further with this module. Homozygous and heterozygous An organism with a pure breeding trait is said to be homozygous for that feature. So AA and aa are both homozygous; AA is pure breeding dominant and aa is pure breeding recessive. The hybrid character is called heterozygous. Aa is an example. To help you to remember these terms better, just remember the word stem: • homozygous – when a trait has the same two factors • heterozygous – when a trait has two different factors. To help you to recognise and use these terms better, complete the following tasks then check your answers. 1 Brown eyes in humans are dominant over blue eyes. Using the letters B for dominant and b for recessive, write the letters for: a) homozygous for blue eyes ____________________________ b) homozygous for brown eyes ___________________________ c) heterozygous for brown eyes __________________________ 2 Predict the likelihood of a couple, heterozygous for brown eyes, having a baby with blue eyes. _____________________________________________________ _____________________________________________________ Check your answers. Genes and alleles The word ‘factor’ is not used much today because it is now known that genes are responsible for transmission of inherited information. The word gene comes from the Greek word genesis that means birth. The term genetics was coined by the famous British biologist William Bateson in 1902. A gene is a unit of genetic material with information about one feature. Alleles are alternative forms of a gene. For example, the height of a pea plant (length of stem) depends on the organism having genes for tallness. Each plant has two genes for this trait. The plant can be pure breeding tall (homozygous, TT) with identical alleles; it can be hybrid tall (Tt) with different alleles; or it can be pure breeding short (tt) with identical alleles. Consider the cross below. 4 Blueprint of life Gill Sans Bold hybrid tall plants X A C B D E F Which plant(s) definitely: 1 has/have identical alleles? ________________________________ 2 has/have different alleles? ________________________________ 3 is/are homozygous? _____________________________________ 4 is/are heterozygous? ____________________________________ Check your answers. Complete Exercise 2.2 now. Genotype and phenotype In Mendel’s experiments, each trait observed was a result of the expression of one or both genes in a pair of genes. (Not only are genes responsible for the transmission of information but they control what an organism looks like.) The appearance of a trait in an organism is called its phenotype. The genetic information that produces the trait in the organism is called its genotype. For example, for the feature of plant height (length of stem), the ‘tallness’ phenotype can be expressed by two genotypes – TT or Tt. The genotype for the recessive short phenotype is tt. Whenever you do problems and calculations involving genotype, use the letters that represent the genes. When describing phenotype, just describe the looks of the trait in the organism. 1 Complete the table below for the trait of seed shape, that involves the alleles R (the gene for round seeds) and r (the gene for wrinkled seeds). Part 2: Patterns of inheritance Phenotype Genotype(s) 15 round seeds wrinkled seeds 2 Why are there only two phenotypes for this trait? _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ Check your answers. Mr Punnett and his squares Reginald Crundall Punnett was born in England in 1875. He was a geneticist who, with William Bateson, discovered genetic linkage. (You’ll learn more about genetic linkage later in the module.) Punnett’s work helped to establish Mendel’s theories. In fact, in 1905 he wrote a book called Mendelism. In 1912, Punnett became a fellow of the Royal Society of London and was named professor of genetics at Cambridge. He is best known for devising the Punnett square for summarising the fusion of gametes in genetic crosses. (Punnett is spelt in a variety of ways in different books, including Punnet, punnett and punnet.) A Punnett square is constructed in the following way. 1 Draw a 3 3 grid. + 6 Blueprint of life Gill Sans Bold 2 Write the alleles for one parent (often the male) in the left column and the alleles for the other parent (often the female) in the top row. For example, Rr Rr R r R r R RR Rr r Rr rr + R r 3 Fill in the empty squares by combining the gametes as shown. When you combine the genes in the gametes, you are finding out the genes that are in the offspring. + So the offspring have genotypes of RR, Rr, Rr and rr. That is, one quarter are homozygous dominant, one half are heterozygous and one quarter are homozygous recessive. Their phenotypes for this trait will be three with the dominant feature to one with the recessive feature, or 3 : 1. Use Punnett squares to solve the following problems. 