DNA and Protein Synthesis PDF

# DNA and Protein Synthesis ## DNA and Protein Synthesis (Mitosis and Binary Fussion) - DNA- is a long molecule found inside the nucleus of a cell. - It's the set of instructions to dictate which proteins to make for certain cell in the body depending on the position in the body. - P...

# DNA and Protein Synthesis ## DNA and Protein Synthesis (Mitosis and Binary Fussion) - DNA- is a long molecule found inside the nucleus of a cell. - It's the set of instructions to dictate which proteins to make for certain cell in the body depending on the position in the body. - Protein make up the function of a cell and the entire organism. - Prokaryotes have DNA like bacteria, but it's a loop and they contain no nucleus - Red blood cells have no nucleus. - non-living pathogens contain DNA as well. - Chromosome- supercooled strand DNA that is wrapped around proteins called histones. - Each human has 46 of these (23 pairs). - Gene- A section of DNA that codes to make a particular protein and controls the characteristics of an organism through alleles. - They carry the instructions for what amino acids are needed to make specific proteins. - We need specific protein depending on where the cell is in the body, like skin cells need to be able to make keratin and white blood cells need to able to make antibodies. - Ribosomes are the sight of protein synthesis. - Genome- all of an organism's DNA ## Inheritance of DNA - DNA is inherited by our parents through chromosomes (23 from our mother and 23 from our father) in our zygote. - As our zygote goes through mitosis and creates more cells, this DNA/chromosome spreads across all cells as these cells have to be genetically identical. - Chromosomes contain genes (a section of DNA that codes for one protein and this protein determines what kind of alleles the gene are). - Chromosomes from one parent will match to a chromosome from the other parent through where the correct gene (for the same features) are placed and they will be a pair (23 pairs). - Genes for the same feature can have different variations called alleles; each feature has 2 alleles, which are whether dominant (capital) or recessive (lowercase) and they determine our phenotype. - When cells go through mitosis, they first have to duplicate the DNA and do so by making the chromosome into an X-shape. - The second branch has the same exact DNA. - The chromosome then lines up in the cell and splits in half so it will be a line. - The chromosomes then go to the poles and divide to become new cells; the two new cells will have the same amount of chromosomes as the parent cell, except they are single lined chromosomes. - But in meiosis, the cell the cell first splits the homologous pairs stay in the X-shapes; this will result in the two new cells having 23 X-shaped chromosomes. - They will then split again, this time the X-shape into a line, now the two new cells made will have 23 line chromosomes. - There is a total of four daughter cells that are all genetically different from the parent, and as they fuse with another gamete from someone else will make the zygote genetically different. ## How Chromosome Look Like - Chromosomes only have one DNA strand, so they only contain the genes that make up that 1 DNA molecule. - So different chromosomes have different DNA and genes. - Human cells contain 46 chromosomes (23 homologous pairs). - One of this pair is from your mother; another is from your father. - For males, they have 22 pairs of chromosomes and 2 two that don’t match up: the X and Y chromosomes. - For females, they have 23 pairs that contain a pair of X chromosomes. - The X and Y chromosomes are the sex chromosome and they determine if a person is a male or a female. - What makes a pair homologous is that they carry the same gene for the same features in the same place in the chromosome. ## What DNA Look Like - DNA is made of two strands twisted together to form a double helix. (this is a sugar phosphate backbone.) - The phosphate group on the outside linked to the sugar molecule that then links to the nitrogen base all of this make up the nucleotide. - DNA has two nucleotides that are bonded by hydrogen. - Inside the two strands are chemical bonds called nitrogen bases (they are made of nitrogen), and contain a small hydrogen bond between them. - There are four different bases: - Adenine (A) - Thymine (T) - Cytosine ALWAYS - Guanine (G) - They always pair in the same way, A-T and C-G, they are complementary base pairings. - Because of this complementary base pair, the amount of adenine in the DNA is equal to the amount of thymine as with cytosine for Guanine. (This changes when there is a mutation) ## Protein Synthesis - Proteins are long chain molecules made up of amino acids joined together by bonds. There are 20 types of amino acids, and they need to be in the right order/sequence to make specific proteins. - Examples of proteins are enzymes (as they require a specific size active sight for the substrate for the enzymes substrate reaction), haemoglobin (as it needs to be the right