Edexcel GCSE Biology: Topic 3 Genetics

• B3.1 Explain some of the advantages and disadvantages of asexual reproduction.

  Advantages:

  -Can produce lots of offspring very quickly because the reproductive cycle is so fast. This can allow organisms to colonise a new area very rapidly.

  -Only one parent is needed which means organisms can reproduce when conditions are favourable without having to wait for a mate.

  Disadvantage:

  -No genetic variation between offspring. So if the environment changes and conditions become unfavourable the whole population could be affected.

  e.g. if no plants are resilient to a disease they'll all get it
• B3.2 Explain some of the advantages and disadvantages of sexual reproduction.


  Advantages:

  -sexual reproduction creates genetic variations within the population which means different individuals have different characteristics. This means that if the environmental conditions change it's more likely that at least some will have the characteristics to survive - over time this leads to natural selection.

  Disadvantages:

  -sexual reproduction takes more time and energy so produce less offspring in their lifetime. Organisms need time and energy to find a mate.

  -Two parents are needed for sexual reproduction which is a problem if individuals are isolated.
• Define:

  Gamete, Zygote, Fertilisation, Haploid, Diploid


  gamete - cell with half the normal number of chromosomes, and only used for sexual reproduction

  zygote - cell formed when two gametes combine

  fertilisation - term to describe the joining of two gametes

  haploid - having half the normal number of chromosomes

  diploid - having the normal number of chromosomes
• B3.3 Explain the role of meiotic cell division. The stages of meiosis are not required.

  Meiosis is a type of cell division that doesn't produce identical cells. It produces gametes

  The gamete-making cell has two sets of chromosomes. It is diploid. The chromosomes replicate (and the copies stick to one another). The cell divides into two and then two again. Each of the final four daughter cells has half the number of chromosomes - this results in genetically different haploid gametes
• Define gene and genotype.

  The genome is the entire DNA of an organism and most cells contain a complete copy of an organism's genome.

  
  

  A gene is a section of a DNA molecule that codes for amino acids
• B3.4 Describe DNA.

  1) Two strands coiled to form a double helix

  2) Strands linked by a series of complimentary base pairs joined together by weak hydrogen bonds

  3) Nucleotides that consist of a sugar and phosphate group with one of the four different bases attacher to the sugar
• B3.6 Investigate how to extract DNA from fruit

  - Mash strawberries and put in a beaker containing a solution of salt and detergent and mix.

  - The detergent breaks down the cell membranes, the salt makes the DNA stick together.

  - Filter the mixture to get froth and big insoluble bits out.

  - Add cool alcohol to filtered mixture.

  - DNA will start to come out of solution as it is not soluble in cold alcohol. It will appear as a white stringy precipitate.

  - Fish it out - this is the DNA.
• H B3.7 Explain how the order of bases in a section of DNA decides the order of amino acids in the protein and that these fold to produce specifically shaped proteins such as enzymes.


  To enable genes to code for proteins, the bases A, T, G and C get together not in pairs but in triplets.

  Each protein is made up of large numbers of amino acid molecules.

  Each triplet of bases codes for one particular amino acid.

  Amino acids are made in the number and order dictated by the number and order of base triplets.

  Finally, the amino acid molecules join together in a long chain to make a protein molecule. The number and sequence of amino acids determines which protein results.
• H B3.8 Describe the stages of protein synthesis, including transcription and translation:

  a RNA polymerase binds to non-coding DNA located in front of a gene

  b RNA polymerase produces a complementary mRNA strand from the coding DNA of the gene

  c the attachment of the mRNA to the ribosome

  d the coding by triplets of bases (codons) in the mRNA for specific amino acids

  e the transfer of amino acids to the ribosome by tRNA

  f the linking of amino acids to form polypeptides.