1 What are the possible genotypes and phenotypes for the offspring of a couple who have brown eyes? The male is homozygous (BB) and the female is heterozygous (Bb). a) Draw a 3 3 grid. (It has been done for you.) b) Write the alleles for one parent in the left column and the alleles for the other parent in the top row. + c) Fill in the empty squares by combining the gametes. d) Now answer the question in the problem. __________________________________________________ __________________________________________________ __________________________________________________ __________________________________________________ 2 What is the genotype and phenotype of a person who has a homozygous brown-eyed mother and a blue-eyed father? a) Draw and complete a Punnett square. Part 2: Patterns of inheritance 17 b) Answer the question in the problem. __________________________________________________ __________________________________________________ __________________________________________________ __________________________________________________ 3 Determine the possible genotypes and phenotypes for the offspring of a couple where the male is blue-eyed (bb) and the female is brown-eyed but has a blue-eyed father. _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ In each of the problems above, the characteristic (eye colour) can have two phenotypes but there are three possible genotypes (BB, Bb and bb). 4 Using this example, explain the relationship between dominant and recessive genes and the phenotype observed. _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ 8 _____________________________________________________Blueprint of life Gill Sans Bold _____________________________________________________ Check all your answers. Complete Exercise 2.3 now. Part 2: Patterns of inheritance 19 Investigating human variation You have been looking at the variation that Mendel studied in pea plants. Variation in humans has also been extensively studied. Complete your own investigation of human variation by reading and following the instructions below. Aim A single pair of genes (alleles) controls certain traits or characteristics in humans. The aim of this activity is for you to examine some of these human inherited traits in yourself and someone you know. Procedure Read the list of some human traits and their descriptions below. Then determine which of these characteristics you possess. Record your phenotype and genotype in the result table on the next page. Then identify and record the information for a friend or family member. List of some human traits 20 • ear lobes may be attached or free. Free ear lobe (G) is dominant over attached (g) • cheek dimples may be present in one cheek only, in both cheeks or there may be more than one dimple in each cheek. Dimples (D) is dominant over no dimples (d) • tooth gap may occur in the centre of the top row of teeth. Frontteeth close together (T) is dominant over a tooth gap (t). • vision short or long sight (V) are dominant over normal vision (v) • eyelashes long eyelashes (S), 10 mm or more, are dominant over short eyelashes, less than 10 mm (s). (Use a small ruler to measure the length of eyelashes) • tongue rolling the ability to roll the sides of the tongue upwards to form a U-shape. This ability is dominant (R) over inability to roll the tongue (r) • widow's peak where the hairline on the forehead comes down to a peak in the middle is dominant (W) over a straight or curved hair line (w) • freckles spots on the skin (freckles) are dominant (F) to no freckles ((f) • mid digital hair hair on the back of the middle segments of the fingers. Hair on the middle segment of one or more fingers (H) is dominant over lack of hair on the middle segments of the fingers (h) • handedness of life right-handedness (P) is dominant over left-handedness Blueprint (p) Gill Sans Bold • second toe length the second toe may be shorter than the big toe, which is dominant (B), or longer than the big toe, (b) • hair form determined by a pair of genes, which are incompletely dominant. Homozygous gene combinations produce straight hair (CC) or curly hair (cc) while the heterozygous combination produces wavy hair (Cc). You’ll learn more about this in Part 3. Results Characteristic For you Phenotype Possible genotype For a friend or relative Phenotype Possible genotype ear lobes cheek dimples tooth gap vision eyelashes tongue rolling widow's peak freckles mid digital hair handedness second toe length hair form Discussion Compare your features with someone else in your family. Can you see any pattern in features that may have been inherited? _________________________________________________________ _________________________________________________________ _________________________________________________________ _________________________________________________________ _________________________________________________________ Part 2: Patterns of inheritance 21 Pedigrees A pedigree is a record of the features of family members. It is often shown as a diagram. You might call this diagram a family tree. It can help you to examine and explain the inheritance of the feature. A pedigree is quite easy to draw if you follow the instructions below. Constructing a pedigree 1 2 Start with the parents. Use the male and female symbols