shape to be able to bind with oxygen) and muscle tissue. (basically anything that needs to bind with something as it needs to be a specific shape for this.) ### How We Get Specific Proteins - Each cell contains all of your DNA in the nucleus, a gene is a section of DNA that will have the instructions to make one protein (a biological molecule made of chains of amino acids). - The order of bases in the genes determines what protein can be made in the cell as these bases determine the amino acid. - The different order in the chain can be mixed to make up different proteins. (this is the information DNA carries). - Only one strand of the DNA molecule codes for the protein; it’s the one the MRNA is attached to. This is called the template strand. - Many of the proteins are enzymes used as biological catalysts to help speed up chemical reactions; some are structural proteins (keratin) or haemoglobin and hormones. ### How Protein Synthesis Works - A sequence of three bases (vertically) called codons, code for one of these amino acids. - This is called triple code, and they will code one amino acid. - This code is universal and a set of codons in all organisms code for the same amino acid. #### Two Steps Involved in Protein Synthesis 1. **Transcription** - Enzymes unzip the DNA at a specific gene. - When the gene is unraveled, a copy of the DNA is made called mRNA (messenger RNA). - It is a copy of the DNA that should be a copy of not the one it is attached to but its complementary pair base to it. - RNA is a very similar molecule to DNA but has differences as it only has one strand (so the one set of bases and they are just sticking out) - RNA uses the base uracil(U) instead of thymine (T). - DNA contains the sugar deoxyribose; RNA contains ribose. (phosphate, ribose, nitrogen bases) - It will detach from the DNA, cause it to coil back up into a double helix and will stay in the nucleus, the message RNA (mRNA) however will leave the nucleus through the nucleus envelope that has pores, it leaves and enters the cytoplasm. 2. **Translation** - The mRNA attached itself to a ribosome in the cytoplasm. - The ribosome (organelle where proteins synthesise occur) read the three bases (codon) at a time. (remember RNA can’t make thymine only uracil that connects to adenine) - Transfer RNA(tRNA) brings the correct amino acid (to the ribosome) on one side and an anti codon which contains the exact complimentary pair sequence to the codon on the messenger RNA; they than attach. - The first tRNA to bind to the mRNA will do so at a start codon which is the bases sequence AUG. (this makes methionine) - After connecting to the codon, The ribosome will move on to the next codon and sequence, and new tRNA will come and attach to this, bringing a second amino acid will bond to the first forming a chain. - The first tRNA molecule is released and goes to collect another amino acid, and other tRNA arrive and add to the amino acid chain until a stop codon tells the ribosome that the protein is complete; it is then released. - And at the end of the mRNA, the ribosome will detach from the RNA, as the protein (chain of amino acids) is now folded in the right way. ## Cell Division - All cells have a short life span and die getting replaced except brain cells. - Reasons for Cell Division: - Growth: organisms grow by making new cells. We all start as one cell called a zygote. - Repairs: if a cell gets damaged, cell division allows it to get replaced (skin cells shed because they are a layer of dead cells, and they need to be constantly replaced to make sure that they are air-tight and provide a barrier for pathogens. - Cloning: to body needs to make a lot of the same cell quickly. ### Asexual Reproduction - For some organisms, it is a way to make new organisms such as strawberry plant or unicellular organisms. - Sexual reproduction produces variation and for plants that have desirable features, it is better to clone it than to plant it seeds. - For plants that produce asexually, there is no gametes involved. - A part of the organism (runners) grows and breaks off the parent plant and continues to grow through mitosis. - So they are genetically identical to the parent plant and other offspring. - The only time the offspring will genetically different from the parent is if there is a mutation. ### Cloning - Cloning describes any procedure that produces genetically identical offspring. #### Cloning Plants - By taking cuts of plants and growing them, all the cutting contain identical genes. As they grow through mitosis, the cutting become a genetical identical plant. - This is done using micropropagation, first the tips of the stem and side shoot are cut to be cloned (called explants). - The explants are trimmed and the surface sterilised to kill any microorganisms. - They are placed in a sterile agar medium that contains nutrients and hormones to encourage growth. - Secondly, explants with shoots are transferred to another culture medium containing a different balance of plant hormones to induce root formation. - Thirdly, when explants have