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Tuesday, May 23, 2017 3.4: Inheritance Keywords: • co-dominance • punnet square • pedigree chart • sex-linked genes Learning Objectives: We are learning…. • How do we show the inheritance of characteristics through diagrams? •How does co-dominance affect the inheritance of characteristics? •How are blood groups in humans inherited? • How are genetic diseases like cystic fibrosis and Huntington’s disease inherited? • What are mutagens and how do they cause mutation? Starter: What do you understand by the term ‘co-dominance? Watch ‘The story of inheritance’ video Genetic crosses: a history Watch ‘Mendel’s discoveries’ video One of the first people to study genetics was an Austrian monk called Gregor Mendel in the 1850s and 1860s. He experimented with thousands of pea plants and established the basic foundations of inheritance, such as dominant and recessive characteristics. Mendel had no knowledge of DNA or genes but he did identify that inheritance is particulate, i.e. it depends on the transfer of separate (discrete) factors from parents to offspring. 2 of 45 © Boardworks Ltd 2009 Genetic crosses: worked example 3 of 45 © Boardworks Ltd 2009 Monohybrid cross 4 of 45 © Boardworks Ltd 2009 Test cross To determine whether an organism showing the dominant characteristic of a trait is homozygous or heterozygous, a test cross can be performed. This involves crossing the organism with another that is homozygous recessive for the trait. TT or Tt? tt × if TT T T t Tt Tt t Tt Tt all tall 5 of 45 if Tt T t t Tt tt t Tt tt If any of the offspring show the homozygous recessive trait in the phenotype, the parent must have been heterozygous. 2 tall, 2 short © Boardworks Ltd 2009 Crossing over 6 of 45 © Boardworks Ltd 2009 Independent assortment of alleles Independent assortment describes the random arrangement and separation of chromosomes (and all the alleles and genes therein) during meiosis. It is random chance how the chromosomes migrate during anaphase I and II. It was after Mendel had studied the inheritance of two characteristics that he described his law of independent assortment: each of a pair of alleles for a particular gene can combine randomly with either of another pair of alleles for a different gene. 7 of 45 © Boardworks Ltd 2009 More on alleles If the alleles for a characteristic are the same, the organism is said to be homozygous for that characteristic. The organism is a homozygote. If the alleles for a characteristic are different, the organism is said to be heterozygous for that characteristic. The organism is a heterozygote. An allele is dominant if it is expressed in the organism’s phenotype, even if the organism is heterozygous. An allele is recessive if it is only expressed in the phenotype when the organism is homozygous. 8 of 45 © Boardworks Ltd 2009 Representing alleles A gene can be represented using a letter: upper case for the dominant allele, and lower case for the recessive allele. For example, the allele for wing length in Drosophila can be either long (L) or short (l). Genotype 9 of 45 Phenotype LL homozygous dominant long wings Ll heterozygous long wings ll homozygous recessive short wings © Boardworks Ltd 2009 Expected offspring The allele that controls eye colour can be expressed as; B = Brown and b = Blue. If two heterozygous dogs with brown eyes are bred together, what would the ratio of brown eyed to blue eyed offspring be? Would this ratio always arise? Explain your answer Gametes are not always produced in equal numbers and the fusion of gametes at fertilisation is random. You may expect these ratios of offspring, but with such a small sample of dogs it may not always occur. What is co-dominance? Codominance results in a phenotype that shows both traits of an allele pair. For example, Red flower + => Red & spotted flower. An example of co-dominance occurs in the snap dragon plant, in which one allele codes for an enzyme that catalyses the formation of a red pigment in flowers. The other allele codes for an altered enzyme that lacks this catalytic activity and so does not produce the pigment. If these alleles showed the usual pattern of dominant and recessive, the flowers would be just two colours: red and white. Codominant alleles Alleles are codominant if they are both expressed in the phenotype of a heterozygote. They can be represented by two capital letters superscript to the letter representing the gene. For example, flower colour in snapdragons Antirrhinum majus. CR = red flowers CW = white flowers Genotype Phenotype CRCR homozygous red flowers CRCW heterozygous pink flowers CWCW homozygous white flowers 12 of 45 © Boardworks Ltd 2009 What are multiple alleles? Multiple alleles is a type of non-Mendelian inheritance pattern that involves more than just the typical two alleles that usually code for a certain characteristic in a species. With multiple alleles, that means there are more than two phenotypes available depending on the dominant or recessive alleles that are available in the trait and the dominance pattern the individual