shown. Draw all the offspring of these parents using the same symbols for male and female. male female parents son 3 For each offspring that has had children, draw in their partner, husband or wife. parents son 22 daughter daughter daughter’s husband Blueprint of life Gill Sans Bold 4 Repeat step 2 for the next generation. F (parents) father mother F1 (first generation) son daughter daughter’s husband F2 (second generation) daughter’s children Notice that the son has no children but the daughter has two sons and one daughter. An example of a family pedigree From the description of the Vassallo family below, draw the pedigree. My mother and father had me first (a boy) and then they had my two sisters. The first born sister has one daughter. The next sister has a son and a daughter. I have two daughters. Check your answer. Other pedigrees Part 2: Patterns of inheritance Some people have more complicated families, as in the example shown below.23 my father my adopted older sister my mother’s second husband my mother me my sister my brother half sister half brother second husband’s first wife step sister Look at the differences between a sister, adopted sister, half sister and stepsister. Now try drawing your own family pedigree in the space below. Using a pedigree to investigate atrait To investigate the inheritance of a feature, you draw up a pedigree then colour in each member who displays that trait. Here are the pedigree symbols to use. Standard set of pedigree symbols male without the trait being investigated male with the trait being investigated female without the trait being investigated female with the trait being investigated 24 Notice how the individuals with the trait are coloured in. Blueprint of life Gill Sans Bold Hair form in the Vassallo family Suppose you were to investigate the inheritance of the trait for curly hair in the Vassallo family. (You drew their family tree in an earlier activity.) The person who wrote the original statement says that both he and his father have curly hair and so does his second daughter and his first sister. His second sister has straight hair but her husband and both their children have curly hair. Shade in the symbols for people with the curly hair trait on the pedigree below. my parents my mother my father me and my sisters me my sisters my niece our children my children my nephew my niece Check your answer. How are pedigrees used? Taronga Zoo and zoos all around the world use pedigrees to record the mating of animals in their breeding programs. In this way, zoos can prevent diseases that occur when closely related organisms reproduce. (This is called inbreeding and tends to cause disease because it is more likely that related parents will carry a recessive gene for the same disease.) To prevent inbreeding, zoos plan outbreeding programs by including unrelated animals from other zoos around the world. The royal family of Europe A famous example of inbreeding is shown in the pedigree below. Royal families traditionally marry other royal people. These means that many closely related people have reproduced together in the royal families of Europe. The pedigree below shows members of the royal families of Europe since Queen Victoria (mid 1800s). It shows all the members with a genetic disease called haemophilia. Haemophilia sufferers are unable to produce Factor 8, which is responsible for the clotting ability of blood. This means that even a small injury can cause death due to excessive bleeding. The inheritance of this trait can be examined and even predicted by studying this pedigree. Part 2: Patterns of inheritance 25 Prince Albert EdwardVII Alexandra Victoria Alice Alfred Queen Victoria Helena Louise Arthur Leopold Bertrice Frederick William Mary George V Irene Victoria Nicholas II of Russia Alice Alexandra Alfonso XII Victoria Leopold Maurice of Spain Eugenia Prince Andrew of Greece ? George VI Margaret Elizabeth Alexis died in Alfonso Rupert (Tsarevich) (Viscount infancy (Crown Trematon) Prince of Asturias) Alice Waldermar Henry of of Prussia Prussia Gonzalo Philip (Duke of Edinburgh) Elizabeth II Charles Anne Andrew Edward Using a pedigree to make predictions Sometimes you can determine the genotype of the parents by looking at the children’s phenotypes as well as those of the parents. A pedigree is a useful tool for solving these kinds of problems. For example, a blue-eyed male (recessive trait) has four children with a brown-eyed female. Two of the children have blue eyes. You can use a monohybrid cross to determine that the mother was a hybrid, or carrier of blue eyes, giving them a 50 : 50 chance of having blue-eyed children. 