grown roots they are transferred to a greenhouse. - The advantages of this is that large numbers of genetically identical plants can be produced rapidly, plants can be produced at any time of the year, genetical modifications can be introduced in thousands of plants quickly after modifying only a few plants.) ### Mitosis - Every cell that has a job to replace/repair or grow needs to all be genetically identical. - It is the process when the nucleus splits, making genetic identical (the same amount of chromosome present) clones of cells. - It makes the same amount of chromosomes in the 2 daughter cells to the parents cells (all 46 chromosomes get onto the cell), whereas in sexual reproduction, only 23 chromosomes by cell division (meiosis). - The parent cell splits into two daughter cells. - Each new cell receives the same type and amount of DNA as the organism cell through the same chromosome being shared. #### How Mitosis Works - Before mitosis, DNA replicates. - Before a cell can divide, the DNA within has to replicate, making the exact same copy of DNA. - It does this by replicating the chromosome (replication). - Then each cell formed will receive the exact amount and type of DNA (making it genetically identical). - Before mitosis, one chromosome has one chromatid (where DNA is stored, a long strand) with one centromere in the middle (a protein that holds 2 chromatids together). - After the replication, the one chromosome (it is count as one as it has only one centromere even though there are two chromatid now) has double its DNA, forming two chromatids in the shape of an X. - This prepares it to split in cell division as the chromosome will split in half and enter separate cells, then each cell will end up with one chromatid, the same as the start. #### Mitosis Process: 1. **Prophase**: - DNA replicates and the nucleus membrane/envelope disintegrates (this happens because the chromosome needs to travel to the poles of the cells, and the nucleus membranes get in the way). - The chromosome becomes visible under a microscope. 2. **Metaphase**: - Chromosomes line up on the equator (middle of the cell), so that they can be split in a partial way. - Spindle fibers form, which are proteins that attach to the centromere in the chromosome (and when stretched, will spilt the chromosome.) 3. **Anaphase**: - Chromosomes split in the middle at the centromere, being pulled toward the poles. - This will separate them into two forming chromatids (half looking chromosomes). - The chromatids are being pulled by spindle fibers toward the poles (they are v-shaped). 4. **Telophase** - Chromatids are now at opposite sides of the cell, and new nucleus membrane is starting to form on both poles with the same amount of chromatids as chromosomes in the parent cell (making them genetically identical). - The cell begins to split down the middle into two; cytokinesis is the split of the cytoplasm from a parent cell into two new daughter cells. - There may be exam questions that will give you a wall of plant cells at the different stage of visible mitosis and ask you which stage is it at. ### Binary Fission - Animal/plant and some fungi are eukaryotic meaning that they have a nucleus. - Bacteria are prokaryotic meaning that they have no nucleus - Binary fission is the bacteria version of mitosis (it can be called that as mitosis is to do with the nucleus spilling and that do have one). - As they are unicellular, their DNA loop replicates. - The DNA replicates (so 2 DNA loops) as well as everything else (ribosomes, plasmids) so there are two of them. - Then they migrate to the diligent part of the cell (poles) and is pinched into 2 different cells (binary fission) to make 2 genetically identical cells. ## Cell division in bacteria - Organisms undergo meiosis and sexual reproduction as it allows genetics variation within a population. - Gametes are sex cells like sperm and egg cells that are formed in the sex organs (can also be pollen and ova in plants made in the anther and ovules). - In meiosis, the cell will divide 2 times (called meiosis I and meiosis II), making four genetically different haploid cells called gametes. - It is genetically different from the parent and the other daughter cells. - The first division, each homologous pair goes into each daughter cell. - The second division, each of the chromosomes splits in the middle and that goes into each daughter cell. - Gametes are formed by meiosis; each cell formed has one chromosome from one homogeneous pair and one sex chromosome, making it a haploid cell (23 chromosomes). - Meiosis is only for sexual reproduction. ### How Meiosis Works 1. **DNA has to replicate first.** - Before replication, the chromosome is in a line shape (one chromatid) with one centromere in the middle. - After replication, it is now in an X-shape with 2 chromatids but still with one centromere holding them together (because there is only one centromere in both, both count as one chromosome) 2. **The homologous pair line up on the equator, and spindle fibers now form and attach to the centromere of each chromosome.