alleles follow when combined together. Most of the time, when multiple alleles come into play for a trait, there is a mix of types of dominance patterns that occur. Multiple alleles and blood groups In humans, the inheritance of the ABO blood groups is determined by a gene, I, which has three different alleles. Any two of these can occur at a single locus at any one time. Blood group Possible genotypes A IA IA or IA IO B IB IB or IB IO AB IA IB O IO IO ABO blood group Some genes have multiple alleles (i.e. more than two), but only two can be present in an individual. For example, the ABO blood group gene (immunoglobulin) in humans. IA produces antigen A on the surface of red blood cells Genotype Phenotype IAIA and IAIO blood group A IB produces antigen B on the surface of red blood cells IBIB and IBIO blood group B I OI O blood group O IO produces no antigen. IAIB blood group AB A and B are codominant and O is recessive to both. 16 of 45 © Boardworks Ltd 2009 Multiple alleles and blood groups Allele A causes the production of antigen A on red blood cells Allele B causes the production of antigen B on red blood cells Allele O causes no production of antigens on red blood cells (universal donor) Alleles A and B are codominant and allele O is recessive to both. Blood group Possible genotypes A IA IA or IA IO B IB IB or IB IO AB IA IB O IO IO Try a cross between individuals with blood group A and an individual with blood group B Diseases can be caused by a number of things, including: • infections eg influenza • poor diet eg scurvy • environmental factors eg asbestosis • spontaneous degeneration of tissues eg multiple sclerosis Some diseases are inherited from our parents through our genes: they are called genetic disorders. They occur because of faulty alleles. Cystic fibrosis is an example of a genetic disorder. What is a carrier? Any condition caused by a recessive allele, such as cystic fibrosis, will only show if recessive alleles are inherited from both parents (cc). Parents with one dominant and one recessive allele for a disorder are known as carriers – they do not suffer the symptoms of the disorder but have the potential to pass on the allele to their offspring. Main Activity: Become the expert…. Working in groups of four. Each member of the group will become an ‘expert’ on one genetic condition. It is your job to study the condition and then report back to your group. By the end of the activity, each person in the group needs to have notes on the following for each of the four genetic conditions: What is the name of the disease? What is the disease caused by? How common is the disease (in Australia)? What are the symptoms of the disease? What is the likelihood of the disease being passed on to offspring? Are there any symptoms for carriers of the faulty genes? How is the disease treated? Cystic fibrosis A 3:1 ratio is typical of the results of a monohybrid cross between two heterozygotes where one of the alleles is dominant and the other recessive. This fact is very useful when considering human genetic disorders and genetic counselling is being given. For example, cystic fibrosis (CF) is a disease that occurs due to a recessive allele of a gene on chromosome 7. The normal allele controls the production of a membrane protein essential for the proper functioning of epithelial cells. 22 of 45 © Boardworks Ltd 2009 Cystic fibrosis: pedigree diagram If a person is homozygous for the recessive CF allele, thick and sticky mucus is produced in their lungs and pancreas, causing breathing problems and malnutrition. Very salty sweat is also a symptom of the disease. Pedigree diagrams show how alleles, like the one for CF, have been inherited through families. unaffected male male carrier (heterozygote) affected male (has CF) 23 of 45 unaffected female female carrier (heterozygote) affected female (has CF) © Boardworks Ltd 2009 Thalassaemia 24 of 45 © Boardworks Ltd 2009 Albinism Albinism is an inherited condition in which there is a lack of the pigment melanin in structures that are normally coloured. Albinos therefore have pinkish skin, deep red pupils and pink irises, photophobia and pale yellow hair. It is caused by a single recessive gene on an autosome, resulting in a malfunctioning tyrosinase enzyme. This prevents tyrosine from being converted into melanin. 25 of 45 © Boardworks Ltd 2009 Dominant allele: Huntington’s disease 26 of 45 © Boardworks Ltd 2009 Codominant crosses 27 of 45 © Boardworks Ltd 2009 What is gene therapy? Traditional drugs act by altering the phenotype of the target cell, and are called phenotypic drugs. Gene therapies deliver selected genes into a patient’s cells and alter the genetic makeup of the cell. They are referred to as genotypic drugs. Initially, gene therapy was envisioned for the treatment of genetic disorders, but it could be used to treat a wide range of diseases, including cancer, arthritis and neurodegenerative diseases. A person with cystic fibrosis has inherited two faulty alleles for a certain gene on one of their chromosomes, chromosome 7. It is hoped that it may one day be possible