Genetics PDF

Genetics A species is a group of organisms that can interbreed and produce fertile offspring. Fertility is a very important aspect because occasionally two different, but closely related species can interbreed and produce an infertile hybrid. An example of this...

Genetics A species is a group of organisms that can interbreed and produce fertile offspring. Fertility is a very important aspect because occasionally two different, but closely related species can interbreed and produce an infertile hybrid. An example of this is a mule, the offspring of a male donkey and a female horse. Other examples include the liger (lion and tiger), the cama (camel and llama), and the zonkey (zebra and donkey). Variation is the differences between individuals of a particular species. These differences can be structural, e.g., height, weight, or colour, but they can also be behavioural, e.g., aggressiveness. An example of variation is Jack Russell and St. Bernard dogs, the same species but have very different physical attributes. Heredity and Gene Expression **Know definitions and an example of each** Heredity is the passing on of characteristics from one generation to the next by genes. For example, brown eyed parents tend to have brown eyed children. A gene is a length of DNA on a chromosome that codes for a protein or controls the development of a characteristic. Genes control traits and characteristics. Gene expression is the process by which the information from a gene is used to produce a feature or a protein. For example, if a person has the gene for brown eyes, they will have brown eyes even if they also have the gene for blue eyes, because brown is dominant to blue. Chromosome structure Each gene has its own specific position on a chromosome called the locus. Most genes have two different options, called alleles. Alleles are either dominant or recessive. A chromosome is composed of two strands called chromatids held together by a structure called the centromere. When chromosomes are spread out, they are called chromatin. Chromatin consists of DNA and proteins. Before division, cells become shorter and thicker as the DNA coils around histones. When chromosomes replicate, they are called chromatids and are held together by a centromere. (1) A chromosome is an organised length of deoxyribonucleic acid (DNA) and proteins called histones. Structure of Deoxyribonucleic Acid (DNA) - James Watson and Francis Crick discovered the molecular structure of DNA in 1951. - DNA is a polymer with thousands of repeating units - Basic shape is a double helix - Sides of the ladder consist of alternating deoxyribose sugar and phosphate groups. - - The rungs consist of complementary bases held together by hydrogen bonds - Fours bases are: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C) - Adenine only pairs with Thymine - Guanine only pairs with cytosine (2) Nucleotides (3) DNA Replication - DNA replication occurs in the nucleus during interphase of mitosis and meiosis - Energy (ATP) and the enzyme DNA polymerase are needed for DNA replication Process - The double helix opens - it untwists and unzips (hydrogen bonds broken by enzymes) prior to replication - Free nucleotides enter nucleus from cytoplasm - New nucleotides form a new chain. Each strand acts as a template to make the opposite strand from complementary nucleotides. - Each new DNA molecule is (i) half new and half old, (ii) identical to the original and each other - Each new piece of DNA rewinds to form a double helix - Identical helices are formed i.e., the original DNA molecule makes 2 exact replicas of itself Replication of DNA is necessary to pass on genetic code/information exactly from generation to generation. Structure of Ribonucleic Acid (RNA) Differences between RNA and DNA - RNA contains the sugar ribose instead of deoxyribose - The base thymine found in DNA is replaced by uracil in RNA - RNA is single stranded while DNA is double stranded (4) There are three different types of RNA: **need to know mRNA** - Messenger RNA (mRNA): Carries the information from the DNA in the nucleus to the ribosomes in the cytoplasm, where the ribosomes use the information to produce proteins - Ribosomal RNA (rRNA): Major part of the structure of ribosomes. Produced in the nucleolus and assembled in the cytoplasm - Transfer RNA (tRNA) – Collects and brings specific amino acids found in the cytoplasm to the ribosomes to be assembled into proteins DNA profiling DNA profiling is a method of making a unique pattern of bands from the DNA of a person, which is used to distinguish that DNA from other DNA. Process - DNA is extracted from cells e.g., blood or semen by breaking up the cell membrane. - DNA amplification can be used if the quantity of DNA is low. Increasing the quantity is done by a technique called the polymerase