** - The nucleus envelope first disappears so that the chromosomes can line up. 3. **The chromosomes will be pulled at the centromere by the spindle fibers, separating the homologous pair toward the two poles of the cell.** - Which one of the pair is on the right and left is completely random, so when the cell splits, they are randomly distributed; so each cell gets a different amount of maternal and paternal chromosomes and so the genetic combination in each cell that undergoes meiosis is different. - And when the chromosome undergoes the second division and splits in the arm of the chromosome, all the cells will be genetically different. 4. **New nucleus membrane forms at the two poles, incase the 23 chromosomes, one in each, and the cell then splits down the middle, creating two different cells that only have half the chromosome.** 5. **Then the 23 chromosome of each new cell then also line up on their respective equators, and the spindle fibers form pulling the two chromatids away from each other and toward their two poles.** 6. **Then cells split into two again, creating 4 haploid cells (23 chromosomes) that are all genetically different.** - All gametes that undergo meiosis don’t have the same combination of alleles as well. - When a male and female gametes fuse (fertilisation), the two nuclei will restore the diploid number to form a cell called a zygote. - This cell has all 46 chromosomes in the homologous pairs with 2 sets of genes for each feature. - And each gene contains a set of alleles. - This is why gametes have to be a haploid to make sure when they fuse that the cell they make has exactly 46 chromosomes. ## Sexual Reproduction and Variation - Offspring made by sexual reproduction have genetic variation as they are not genetically identical to their parents but a mix of them. - This is because the DNA of each gamete is different. - Fertilisation can happen at random as any sperm can fertilise any egg. - The same applies to plants as the embryo (inside seeds) made during sexual reproduction will be genetically different to the parents as different pollen grains fertilise different egg cells. - And any individual made will be genetically different from the rest except identical twins. - As identical twins form the same zygote, then the zygote divides by mitosis and the two genetically identical cells formed do not stay together. - Instead, they separate, and each cell acts like its own individual zygote and develops into embryos. - Because they came from genetically identical cells, their zygote and then the embryo are genetically identical. - Non-identical twins come from different zygotes and are not genetically identical. ## Variation - Phenotype is not impacted by genes alone; it’s also impacted by the environment the organism grows up in. - Genetically identical twins may be genetically identical but can look somewhat different like having different height/physical features; this is due to environmental factors. - And plants that have a tall gene may not grow to the same height due to some getting more sunlight and will photosynthesis better; they may not receive the same amount of soil and water or carbon dioxide, and so will not photosynthesis will enough. ## Mutations - A mutation is when there is a change in the base sequence (nitrogen bases) in the DNA where the wrong nucleotide is used, and this has a domino effect because this will change the mRNA that will form, and so that will change the (the wrong tRNA will join) amino acid sequences of the protein change and further change the species genetically too. - This change in the genetic code will change the final protein. - There are three different types of mutations that can happen: 1. **Insertion:** - The nucleotide is inserted twice. - So the last base of the last triplet becomes the first base of the next triplet, and will effect any triplet after the change by adding this new base, the whole code shifts. - Now it will effect the way that the mRNA will be read by the ribosome (as it will now have a different complementary pair) and this will code a different amino acids which will code for a different protein. - So, the genetic code of the organism is then changed. 2. **Deletion:** - A nitrogen base gets removed from the genetic code. - By deleting it and replacing it with the first base of the next triplet, this will make all the code shift backward. - This also affects the way the mRNA of the code will form and be read. - This will also code for a complete different protein. 