1 Here is an example of a family whose pedigree shows members with blue eyes (the coloured in members of the pedigree). parents F children F1 grand children F2 26 2 1 3 10 5 4 11 12 6 13 8 7 14 15 16 9 18 17 a) Predict the phenotype for 10, 11 and 12. Explain your answer. Blueprint of life __________________________________________________ Gill Sans Bold __________________________________________________ __________________________________________________ __________________________________________________ b) Predict the genotype for 6. Explain your answer. __________________________________________________ __________________________________________________ __________________________________________________ __________________________________________________ 2 Study the pedigree below. F 2 1 F1 3 5 4 6 8 7 9 F2 10 11 12 13 14 15 16 17 18 a) Is the trait being investigated dominant or recessive? Explain. __________________________________________________ __________________________________________________ __________________________________________________ b) What are the chances of each of the first generation having the recessive genotype? __________________________________________________ __________________________________________________ __________________________________________________ __________________________________________________ __________________________________________________ c) If the parents just kept on having children, what chance would there be of the dominant trait eventually showing through? __________________________________________________ __________________________________________________ __________________________________________________ Part 2: Patterns of inheritance 27 d) What could be the possible genotypes for each of the members numbered below? 2 _______ 4 ______ 8 _______ 11 _______ 3 _______ 6 ______ 9 _______ 15 _______ Dominant traits can get a bit trickier because you need to determine whether the character being investigated is homozygous or heterozygous. Studying a pedigree of the family can help you to do this. 3 Consider this example. F 2 1 F1 3 5 4 6 8 7 9 F2 10 11 12 13 14 15 16 17 18 a) The trait being studied in this pedigree is recessive. The parents (1 and 2) do not have the trait yet two of their children (5 and 7) do. Explain the inheritance of the recessive trait in 5 and 7. __________________________________________________ __________________________________________________ b) Is 9 homozygous or heterozygous? Explain. __________________________________________________ __________________________________________________ c) Is 6 homozygous or heterozygous? Explain. __________________________________________________ __________________________________________________ d) How could you determine if 4 is homozygous or heterozygous? __________________________________________________ __________________________________________________ Check all your answers. Complete Exercise 2.4. 28 Blueprint of life Gill Sans Bold Hybridisation By now you should have a bit of an idea of what hybrid means in genetics. You learnt that the hybrid form of a trait results from the inheritance of two different alleles. In horticulture and agriculture, hybrid has a much less specific meaning. A hybrid is produced from parents with very different characteristics. Offspring produced from two quite different varieties of a species or even two different species (as in the case of a mule) will be hybrids of those parents. Producing hybrids is a very common practice amongst plant breeders. For example, Triticale is a crop plant formed by the hybridisation of wheat and rye. It has the advantages of being high yielding, drought and disease resistant. It also produces fertile offspring. Triticale is an example of a species produced by crossing wheat and rye. (Photo: J Haeusler) Hybridisation is the practice of selecting parent stocks with different characteristics on purpose in order to obtain more desirable features in the offspring. There are many advantages of using hybridisation. As soon as farmers began to produce hybrid crops, their productivity increased. Plant nurseries sell lots of popular varieties that are hybrids. The disadvantages of hybridisation stem from the nature of hybrids. Propagation is usually hampered by the fact that the offspring have such diverse hereditary characteristics that they may be infertile or not have the desired features from their parents. An open-ended investigation of plant hybrids Visit your nearest plant nursery or ring them. Ask if you may speak with a nurseryman Part 2: Patterns of inheritanceor someone who knows about hybrids. 29 Ask the following questions and make a basic summary of the responses in a table. (You need to draw up a table below to record your answers.) • What are the names of at least five different hybrid plants on sale at the moment? • What feature of a hybrid makes the plant more desirable than its parent stock? • Are the hybrids fertile or sterile? Now use the information you have collected to identify an advantages and/or disadvantage of hybridisation. _________________________________________________________ _________________________________________________________ _________________________________________________________ _________________________________________________________ _________________________________________________________ _________________________________________________________ 30 Blueprint of life Gill Sans Bold Suggested answers Observing traits 1 Plants with round seeds were crossed with plants with wrinkled seeds. This is usually written in the form: round seeds wrinkled seeds. 