chain reaction (PCR) - Restriction enzymes are used to cut the DNA wherever a specific base sequence occurs (mostly junk genes which are highly variable). This base sequence occurs at a different set of positions in everyone. The sets vary in size (length) and number - Gel electrophoresis - the fragments are separated, using an electric current, along a piece of gel. The smaller the fragment the faster they travel along the gel - The DNA is then transferred onto a nylon membrane for ease of use. The fragments are made visible by attaching radioactive pieces of DNA to them that produce a bar-code-like design on an X-ray film Uses DNA profiles can be used to identify criminals from crime scenes or fathers in paternity cases (5) Genetic Screening **Process not required** Genetic screening is checking for the presence or absence of a particular allele or gene. A length of DNA with the same base sequence as that of a particular allele labelled either with a radioactive or fluorescent marker is added to a sample of a person’s DNA. If the marker sticks to the DNA, it shows that the gene is present. This can be used to check for the presence of genes for cystic fibrosis or hemochromatosis, which are two inheritable diseases common in Ireland. Mandatory Practical – To isolate DNA from plant tissue - Chop up a kiwi or an onion -Add sodium chloride (salt) to washing up liquid in distilled water - Add the kiwi to the washing up liquid and salt solution - Place this solution in a water bath at 60o C for 15 minutes (denatures the enzymes which stops the DNA being digested) - Place the solution in a water bath that is ice cold for 5 minutes - Place the solution into a blender for 3 seconds - Filter the solution using coffee filter paper - Using a syringe place some of the filtered solution into a boiling tube - Add protease enzyme which helps break down the proteins around the DNA - Add ice cold ethanol slowly down the side of the boiling tube - Using a glass rod, DNA should attach to it as it is twisted. Protein Synthesis Protein synthesis is the process of building proteins out of their building blocks (amino acids) Process Genes (Information Store) - The information needed to produce the protein is stored in the nucleus - The nucleus is surrounded by the nuclear membrane, which is selectively permeable and keeps the information safe - The genes are stored in the chromosomes. All the words used are three-letter words called triplets - The triplets say which amino acid to use, and the sequence to assemble them in Transcription (Copying the information) - DNA is unzipped by the enzyme DNA helicase and the complementary bases automatically fit against the corresponding base on the DNA - Uracil is used in making RNA - An enzyme called RNA polymerase forms the mRNA strand Movement of mRNA - The single stranded mRNA moves out of the nucleus into the cytoplasm through the nuclear pores - The cytoplasm contains numerous ribosomes, which are mode of rRNA and protein - The two RNA components of the ribosome are made in the nucleolus and assembled in the cytoplasm - The ribosomes in the cytoplasm attach to the mRNA using rRNA Transfer RNA (tRNA) - Each tRNA is a length of RNA shaped like a cross - Has a sequence of three bases at one end called an anticodon, and a specific amino acid at the other - The three bases of the anticodon are complementary to a codon on the mRNA, which matched a triplet on the DNA, thus the sequence of amino acids matches the sequence coded for by the DNA - Each tRNA collects its specific amino acid from the cytosol - The free floating tRNA then brings amino acids to the ribosome Translation (Making the protein) - As each ribosome moves along the mRNA, it comes across a start codon, which tells it to start assembling the protein - The tRNA with the complementary anticodon places the appropriate amino acid in sequence determined by the mRNA codons - In the protein the amino acids are joined to each other by a peptide bond - Once the amino acid has been delivered, the tRNA that brought it is set free to go and find its particular amino acid again - When the ribosome reaches a stop codon, it stops making the protein and sets it free. The ribosome can now attach to another mRNA - The protein folds into the correct shape to carry out its function Distribution of proteins Once the protein has been made, it can either be used in the cell or packaged and released to carry out its function elsewhere Genetic Inheritance Gregor Mendel was a 19th century Augustinian monk and teacher who joined a monastery in what is not the Czech Republic. Between 1858 and 1866, Mendel bred peas and brought a fresh approach to the process, elevating it to a scientific footing. This allowed him to deduce principles that had eluded others. His work was ahead of its time and not accepted by the