to repair the faulty alleles using gene therapy, perhaps by putting the normal allele into the cells of the lungs. “Genetic disorders should not be corrected using gene therapy. Some people are born weaker than others and consequently die....it’s called survival of the fittest.” A couple of other Inherited conditions… Progeria • Progeria is caused by a single tiny defect in a child's genetic code. • On average, a child born with this disease will be dead by age 13. • Symptoms include premature baldness, heart disease, thinning bones and arthritis. • Progeria is rare, only around 48 people living with it in the world. • However, there is a family that has five children with the disease A couple of other Inherited conditions… Human Werewolf Syndrome (Congenital Hypertrichosis Lanuginosa) • Inerited – a genetic mutation occurs spontaneously. • People who suffer from it are completely covered in hair except in the palms of their feet and hands. • No treatment Sex determination Sex in mammals is determined by two chromosomes, known as the sex chromosomes or heterosomes. The X chromosome is larger and contains about 2000 genes, whereas the Y chromosome contains fewer than 100. Females (XX) are the homogametic sex. Males (XY) are the heterogametic sex. 32 of 45 © Boardworks Ltd 2009 How is sex determined in other organisms? Sex Chromosomes X-O: Grasshoppers, roaches, and other insects have a similar system for determining the sex of an individual. Adult males lack a Y sex chromosome and have only an X chromosome. They produce sperm cells that contain either an X chromosome or no sex chromosome, which is designated as O. The females are XX and produce egg cells that contain an X chromosome. If an X sperm cell fertilises an egg, the resulting zygote will be XX or female. If a sperm cell containing no sex chromosome fertilises an egg, the resulting zygote will be XO or male. How is sex determined in other organisms? Sex Chromosomes Z-W: Birds, insects like butterflies, frogs and some species of fish have a different system for determining gender. In these animals it is the female gamete that determines the sex of an individual. Female gametes can either contain a Z chromosome or a W chromosome. Male gametes contain only the Z chromosome. Females of these species are ZW and males are ZZ. How is sex determined in other organisms? Parthenogenesis: What about animals like most kinds of wasps, bees, and ants that have no sex chromosomes? How is sex determined? In these species, fertilisation determines sex. If an egg becomes fertilised it will develop into a female. A non-fertilised egg may develop into a male. The female is diploid and contains two sets of chromosomes, while the male is haploid. This development of an unfertilized egg into an individual is called parthenogenesis. Caster Semenya is a South African middle-distance runner and world champion. Semenya won gold in the women's 800 metres at the 2009 World Championships with a time of 1:55.45 in the final. Following her victory at the 2009 World Championships, it was announced that she had been subjected to gender testing….. Gender testing While it would seem a simple case of checking for XX vs. XY chromosomes to determine whether an athlete is a woman or a man, it is not that simple. Fetuses start out as undifferentiated, and the Y chromosome turns on a variety of hormones that differentiate the baby as a male. Sometimes this does not occur, and people with two X chromosomes can develop hormonally as a male, and people with an X and a Y can develop hormonally as a female. Women who test in the male range for testosterone, and whose bodies respond to the hormone, may not be eligible to compete as females. Hyperandrogenism Back to slides The presence of male and female sex hormones influences the sexual development of a person. While both sexes have male and female sex hormones, there is a predominance of male sex hormones (androgen) in men and female sex hormones (oestrogen and progesterone) in women. Women, therefore, should have more oestrogen and progesterone than androgens in their ovaries. Androgens are related to masculine traits like large muscles and a low-sounding voice. When a woman's body produces more androgens than estrogens, the disturbance in her hormonal balance may lead to a condition known as hyperandrogenism. Hyperandrogenism usually occurs from over production of male hormones in a woman's ovary and is associated with other health problems, including cysts in the ovaries. Sex linkage Human cells contain 23 pairs of chromosomes for a total of 46. There are 22 pairs of autosomes and one pair of sex chromosomes. The sex chromosomes are the X chromosome and the Y chromosome. Sex linkage refers to the carrying of genes on the sex chromosomes. These genes determine body characters and have nothing to do with sex. The X chromosome carries many such genes, the Y chromosome has very few. Features linked on the Y chromosome will only arise in the heterogametic (XY) sex, i.e. males in mammals, females in birds. Features linked on the X chromosome may arise in