2 The offspring plants all produce round peas. 3 The hybrid character is round peas (which all have the round factor and the wrinkled factor). Dominant and recessive traits 1 The recessive factor is not destroyed, only hidden in the first generation. It shows up in the second generation because the recessive factor is passed on, unchanged, to some offspring. 2 The ratios are shown in the most simplified form. Expt Trait examined Dominant factor Recessive factor Ratio in second generation 1 seed shape round wrinkled 3:1 2 seed colour yellow green 3:1 3 flower colour violet-red white 3:1 4 form of seed pods inflated constricted 3:1 5 colour of seed pods green yellow 3:1 6 position of flower axial terminal 3:1 7 length of stem long short 3:1 3 Yes, the ratios for traits in all the second generations are very similar. 4 dominant : recessive = 3 : 1 5 A large number of peas were needed to give a large sample size. This means that enough information could be gathered to provide a reliable ratio. Part 2: Patterns of inheritance 31 Explaining Mendel’s observations Form Factors pure dominant RR pure recessive rr hybrid Rr 1 Tt 2 long-stemmed : short-stemmed = 787 : 277 = 3 : 1 3 One quarter will be TT. Two quarters (one half) will be Tt. One quarter will be tt. Monohybrid crosses aa Aa Aa aa Aa Aa Half of the offspring are Aa hybrids, showing the dominant feature. Half of the offspring are aa, showing the recessive feature. Homozygous and heterozygous 1 a) bb b) BB c) Bb 2 Both parents have Bb genes. Therefore, their offspring are likely to be a quarter BB, a half Bb and a quarter bb. That is, there is one chance in four, or a 25% chance, that a baby will have blue eyes. Genes and alleles 1 F (C, D and E may have identical alleles but you cannot be certain) 2 A and B (C, D and E may have different alleles but you cannot be certain) 3 F (C, D and E may be homozygous but you cannot be certain) 4 A and B (C, D and E may be heterozygous but you cannot be certain) Genotype and phenotype 1 32 Blueprint of life Gill Sans Bold 2 Phenotype Genotype(s) round seeds RR, Rr wrinkled seeds rr The R gene is dominant so RR and Rr both produce round seeds; the recessive gene is hidden in the hybrid form. The homologous recessive gene pair, rr, only produces wrinkled seeds. Mr Punnett and his squares 1 B b B BB Bb B BB Bb + Half of the offspring have a genotype of BB and half are Bb. This means that all the offspring will have the phenotype of brown eyes (because B is the dominant gene). 2 B B b Bb Bb b Bb Bb + The person will be brown-eyed with a genotype of Bb. 3 B b b Bb bb b Bb Bb + Part 2: Patterns of inheritance 33 The offspring will have either blue eyes with a genotype of bb or have brown eyes with a genotype of Bb. 4 Two recessive genes (bb) produce the blue-eyed phenotype. However, the dominant gene hides the recessive gene, so the hybrid form (Bb) causes the brown-eyed phenotype. Two dominant genes (BB) also produce the brown-eyed phenotype. An example of a family pedigree my parents my mother my father me and my sisters me my sisters my niece our children my children my nephew my niece Using a pedigree to investigate a trait my parents my mother my father me and my sisters me my sisters my niece our children my children my nephew my niece Using a pedigree to make predictions 1 a) 10, 11 and 12 will have blue eyes. 3 and 4 have blue eyes so they must both have genotypes of bb since the blue-eyed gene is recessive. bb bb will produce all bb offspring. b) Male 6 is brown-eyed so his genotype is either BB or Bb. He has a blue-eyed son so 6 must be heterozygous (Bb). 2 34 a) The trait shown in the pedigree is dominant. If it were recessive, all the people in the pedigree would have the characteristic since 1 and 2 have the trait. However, 1 and/or 2 have hidden genes that are expressed in the pedigree so they must be Blueprint of life hybrids showing the dominant phenotype. Gill Sans Bold b) All of the children in F1 (4, 5, 7 and 8) have the recessive genotype. To produce a recessive genotype, both parents (1 and 2) must be hybrids. Thus, the chances of each person in the first generation having the recessive genotype are one chance in four, or 25%. c) A big chance! The expected ratio of dominant : recessive = 3 : 1. In a large enough family, this ratio should be observed. d) 2 Aa 4 aa 8 aa 11 Aa 3 Aa 6 Aa or AA 9 Aa 15 Aa Part 2: Patterns of inheritance 35 3 a) Both parents must be heterozygous for the trait. A quarter of their children would be homozygous dominant and half would be heterozygous; all these children would show the dominant phenotype. A quarter of their children would be homozygous recessive, like 5 and 7, and exhibit the recessive phenotype. b) Person 9 must be heterozygous. He carries the recessive gene since he has passed it to his elder son and to his daughter. He exhibits the dominant phenotype so he must also have the dominant gene. c) Person 6 displays the dominant phenotype so he could be homozygous dominant or heterozygous. There are not enough children in the family to be sure that one child will not display the recessive trait. d) Look at person 4’s children. If she has any children with the recessive trait (which she does) then she must carry the recessive gene hidden by a dominant gene. She is heterozygous. If she had many children and none of them had the recessive trait, then perhaps she could be homozygous. However, she could still be heterozygous if person 3 is homozygous dominant. You would need to investigate person 3 to find out whether they are homozygous or heterozygous before you could make a decision. 