scientific community until after his death in 1884. He became known as the father of modern genetics. Statistical analysis of his work in the 20th century suggested that some of his results were too good to be true. However, Mendel knew exactly what was happening in his breeding work, because he picked suitable characteristics to confirm his hypothesis, and designed experiments to demonstrate this. Mendel’s concepts Each characteristic is controlled by a fair of factors known as genes. One pair of genes is inherited from each parent. There are two options to a characteristic, these are called gene alleles. Normally one option is stronger than the other, and the strongest one is said to be dominant, while the weaker one is recessive. - Dominant allele is the allele that is always expressed if it is present - Recessive allele is the allele whose expression is masked by the dominant allele Law of Segregation – Mendel’s First Law This states that each characteristic is governed by a pair of factors (genes). These separate at gamete formation and each gamete only receives one of each pair of factors. - Phenotype is the physical expression of the genotype or genotype plus environment. - Genotype is the genetic make-up of an individual. When carrying out genetic crosses, we use letters to symbolize the genes. The letter used is normally the first letter of the dominant feature. In the case of pea- seed colour, we use Y for yellow and y for green. In the case of pea-seed shape, we use capital R for round seeds and r for wrinkled seeds. If a pea has two identical alleles, e.g., TT or tt, it is said to be homozygous. Homozygous describes the situation when both alleles are identical. If the pea has two different alleles, e.g., Tt, it is said to be heterozygous. Heterozygous describes the situation when both alleles are different. Monohybrid Cross This feature is looking at one feature at a time, e.g., seed colour, height, or sex There are three possible types of cross; - Homozygous dominant crossed with homozygous recessive - Heterozygous crossed with homozygous recessive - Heterozygous crossed with heterozygous Homozygous dominant crossed with homozygous recessive Parental Phenotypes Homozygous X Homozygous dominant recessive Parental Genotypes YY yy Meiosis (Chromosome Separates) Y Y y y Gamete Genotypes y y Y Yy Yy Y Yy Yy All possible random fertilisations using the Punnett Square F1 progeny phenotypes Yy Yy Yy Yy F1 progeny phenotypes yellow yellow yellow yellow Phenotype ratio All Yellow In peas, the allele for yellow seed is dominant to green seed. A pure-breeding pea plant with yellow seeds is crossed with a pea plant that produces green seeds. Procedure - Write down the parents phenotypes - Under this, write their genotypes. (Be careful to use Capital and small letters correctly) - Always write the dominant character first when writing the genotype - Write meiosis, and the alleles are separated, with one going to each gamete - Draw a circle around the gametes - Construct a punnet square and place the gametes as shown above - Put them together, remembering that the capital letter must always go first - Write down each type of progeny genotype - Write down the phenotype of each genotype - Work out the ratio of phenotypes Heterozygous crossed with homozygous recessive Parental Phenotypes Heterozygous X Homozygous recessive Parental Genotypes Rr rr Meiosis (Alleles Separates) R r r r Gamete Genotypes r R R Rr Rr r rr rr All possible random fertilisations using the Punnett Square F1 progeny phenotypes Rr Rr rr rr F1 progeny phenotypes Round Round Wrinkled Wrinkled Phenotype ratio 1 (50% round) : 1 (50%) wrinkled - In peas, the allele for round seed is dominant to wrinkled seed - A heterozygous peas plant with round seeds is crossed with a pea plant that produces wrinkled seeds - The letter R is used because round is dominant to wrinkled - The heterozygous plant is Rr - Wrinkled seed is recessive, so the plant has to be homozygous for this feature Heterozygous crossed with heterozygous Parental Phenotypes Heterozygous X Heterozygous Parental Genotypes Tt Tt Meiosis (Alleles Separates) T t T t Gamete Genotypes T T T TT Tt t Tt tt All possible random fertilisations using the Punnett Square F1 progeny phenotypes TT Tt Tt tt F1 progeny phenotypes Tall Tall Tall Dwarf Phenotype ratio 3 Tall : 1 Dwarf - In peas, the allele for tall stem is dominant to the allele for dwarf stem - A heterozygous tall pea plant is crossed with a heterozygous tall pea plant - Each plant is tall, and tall is dominant, so it has a T - Each is heterozygous, which tells us they have two different alleles, so each must have a t - The genotype of each parent is therefore Tt Filial generations First Parental (P1) Phenotypes Purple Flowers X White Flowers P1 Genotypes PP pp Meiosis P P p p P1 Gamete