either sex. Hyperandrogeny Sex linkage Genes are located on the sex chromosomes are described as sex linked. The study of their inheritance involves examining both the sex of the offspring and the genetic trait of interest. X-linked diseases Haemophilia Duchenne muscular dystrophy Red–green colour blindness 40 of 45 Y-linked diseases Rare and debatable! It is argued that there is little room on the Y chromosome for anything other than genes controlling testes formation and function. © Boardworks Ltd 2009 Sex linkage: X linkage X linkage is more common because: the X chromosome is larger part of it does not have a homologous section on the Y chromosome, therefore only one allele of a gene will be present and so will always be expressed. homologous regions do not carry sex-determining genes non-homologous regions carry sex-determining genes and other genes 41 of 45 © Boardworks Ltd 2009 H – allele for production of clotting protein h – allele for non-production of clotting protein Xh XH (carrier female) Xh Y (sufferer male) How does sex-linkage work? Because the X chromosome is much longer than the Y chromosome, for much of the length of the X chromosomes there is no equivalent homologous portion of the Y chromosome. Those characteristics controlled by recessive alleles on the X chromosome appear more frequently in the male – there is no homologous portion on the Y chromosome that might have the dominant allele. The recessive allele will always be expressed. Haemophilia – the royal disease Haemophilia figured prominently in the history of European royalty in the 19th and 20th centuries. Britain's Queen Victoria, through two of her five daughters (Princess Alice and Princess Beatrice), passed the mutation to various royal houses across the continent, including the royal families of Spain, Germany and Russia. Victoria's son Prince Leopold, Duke of Albany suffered from the disease. For this reason, haemophilia was once popularly called "the royal disease". Tests of the remains of the Romanov imperial family show that the specific form of haemophilia passed down by Queen Victoria was likely the relatively rare Haemophilia B. Sex linkage: haemophilia 44 of 45 © Boardworks Ltd 2009 Haemophilia in the Royal Family 45 of 45 © Boardworks Ltd 2009 Treatment of haemophilia Though there is no cure for haemophilia, it can be controlled with regular infusions of the deficient clotting factor, i.e. factor VIII in haemophilia A or factor IX in haemophilia B. Factor replacement can be either isolated from human blood serum, recombinant, or a combination of the two. Some haemophiliacs develop antibodies (inhibitors) against the replacement factors given to them, so the amount of the factor has to be increased or non-human replacement products must be given, such as factor VIII from pigs. The long-term outlook for haemophiliacs? Like most aspects of the disorder, life expectancy varies with severity and adequate treatment. People with severe haemophilia who don't receive adequate, modern treatment have greatly shortened lifespans and often do not reach maturity. Prior to the 1960s when effective treatment became available, average life expectancy was only 11 years. By the 1980s the life span of the average haemophiliac receiving appropriate treatment was 50–60 years. Today with appropriate treatment, males with haemophilia typically have a near normal quality of life with an average lifespan approximately 10 years shorter than an unaffected male. Haemophiliacs are more likely to die from HIV/AIDS and hepatitis than severe heamorrhaging due to contaminated blood products. Sex linkage and colour blindness Did you know? One person in twenty is colour blind Red/green colour blindness is a common hereditary condition which means it is usually passed down from your parents. Colour blindness is usually passed from mother to son on the 23rd chromosome, which is the sex chromosome. The X chromosome is the sex chromosome: males have an X chromosome and a Y chromosome and females have two X chromosomes. For a male to be colour blind the faulty colour blindness gene only has to appear on his X chromosome. For a female to be colour blind it must be present on both of her X chromosomes. This is why red/green colour blindness is far more common in men than women. Inheritance of colour blindness allele – complete the family trees In the back of your books 1. 2. Hint 3. 5. 4. 