3. **Substitution:** - This is when nitrogen base gets swapped for another nitrogen base in this the swap is random and only changes the triplet that it is in, the rest of the codes stay the same, and this new triplet may code for another amino acid, and this will cause a change in the protein structure. - This change may result in the alteration to the function or total lack of function. - But this code may not always code for different amino acids, as most amino acids have more than one code. (In this case the function will not change.) - There are four nitrogen base in total and only 3 are read to code for one amino acid, so there is a 4 to the power of 3 =64 different variety of amino acid that can made from the triplet. - But we only have 20 different amino acid options; this is called a redundancy in the code, as we have 64 possibilities but only 20 amino acids. - This is because some of the different triplet can code for the same amino acid, so in this case the ATC code could code the same amino acid as the ATT code. 4. **Inversion:** - This is like substitution, only affecting one of the triplets where a sequence of base is reversed, and this may code for a different amino acid and create a change in the protein structure. ### Mutation Affects - Most mutations don’t really have an affect on the body. This can be because: - If it happens in certain body cells will only affect that particular cell, and if it was harmful the cell will die and the mutation will be lost with it. - The new codes code for the same amino acid (there’s still a mutation in the DNA but the protein is still the same, and so there is no consequent for the cell). - The mutation happens in the recessive allele. - This will not affect the organism as the dominant allele will code for what the organism will look like (eg. brown eye allele). - The mutation happens in a minor section of DNA. - The majority of the DNA are not genes (code for proteins). - This is called junk DNA and is not needed for the formation of a protein, so if a mutation happens there, the organism is not affected. - For mutations that do affect the body, some of them only have a small effect, slightly changing a person’s characteristics, but some mutations can have a significant effect on a person’s phenotype (a set of observable characteristics). - These mutations can be both beneficial and harmful: - An example of a small change is enzymes (because they are a protein), can be mutated, and this can cause the active site that connects to the substrate to be the wrong shape, and so the substrate can no longer fit in, meaning there would be no reaction. - However, if the mutation happens somewhere else on the enzyme (not the active site) the reaction still might work. - A big effect can be something like the cause of cancer to develop. - If a mutation happens in a specific area in the DNA, it can cause uncontrolled cell division (so mitosis will keep happening) and will not stop; this can cause a tumor to develop. - Some beneficial mutations can introduce a new allele into the organism that gives a selective advantage. - For example, a big change to bacteria has made them antibiotic resistant, and so they can withstand the antibiotic and not get killed. - That one bacteria with the resistant survives, its offspring are genetically identical so they all will then have that mutation. (Natural selection) - And if a new allele is introduced, this will increase variation in the organism, as there will be more combinations of DNA possible. ### How mutations are inherited and what factor increase them - Some mutations are inherited or develop stronger due to environmental factors. - If a mutation happens in body cells, the body cell usually dies and it will not get passed on. - However, if the mutation occurs in the sex cell (gametes) it can get passed on, and this is the only way it can get passed on. - This happens by the mutation taking place during the production of gametes (meiosis) or in the gametes. #### Environmental factors that influence mutation - During the replication of DNA mutations can randomly occur; these are called spontaneous mutations, and their frequencies can increase due to two environmental factors: 1. Chemical called mutagens often found in cigarettes smoke and can cause lung cancer. 2. Another form of mutagens is ionising radiation such as gamma ray, X-rays and UV rays that are able to damage DNA as they are so small that they can penetrate your skin and hit DNA in the nucleus and make a mutation occur. ## Genetic Inheritance - Only genetic characteristics that influence the phenotype can be passed down to future generations. - Environmental characteristics cannot be passed on. ### Genes and Alleles - **Genes** code for a protein, and each gene contributes to a particular feature in the body. - Genes have more than one form, which results in alternative forms for each feature; these are called **alleles**. - We have two alleles per feature inherited by parents. - Homologous pairs of chromosomes may carry the same genes in the same place, but have different alleles. - Because the pair of chromosomes contains 2 of the same gene, we can say that a feature needs two genes to code for it. #### Alleles - Alleles can be either **dominant**, shown as a capital letter (Eg. AA) and will shown in the phenotype, whether there is one or 2 of them. Or **recessive**, shown as a lowercase letter (Eg.aa), which requires two of them to be present in each feature. - As you have two alleles, they can either be **heterozygous** (Eg.Aa) or **homozygous** (Eg.Aa). - **Genotype**: the alleles