Genotypes First Filial (F1) Genotypes All Pp F1 Phenotypes All Purple Now the F1 generation are self-fertilised and become the second parental generation (P2) P2 Phenotypes Purple X Purple P2 Genotypes Pp Pp Meiosis P p P p P2 Gamete Genotypes P P P PP Pp p Pp pp All possible random fertilisations using the Punnett Square Second filial F2 Genotypes PP Pp Pp pp F2 phenotypes purple purple purple white F2 Phenotype ratio 3 purple : 1 white The parents involved in a cross are the first parental generation (P 1), while the ‘sons’ and ‘daughters’ produced by a cross are called the first filial generation (F1), from the latin filius, and filia, meaning daughter. The crosses above produce the first filial generation. If these offspring are cross- fertilised, they are called the second parental generation (P 2), and their offspring are called the second filial generation (F 2). This is what Mendel did, and it was this that led him to his deduction about pairs of factors. Sex Determination In most mammals, including humans, XX is female and XY is male. Sex is determined by a monohybrid cross. The cross below shows that there is a 50:50 chance of a baby being a boy or a girl. However, it does not work out exactly like this. Some parents have three boys and no girls, or seven girls and no boys, or any other combination. This shows that genetics is all about chance. Parental Phenotypes Female XX Male XY Parental sex chromosomes XX XY Meiosis (Alleles Separates) X X X Y Gamete sex chromosomes X Y X XX XY X XX XY All possible random fertilisations using the Punnett Square Progeny sex chromosomes XX XY XX XY Progeny phenotypes Female Male Female Male Phenotype ratio 1 (50%) male : 1 (50%) female Incomplete Dominance In cows, a parent with a red coat and a parent with a white coat produce offspring with a mottle colour called roan. Neither allele is fully dominant since both show in the phenotype. This is called co-dominance or incomplete dominance. This is easily recognisable because some of the offspring have a feature which is intermediate between the two parents. Dihybrid Cross A dihybrid cross is the study of crosses involving two characteristics at a time. From studying this type of cross, Mendel devised his second law. Law of Independent Assortment – Mendel’s Second Law Mendel’s second law states that during gamete formation, any member of a pair of factors has an equal chance of entering a gamete with either member of any other pair of factors. Homozygous recessive x heterozygous for both characteristics - In peas, red flower is dominant to white flower. For example, the crossing of peas that are tall, have red flowers, and are heterozygous for both characteristics, with other pea plants, such as those that are short and have white flowers - The heterozygous tall peas with red flowers have the genotype Tt Rr, so the T has an equal chance of combing with R or r to give gametes TR or Tr - The homozygous recessive dwarf white flowered peas tt rr can only produce gametes that are tr - There are parental phenotypes dwarf white and tall red, but also new parental phenotypes: in this case, tall white and dwarf red. Thus, independent assortment has led to variation. Parental 1 (P1) phenotype Dwarf white x Tall Red Parental 1 (P1) genotype tt rr TtRr Meiosis Gamete 1 (P1) genotypes tr TR Tr tR tr TR Tr tR Tr tr TtRr Ttrr ttRr Ttrr First filial generation (F1) Genotypes TtRr Ttrr ttRr ttrr Phenotypes 1 tall red : 1 tall white : 1 dwarf red : 1 dwarf white Phenotype ratio 1: 1: 1: 1 Dihybrid cross to the F2 Generation First parental phenotypes (P1) Tall Red Dwarf White P1 genotypes TTRR ttrr Meiosis P1 gametes TR TR Cross tr tr First filial generation (F1) All TtRr now F1 becomes parental 2 (P2) Tall red Tall red TtRr TtRr Meiosis TR Tr tR tr TR Tr tR tr TR Tr tR Tr TR TTRR TTRr TtRR TtRr Tr TTRr TTrr TtRr Ttrr tR TtRR TtRr ttRR ttRr tr TtRr Ttrr ttRr ttrr Second filial generation (F2) Genotypes TTRR TTrr ttRR ttrr Phenotypes 9Tall Red 3Tall White 3Dwarf Red 1Dwarf White Phenotype Ratio 9: 3 : 3 :1 Above shows a dihybrid cross between a homozygous dominant pea plant and a homozygous recessive pea plant taken to the second filial generation. The ratio of phenotypes of the F2 is 9:3:3:1. That means, 9 are showing dominant features, 3 are showing one dominant feature and one recessive feature, 3 are showing the other dominant and the other recessive feature, 1 is showing both recessive features. Linkage Linked genes are genes that are located on the same chromosome and are therefore inherited together. The characters Mendel examined happened to be on separate chromosomes. That is why he observed independent assortment. If, however, the genes are on the same chromosomes, they will be inherited together. For example, consider the following parental nuclei of a fruit fly. Both father and