6. Pedigree charts Pedigree charts are used to trace the inheritance of sex-linked characteristics such as haemophilia and red-green colour blindness. See if you can make sense of this one. Female sufferer Male Female Could you add anything to the key for this diagram? Male sufferer The pedigree chart explained I II III Complete the pedigree charts exam questions So what are mutations? Any change in the structure or the amount of DNA of an organism is called a mutation. Most mutations occur in somatic (body) cells and are not passed from one generation to the next. Only those mutations which occur in the formation of gametes can be inherited. These mutations produce sudden and distinct differences between individuals. Michael Berryman is an American actor who has appeared in many horror films. He was born with a rare genetic condition which prevents him from developing hair, sweat glands or fingernails and claims to have had "twenty-six birth defects." An example of how mutagens cause damage to the genes controlling the normal cell cycle is shown below. After exposure to UV light, adjacent thymine bases in DNA become cross-linked to form a ‘thymine dimer’ (lesions form between bases). This disrupts the normal base pairing and throws the controlling gene’s instructions into chaos. Changes in gene structure – point mutations Changes in the structure of DNA which occur at a single locus on a chromosome are called gene mutations or point mutations. Any change in the sequence of nucleotides will produce the wrong sequence of amino acids in the protein it makes. This protein is often an enzyme. Why might this be a problem? Complete the card sort to find out how sickle cell anaemia is caused. Types of gene mutations – the causes Chernobyl - The accident In the early hours of 26 April 1986, one of four nuclear reactors at the Chernobyl power station exploded. Moscow was slow to admit what had happened, even after increased radiation was detected in other countries. The lack of information led to exaggerated claims of the number killed by the blast in the immediate area. Contamination is still a problem, however, and disputes continue about how many will eventually die as a result of the world's worst nuclear accident. Chernobyl - The environment The disaster released at least 100 times more radiation than the atom bombs dropped on Nagasaki and Hiroshima. Much of the fallout was deposited close to Chernobyl, in parts of Belarus, Ukraine and Russia. More than 350,000 people resettled away from these areas, but about 5.5 million remain. Contamination with caesium and strontium is of particular concern, as it will be present in the soil for many years. After the accident traces of radioactive deposits were found in nearly every country in the northern hemisphere. But wind direction and uneven rainfall left some areas more contaminated than their immediate neighbours. Scandinavia was badly affected and there are still areas of the UK where farms face post-Chernobyl controls. Chernobyl – The present day The sarcophagus encasing Chernobyl was built in haste and is crumbling. Despite strengthening work there are fears it could collapse, leading to the release of tonnes of radioactive dust. Work has begun on a £600m replacement shelter designed to last 100 years. This New Safe Confinement will be built on site and then slid over the sarcophagus. Video - Chernobyl: The catastrophe that never ended Despite the lasting contamination of the area, scientists have been surprised by the dramatic revival of its wildlife. Wild horse, boar and wolf populations are thriving, while lynx have returned to the area and birds have nested in the reactor building without any obvious ill-effects. Quizzes and activities Quick quiz – Use the whiteboards to answer the following questions (draw punnet squares) 1 mark = correct genotype 1 mark = correct phenotypes 1 mark = ratio of phenotypes Question 1: A homozygous dominant green plant is crossed with a homozygous recessive yellow plant Question 2: Two heterozygous carriers of cystic fibrosis want to know the possibility of their children inheriting the disease. Question 3: Two pea plants are crossed – green and yellow. One is a pea plant which is homozygous for green peas. The other is a heterozygous pea plant. Question 4: Huntington’s disease is caused by the presence of a dominant allele. Two parents, one of whom is a sufferer and the other who is normal, want to know what the likelihood is of their children inheriting the disease is. Plenary: 1. If a female is a carrier for haemophilia X H X h and is married to a man with haemophilia X h Y , what is the probability that she will have a daughter with haemophilia? 25% of the offspring are daughters with genotype X h X h 2. A normal female marries a man who has haemophilia . (You’ll need to figure out the genotypes). What percentage of their sons will have haemophilia? 0 sons will have haemophilia, all inherit the normal allele from mother 3. If a female has haemophilia and is married to a normal man. What percentage of her sons will have haemophilia? 4. What percentage of her daughters will have haemophilia? Plenary: Plenary: Autosome means the 22 non sex determining chromosomes. Plenary: Suggest a genetic explanation for the difference in life expectancy of females and males (3) Males have XY / Females have XX Males have only one allele for some genes – the homologous portion is missing These alleles are expressed Harmful alleles increase the chance of early death / genetic conditions Males develop testes due to XY chromosomes Testes produce testosterone Testosterone causes males to take more risks Glossary 70 of 45 © Boardworks Ltd 2009 What’s the keyword? 71 of 45 © Boardworks Ltd 2009 Multiple-choice quiz 72 of 45 © Boardworks Ltd 2009