each cell has for a certain feature. - **Phenotype**: the appearance/characteristic of an organism is called it’s phenotype, and it’s the result of both its allele/gene and the environment it grows up in. - **Monohybrid inheritance**: inherited traits that involve one gene. - **Polygenic inheritance**: traits that are controlled by multiple genes that work together, such as skin color. They produce a large range of phenotypes. ### Alleles and Punnett Square - **Gametes** are formed by meiosis and are **haploid**. - The sperm and egg cells contain only half the chromosome (with no pair, only single) as this is because when they fuse (fertilisation), that will form the restored diploid number (46) in the zygote. - Gametes (either sperm or egg cells) contain 1 allele of a feature as they only contain 1 chromosome with 1 gene (with one allele), and no matching pair. - And when it fuses with another, it will receive the two genes/alleles to determine a characteristic. - **Tall pea plants** are caused by a **dominant allele**. - **Short pea plants** are caused by a **recessive allele**. - **Cross** between a true breeding pair, one that has **homozygous dominant** (AA) and **homozygous recessive** (aa) (as this is the only way to get the characteristic of recessive is that you have both). - **Punnett Square**: the offspring of two parents. - **On one side**: if the tall plant (AA) and the other with short plant (aa) and we can see that the offspring each have heterozygous genotype (Tt) This will result in them all being tall as the dominate allele (Tt) will dictate that phenotype, but they will all carry 1 recessive allele. - **In another case**: where the genotype of two plant are two heterozygous, the result will be: - **From this**: We can see that the genotype of the offspring are TT (homozygous dominant), two Tt (heterozygous dominant), and tt (homozygous recessive) - **From this**: we can write a ratio of the genotype: dominant: recessive - 3:1 - **We can write**: a percentage of the phenotype 75% tall and 25% short. - We can see an explanation in the Mendall experiment. ### Mendall Experiment - It talks about how gene and allele are passed down (they are usually through a family tree) - The first generation are called the P generation (they are usually two true breeding pairs, plant that have homozygous allele, where the offspring if would be only one phenotype if bred with the same genotype.) - Transferring the pollen over from one plant to another, not allowing it to self-pollinate. - The F1 generation of a true breeding pair will always look like only one of their parents (the dominant one), and take on their phenotype. - If you bred homozygous recessive and dominant, all the offspring will have a heterozygous genotype. - Sex cells formed by meiosis only have one allele. - So for all the F1 generation, because they are heterozygous, it’s either T or t, that the gametes are carrying. - The F2 generation were allowed to self-pollinate as self-pollination is random, as any pollen grain carries any genotypes which can fertilise an egg cell (fertilisation is random). - The ratio of dominant to recessive was 3:1. - This is because if you mix the Tt genotype of the F1 generation with TT, tt and Tt, you will get a total of 12 square with the different genotypes, where 9 were dominant and 3 were recessive. - We can simplify to the ratio of 3:1. - Remember, that this 3:1 ratio is a predicted ratio (only really met when there are large numbers involved). - In breeding experiments, actual offspring are unlikely to have this result due, not every pollen will fertilise some ova, or some seedlings may die before maturing. ## Sex determination - **Sex determination**: Whether we are female or male is controlled by the present or absence of the Y chromosome. - Females have 44 chromosomes and two X chromosomes (except egg cell) - Males have 44 chromosomes and an X and Y chromosome (except sperm cells). - If we look at the phenotype of both parents, male (genotype XY) and female (genotype XX), then the ratio of the genotype XY to XX is 50%:50%. - This is a predicted genetic ratio that is only really met when there are large numbers involved. ## Co-dominances - Is when two different alleles can be expressed in the phenotype (both contribute to the phenotype). - Genes can show a range of dominance from total to equal. - For example, a red flower and a white flower, some of the offspring being pink colored. - Red flower (RR) white flower(rr) > pink flower (Rr) - If the P generation are both homozygous then that all the F1 generation will be pink as all will have the genotype (Rr) - Note in the second image it is more accurate as the alleles for red and white are both capital as neither allele is dominant over the other as they are both expressed equal in the phenotype. - In the F2 generation where both parents have a heterozygous genotype of the pink RW. - The results are: - There is 1 individual with the RR genotype, 2 with the RW genotype and 1 with the WW genotype. - So the ratio of red:pink:white is 1:2:1. ### Working