mother have a pair of chromosomes with alleles for two different genes: Fruit flies can have long wings or vestigial (short) wings and a narrow abdomen or a wide abdomen. Long wings are dominant to vestigial wings and wide abdomens are dominant to narrow abdomens. - If a long-winged, wide-abdomen individual is crossed with a vestigial-winged, narrow-abdomen individual, all the offspring will have long wings and wide abdomens. - If the offspring of this are cross-mated, one may expect all combinations of wing length and abdomen width, but this is not the case. This is because the two genes are linked. Since the two alleles are linked, they behave as one and do not assort independently. The result is that only parental phenotypes are shown in the offspring. - A small proportion of non-parental phenotypes do occur due to a phenomenon known as crossing over. This occurs during the production of gametes by the process of meiosis. This involves the swapping of sections of homologous chromosomes as the chromosomes are pulled apart. **Don’t need to know crossing over in detail** Long Wing Vestigal Wing Long Wing Long Wing Wide Abdomen X Narrow Abdomen Wide Abdomen X Wide Abdomen V V v v V v V v A A a a P2 A a A a  V v V v V v A a G2 A a A a F1 individuals crossed with each other F1 V v V v A a A a Long Wing Wide Abdomen V V V V v A A A A a V V v v v a A a a a F2 Genotypes V V V v V v v v A A A a A a a a F2 Phenotypes Long Wing Long Wing Long Wing Vestigial Wing Wide Abdomen Wide Abdomen Wide Abdomen Narrow Abdomen F2 Phenotype Ratio 3 Long Wing Wide Abdomen : 1 Short Wing Narrow Abdomen Sex Linkage Sex-linked refers to genes found on the X or Y chromosome. These genes are called heterosomes because they are different shapes and sizes. The Y chromosome is relatively empty, but it does contain genes to do with sperm production. The other 22 pairs of chromosomes are called autosomes, as each pair is the same size and shape. Sex-linked characteristics are determined by genes that occur on the X chromosome. For example, haemophilia and colour blindness; - Haemophilia: A group of diseases where a person’s blood does not clot properly. Caused by a lack of a particular protein - Colour blindness: An inherited condition where people cannot see the colours red and green properly. - In both conditions, the gene for the normal condition is dominant while the gene for the abnormal condition is recessive Carrier is a female who has an allele for the abnormal condition but does not show it. Phenotypes and Genotypes in Males & Females with regards to Haemophilia Females - XNXN has normal blood clotting - XNXn has the normal blood clotting because the dominant normal allele masks the abnormal allele. This person is a carrier because she carries the condition - XnXn has haemophilia. She has two recessive alleles Males - XNY- has normal blood clotting because even though he has only one allele, it is normal so it is expressed - XnY- has haemophilia because even though he has only one allele, it is abnormal so it is expressed Parents: Female carrier X Male normal XN Xn XNY- Gametes: XN Xn XN Y- F1 X N XN XNY- XN Xn XnY- Phenotypes Female Male Female Male Normal Normal Carrier Haemophiliac - 25% chance of producing a haemophiliac child - 50% chance of producing a haemophiliac son. - It is the mother that determines if the son is haemophiliac or not, since the father always passes the Y chromosome to his son. Non-nuclear Inheritance - Although the majority of DNA in a cell is found in the chromosomes, there is also DNA found in the nucleus in mitochondria and chloroplasts. This is called Non- nuclear DNA - This DNA is always passed on by the female in the cytoplasm of the ovum (egg). - In animals, the sperm only provides the chromosomal materials. Mitochondrial DNA is used by the sperm to provide energy so it can swim to the egg. Only the nuclear DNA enters the egg - In plants, the pollen grain only contains nuclear DNA. When the male gametes are formed, there is no mitochondrial or chloroplast DNA present Family tree of sex-linked crosses **Important to know all combinations** XNY- A Xn X n XnY- B XNXN XnY- Xn XN XNY- C XnXN XNY- XNY- XnXN Xn XN XnY- XNY- XNXN XnXN Affected Female Carrier Female Normal Female Normal Male Affected Male Xn Abnormal Allele XN Normal Allele - Cross A produces 2 affected males and 2 carrier females - Cross B produces 2 normal males and 2 carrier females - Cross C produces 1 affected male, 1 normal male, 1 normal female, and 1 carrier female Ratios **Important to know all combinations** Monohybrid and Dihybrid crosses Ratio Parental Genotypes  All one phenotype Homozygous dominant with homozygous recessive or heterozygous 3:1 (75%:25%) Heterozygous with heterozygous 1:1 (50%:50%) Heterozygous with homozygous recessive 1:2:1 (25%:50%:25%) Heterozygous co-dominant with