36 Blueprint of life Gill Sans Bold Exercises – Part 2 Exercises 2.1 to 2.4 Name: _________________________________ Exercise 2.1: Recognition of Mendel’s work a) Outline the experiments carried out by Gregor Mendel. Include information about each of the following in your answer. • What kind of organism, and how many of them, did he study? • What kinds of features did he study? • How many features did he study at one time? • How did he perform his investigations? (What was he studying?) • What did he do that was similar to other researchers? • What did he do that was different? _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ b) Mendel’s research was not recognised for about 35 years after it was published. What are three reasons why this happened? • ____________________________________________________ ____________________________________________________ ____________________________________________________ Part 2: Patterns of inheritance 37 • ____________________________________________________ ____________________________________________________ ____________________________________________________ • ____________________________________________________ ____________________________________________________ ____________________________________________________ c) What was the main factor that led to the rediscovery of Mendel’s work? _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ 38 Blueprint of life Gill Sans Bold Exercise 2.2: Genes and alleles Here is a diagram representing peas produced by pea plants with different genetic information. round (RR) a) round (Rr) wrinkled (rr) What trait is shown? _____________________________________________________ _____________________________________________________ b) How many genes are responsible for this trait? What are they? _____________________________________________________ _____________________________________________________ _____________________________________________________ c) Are these genes alleles? Explain. _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ d) Circle the group(s) of peas above that are homozygous for this trait. Explain why you chose these peas. _____________________________________________________ _____________________________________________________ _____________________________________________________ Part 2: Patterns of inheritance 39 Exercise 2.3: Mr Punnett and his squares The trait of plant height is controlled by two genes – T for tall and t for short. a) Determine the possible genotypes and phenotypes for the following crosses. Show all your working using Punnett squares. X homozygous tall short _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ X heterozygous tall 40 heterozygous tall Blueprint of life Gill Sans Bold _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ b) Why are the pea plants either tall or short in each of these crosses? _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ Part 2: Patterns of inheritance 41 Exercise 2.4: Using a pedigree to make predictions One form of deafness in humans is due to a dominant gene, D. Normal hearing depends on a recessive gene, d. The D gene is very uncommon in Australia’s population. Joe, who is deaf and has the D gene, marries Sue, who has normal hearing, and they have four children. The eldest child, John, and the eldest daughter, Kim, are deaf. The other two daughters, Kate and Julie, have normal hearing. John is married to a deaf woman, Sally, and they have three sons. Julie is married to a hearing man, Sam, and they have a daughter then a son. a) Draw a pedigree for this family, showing the trait of deafness. b) What is the chance of Julie and Sam having a deaf child? Explain. _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ c) Would you expect John and Sally’s children to be deaf or hearing? Explain. (There are three possibilities that you need to discuss in your answer.) _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ 42 _____________________________________________________Blueprint of life Gill Sans Bold _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ _____________________________________________________ d) How do you think the pedigree you have drawn could help this family? _____________________________________________________ _____________________________________________________ _____________________________________________________ Part 2: Patterns of inheritance 43