out genotype- Test cross - You use a test cross to work out an organism’s genotype; we only really need this for dominate phenotype. - We don't know by looking at the organism whether they carry the TT or Tt genotype unlike recessive homozygous. - To work the genotype, you have to control the genotype that we can breed with, because we can only tell recessive homozygous. - The dominant homozygous can only breed with that. 1. **Draw a Punnett square** for both possibilities (TT or Tt) breeding with tt. - From this, you will see that if tt is breeding with TT, the only possibilities for the offspring will have the dominant phenotype as all will be Tt. - If it is Tt with tt, then 50% will have the dominant phenotype and 50% will have the recessive phenotype as the square show 2 Tt and 2 tt. 2. **Breed the two organisms and see which possibility matches with the offspring phenotype** ### Pedigree Tree Diagram - We can use the thinking of the test cross to work out the genotype of individuals in a family to work out if they agree affected by a recessive or dominant genotype disease. - The rule is that everybody affected by the disease is coloured in. - The order from left to right is the birth order and anybody has a line through them then they have died, and squares are males. #### Recessive Allele Disease - For genetic diseases like cystic fibrosis (CF) are carried by recessive homozygous alleles. - Because of this, the genotype is visible from the phenotype - A healthy individual is homozygous dominant or heterozygous (but they will be carried to the disease). - Rules: - Find a generation when both parents are unaffected, and at least 1 child is affected. - From this, you can tell the unaffected parent must be carriers with genotypes Tt, and we already know the genotype of an affected child. - In this case 8 and 9 are carriers with heterozygous genotype (carrying one recessive CF allele) as they have produced an affected child (homozygous recessive) whilst being unaffected. #### Dominant Allele Disease - Polydactyly is a condition when a person develops an extra digit and is determined by a dominant allele. - So anyone with a homozygous recessive allele will have a normal number of digits. - Because of this, anybody with the genotype DD will have the condition with two dominant polydactyly alleles, and anyone with Dd will also have polydactyly (but one dominant polydactyly and recessive normal allele), dd will have a normal number (two recessive normal digit alleles). - From this, we can see individual 1 and 2 are affected by having two unaffected child(homozygous recessive), so they can only be heterozygous. - So from this, we can tell that because both affected individuals are heterozygous and show the condition then the condition is the dominant allele. - We can also tell that anybody that has polydactyly that has parents that have a normal number of digits must be heterozygous. (And so determine the genotype of other individuals). - But for individual 5, 8 and 12, we don’t know if they are homozygous or heterozygous. - As 5 can be either Dd or DD as they are affected either way, and 8 and 12 are affected, and have children that are all affected by people who aren't so could be either DD or Dd. ## Summary of Key Terms - **Gene**: a section of DNA that codes for a protein (we inherit 23 from our mother and 23 from our father). - **Allele**: different version of the same gene (we inherit 2 alleles per feature from our parents). - **Dominant**: Only 1 allele is needed to be expressed in the phenotype - written as a capital letter. - **Recessive**: 2 of the same allele are required to express the characteristic in the phenotype - Eg. aa - **Homozygous**: an organism that has the 2 identical allele of the same gene - Eg. AA - **Heterozygous**: an organism that has the different allele of the same gene - Eg. Aa - **Genotype**: the genes of an organism/cell that is expressed as two letters - **Phenotype**: the visible characteristic of the organism that is included by both the genotype and the environment. - **Codominance**: both alleles are expressed equally in the phenotype. - **Mutations**: a change in the DNA base sequence. ## Natural Selection - **Charles Darwin** noted that there must be competition for resources between species, and many individuals will die. - The ones who were best suited for their environment would survive and pass down their traits. - As organisms generally produce more offspring than needed to replace them, despite this over-reproduction, populations remain stable. - The theory of natural selection is that some factors in an environment select which form of a species survives and reproduces. - There is variation in the offspring of a species. - Changing conditions of an environment (selection pressures) favour one form over another (selection advantage). - The frequency of the favoured form will increase under these conditions as they are more likely to breed and pass on beneficial alleles/genes. - The frequency of the least adapted form will decrease under these conditions. -