heterozygous co- dominant 1:1:1:1 (25%:25%:25%;25%) Heterozygous dihybrid with homozygous recessive dihybrid 9:3:3:1 Heterozygous dihybrid with heterozygous dihybrid Sex-linked All offspring normal, all female carriers XNY- with XNXN 50% normal males:50% carrier XNY- with XNXn females 1 affected male, 1 normal male, 1 normal XNY- with XNXn female, 1 carrier female All female carriers and all male XNY- with XnXn offspring are affected All offspring affected XnY- with XnXn Possible gamete genotypes in a dihybrid cross Parental TT TT TT TrR Tt Ttr ttR ttR Ttr Genotyp RR Rr rr R Rr r R r r es Gamete TR TR, Tr TR, TR, Tr, tR tR, tr Genotyp Tr tR Tr, tr tr es tR, tr Natural Selection and Evolution Variation Variation is the differences between individuals of a species. Causes of Variation - Sexual reproduction: The independent assortment of homologous chromosomes during meiosis ensures genetic variation among gametes - Mutations: This is any sudden change in the amount or structure of DNA Types of Mutations - Chromosome Mutations: A change in chromosome number or alteration of genes within a chromosome. This is potentially more harmful because of the number of genes involved. A change in chromosome number tends to be harmful in animals and humans but beneficial in plants e.g., Down Syndrome. Down Syndrome is a result of having 47 chromosomes (an extra ‘number 21’ in every body cell). This happens because one gamete had an extra copy of this chromosome. During meiosis, homologous chromosomes failed to separate and hence, two of the gametes had no number 21 and two had two copies of the chromosome. - Gene Mutations: A change in bases in the gene. This alters the amino acid sequence of the protein controlled by that gene. The bases may be altered by deletions, insertions, and substitutions. They usually make the gene non-functional or recessive, e.g., Cystic Fibrosis. Cystic Fibrosis is the inability to remove mucus from the lungs. The gene codes for a protein found in cell membranes that controls the flow of chloride ions into and out of the cell. This results in thick mucus that clogs the lungs and stops the correct functioning of the pancreas and liver. Causes of Mutations - Spontaneous mutations: Faulty DNA replication making ‘mistakes’ or, when DNA fails to repair properly - Mutagens: Agents that cause mutations They can speed up the spontaneous rate of mutation e.g., a. Ionising radiation such as X-rays, UV rays, cosmic rays, //gamma rays. These harm DNA indirectly and their effect can accumulate in the body over years. They can also harm gametes. UV alters DNA structure directly and cause mutations in skin cells, e.g., skin cancer b. Chemicals e.g., formaldehyde, tobacco smoke, caffeine and many drugs, preservatives, and pesticides. Many are carcinogenic (cancer-causing) - Some viruses and ageing may cause mutations. Hepatitis B virus can cause liver cancer Evolution Evolution is an inheritable change in a species in response to a change in the environment by natural selection over a long period of time. All the evidence for evolution is circumstantial i.e., it appears to fit the facts of evolution if evolution is true. Evolution can never be proved. The mechanism by which evolution may have occurred was proposed by Charles Darwin and Alfred Russell Wallace in 1858. In 1859, the book ‘On the Origin of Species by means of Natural Selection’ was published. Darwin collected evidence on his ship, The HMS Beagle, for 5 years, beginning in 1832. Theory of Natural Selection Observations - Overbreeding: More offspring are produced (e.g., a tree produces many seeds) than the environment can support - Population numbers stay the same - Inherited variations occur in populations: No two members of the same species ever look alike (except identical twins) Conclusions - Struggle for existence: Overbreeding gives rise to competition or a struggle to survive. - Survival of the fittest: Those individuals with favourable variations are better adapted to the environment and have a better chance of survival. This is the ‘survival of the fittest’ e.g., some deer run faster than others and can escape from predators, improved hearing versus deafness in rabbits (improved hearing will survive where the others would be killed off because they couldn’t hear predators). The better animals survive and reproduce - Origin of species: Over a period, accumulation of advantageous variations will result in offspring widely divergent from their original type. They will no longer interbreed and thus form a new species Evidence for Evolution **Know in detail** Observing one trait – height of horse - 60 million years ago 0.4 m tall (size of a fox) - 30 million years ago 0.6m high (German shepherd height) - 10 million years ago 1m high (Great Dane) - 1 million years ago Equus, modern horse = 1.6m tall. The evolution of the horse shows that over 60 million years they have: -grown in size - gone from having four toes to walking on a single toe (running on hard ground to escape predators). - Molar teeth - from low-crowned for eating soft foliage to hard ridges for chewing grass. - The structural changes are related to the different environments they adapted to i.e., marshy wooded areas changing to dry grassland. Natural selection worked causing the extinction of the older less well-adapted forms. Fossil records cannot provide a complete description of evolution as: - generally, fossils are only found for the harder structures of plants and animal - older fossils are difficult to date - many fossils will never be discovered - many fossils are destroyed by erosion Genetic Engineering Genetic Engineering is the manipulation and alteration of genes. It means that it is possible to combine genes from one species with genes from another unrelated species, which does not normally occur in nature because different species rarely mate. Genetic engineering breaks down what is known as the species barrier. As a result, it is possible to insert; - Human genes into bacteria and other animals - Bacterial genes into plants Process of Genetic Engineering of Insulin with Bacterium **Know in detail** - Isolation: The gene for human insulin production is called the target gene, and when found is cut from its chromosome. It is then separated from all the other genes - Cutting: Cutting is achieved using a special enzyme called a restriction enzyme that cuts DNA at a particular sequence of base pairs. It involves cutting two things: a. The chromosome that contains the desired gene, this is called the target gene. In this case, it is the human insulin-producing gene b. A special genetic structure found in bacteria, called a plasmid. This is called the cloning vector. Both the chromosomal and the bacterial DNA are cut at the same base sequence so that they can later be joined together -Ligation: The insulin gene and the plasmids are then mixed. An enzyme called DNA ligase causes the foreign DNA to join with the new plasmid DNA. This results in the insulin gene inserting itself across the cut in the plasmid, producing a new type of DNA called recombinant DNA - Transformation: The recombinant DNA is mixed with the bacteria species chosen as the producer. - Expression: The bacteria are cultured under conditions that favour their multiplication. All the cells produced are genetically identical copies, so this is example of cloning (6) Applications of Genetic Engineering **Know in detail** Plant - Weedkiller-resistant crops: Inserting a bacterial gene for herbicide resistance into crop plants, so that when the herbicide is sprayed it will kill weeds, but it will not kill the plant. Animals - Sheep produce a protein to treat emphysema. A human gene for this protein (AAT) has been inserted into sheep DNA and they can then produce the protein in their milk. Microorganisms - Bacteria make insulin: Inserting the gene for human insulin into a bacterium which then produces human insulin for use by diabetics. This overcomes the danger of people producing antibodies to the pig insulin.  Exam Questions 2014 – HL – Section C – Question 10 (c) The diagram shows part of the genotype of an individual of the Aberdeen Angus cattle breed. This breed is unusual in that the allele for the polled (hornless) condition is dominant to the one for the horned condition. (i) What term is used to describe the allele pair Pp? Heterozygous (ii) Is this a sex-linked condition? Explain your answer. No, because the alleles are not present on the sex chromosomes (iii) What is the phenotype and sex of the animal whose partial genotype is shown above? Hornless Male (iv) Draw a diagram, similar to the one shown, to describe an Aberdeen Angus which, when crossed with the one above, would ensure the production of a polled calf. (v) Name a group of organisms in which the XY chromosome pair gives rise to a different sex than in cattle. Birds 2013 – HL – Section A – Question 6 6. (a) (i) In DNA, nitrogenous bases occur in complementary pairs. Explain the term complementary as used here. Complementary means one base matches with another specific nitrogenous base (ii) In each case, name the complementary base in RNA for: 1. Adenine - Uracil 2. Cytosine - Guanine (iii) Name a carbohydrate that is a component of nucleotides - Ribose (iv) Name a component of a nucleotide that is neither a carbohydrate nor a nitrogenous base - Phosphate (b) (i) What does the ‘m’ stand for in mRNA? Messenger (ii) Give one difference between RNA and DNA, other than the nitrogenous bases. RNA is single stranded, DNA is double stranded (iii) Give the role of the enzyme RNA polymerase – RNA polymerase synthesises RNA 2013 – HL – Section C – Question 11 11. (a) (i) Give a source of evidence for evolution. Palaeontology (ii) Briefly outline the evidence from the source referred to in (i). Fossils records show a detailed history of the timeline of evolution and how organisms have adapted and changed over millions of years (b) (i) Human males and females differ in one of their twenty three pairs of chromosomes. What name is given to this pair of chromosomes? Sex Chromosomes (ii) Draw this pair of chromosomes for a human male and for a human female and label them appropriately. (iii) Using the chromosomes referred to in part (b) (ii), show, using a Punnett square or otherwise, that a child stands an equal chance of being male or female. Gametes X Y X XX XY X XX XY F1 offspring are in the ratio of 1:1. There is an equal chance that the child will be male or female. (iv) 1. What is meant in genetics by the term sex linkage? Sex linkage is whee genes are present on a sex chromosome 2. Name two common sex-linked traits. - Colour blindness - Haemophilia 2012 – HL – Section A – Question 6 6. (a) In genetics, what is meant by the term variation? Variation refers to differences amongst members of the same species (b) Variation can result from mutation. Name one other cause of variation. Sexual Reproduction (c) Name two types of mutation. (i) Gene Mutation (ii) Chromosome Mutation (d) Name two agents responsible for increased rates of mutation. (i) UV radiation (ii) Cigarette smoke (e) Briefly explain the significance of mutation in relation to natural selection. Mutations lead to new genotypes and hence new phenotypes. Most of these new phenotypes will be disadvantageous to organisms. However, occasionally a new phenotype occurs that gives an organism an advantage over the rest of the population. Therefore, this organism will be more likely to survive and pass on the new phenotype to its offspring. In this way mutations are important to natural selection 2012 – HL – Section C – Question 10 10. (a) (i) Nucleic acids are composed of subunits called nucleotides. Each nucleotide is formed from a sugar, a phosphate group and a nitrogenous base. Name the two types of nitrogenous base found in DNA. Purines and Pyrimidines (ii) Give both of the specific base pairs in DNA structure. Adenine – Thymine Cytosine - Guanine (b) In the sweet pea plant the texture and colour of the testa (seed coat) are governed by two pairs of alleles, which are not linked. The allele for smooth (S) is dominant to the allele for wrinkled (s) and the allele for yellow (Y) is dominant to the allele for green (y). (i) State the Law of Segregation and the Law of Independent Assortment - Law of Segregation: Each cell contains two factors governing each trait and these factors separate at gamete formation - Law of Independent Assortment: Members of one pair of factors, can combine with either member of another pair of factors during gamete formation (ii) Using the above symbols, and taking particular care to differentiate between upper case and lower case letters: 1. give the genotype of a pea plant that is homozygous in respect of seed texture and heterozygous in respect of seed colour. SSYy 2. state the phenotype that will result from the genotype referred to in 1. Smooth and yellow (iii) What phenotype will be produced by the genotype SsYy? Give another genotype that will produce the same phenotype. Do not use a genotype that you have already given in response to part (ii) 1. SsYy – Smooth and Yellow (iv) If the allele for smooth were linked to the allele for green and the allele for wrinkled were linked to the allele for yellow, give the genotypes of the two gametes that parent SsYy would produce in the greatest numbers. Sy and sY (c) (i) What is meant by the term genetic engineering? The artificial manipulation of genes (ii) In genetic engineering all or some of the following procedures may be involved. Isolation; The removal of DNA from the cells of interest Cutting (restriction); The removal of a gene from a piece of DNA using a restriction enzyme Transformation (ligation); The uptake of recombinant DNA by a cell Introduction of base sequence changes; The sequences of bases in the new piece of DNA is different in the cell that has been transformed Expression. Activation of the inserted gene and production of its product Briefly explain each of the above terms in the context of genetic engineering. (iii) Give one application of genetic engineering in any two of the following. 1. An animal. Mice have been genetically modified to glow fluorescent green under UV light 2. A plant. Maize has been genetically modified to be resistant to weed killers 3. A micro-organism. E.coli bacteria have been genetically modified to produce human insulin References 1. Vedantu.com 2. Technologynetworks.com 3. Theory.labster.com 4. BYJUS.com 5. Ib.nioninja.com.au 6. Studyrocket.com