  Transcription:

  RNA polymerase binds to non-coding section of DNA located in the from of the gene. This moves along the DNA strand and creates a complimentary mRNA strand which genetically matches the strand of DNA - NEVER THYMINE produces Uracil. mRNA molecule moves out of the nucleus and joins with ribosomes

  Translation:

  Once the mRNA is bound to the ribosome, the protein can be assembled. tRNA (transfer RNA) transfers amino acids to the ribosomes. Every 3 bases on mRNA is called a codon - tRNA has the anticodon which connects the amino acid. The ribosome moves along the mRNA and tRNA brings specific amino acid to match the codon in the correct order. The amino acids are joined together by the ribosome. This makes a polypeptide (protein)
• H B3.9 Describe how genetic variants in the non-coding DNA of a gene can affect phenotype by influencing the binding of RNA polymerase and altering the quantity of protein produced.

  If the RNA polymerase don't bind properly, it can effect how much mRNA is transcribed and therefore how much protein is produced. Depending on the proteins function, the phenotype of the organism may be affected by how much is made. Genetic variants in non-coding regions can still be affect the phenotype of an organism, even if they don't code for proteins themselves
• H B3.10 Describe how genetic variants in the coding DNA of a gene can affect phenotype by altering the sequence of amino acids and therefore the activity of the protein produced.

  RNA polymerase attaches to DNA bases in front of a gene. A mutation in this non-coding region may result in RNA polymerase not binding well, reduces transcription

  WTF ASK
• B3.11 Describe the work of Mendel in discovering the basis of genetics and recognise the difficulties of understanding inheritance before the mechanism was discovered.


  Mendel crossed tall pea plants and dwarf pea plants - offspring produced were all tall pea plants. Then bred two tall off spring together. He found that when offspring from the first cross where crossed together, three tall offspring and one dwarf. Produced a 3:1 ratio of tall:dwarf plants.
• B3.11 What had Mendel showed?

  Height characteristics in pea plants were determined by separately inherited "hereditary units" passed on from each parent. The ratios of tall and dwarf in the offspring shows that the unit for tall plants, T, was dominant over the unit for dwarf plants, t.
• B3.11 Mendel's three important conclusions

  1) characteristics in plants are determined by "hereditary units"

  2) Hereditary units are passed onto offspring unchanged from both parents, one unit from each parent

  3) Hereditary units can be dominant or recessive - if an individual has both the dominant and the recessive unit for a characteristic, the dominant character is expressed.
• B3.11 Recognise the difficulties of understanding inheritance before the mechanism was discovered.

  At the time Mendel's work was cutting edge and new. Now we know that hereditary units are genes but at the time scientists didn't have the background knowledge to properly understand Mendel's findings - no idea about DNA, genes, chromosomes.

  It wasn't until after his death that people realised how significant his work was and that mechanism of inheritance could be fully explained.
• B3.12 Explain why these are differences in the inherited characteristics as a result of alleles

  Genes for the same characteristic can contain slightly different instructions that create variations. Different forms of the same gene are called alleles. There are two copied if every chromosome in a body cell nucleus, a body contains two copies of every gene. Each copy of a gene may be a different allele. There are many alleles and the different combinations of alleles in each person gives each of us slightly different characteristics.
• B3.13 Explain the terms: dominant, recessive,

  Dominant - dominant alleles overrule recessive alleles (represented as a capital letter e.g. D). If alleles are DD or Dd then dominant characteristics will show

  Recessive - represented with lower case letter e.g. d. Recessive traits will only shown if both alleles are recessive "dd".
• B3.13 Explain the terms: homozygous, heterozygous,

  If both alleles are the same, an organism is homozygous for that gene. e.g DD or dd.

  If alleles are different, an organism is hetrozygous for that gene e.g Dd .
• B3.13 Explain the terms: genotype, phenotype and zygote.

  The alleles in an organism are its phenotype. What the organism looks like are its genotype.

  A zygote is a fertilised egg cell
• B3.14 Explain monohybrid inheritance using genetic diagrams.

  The inheritance of a single characteristic is called monohybrid inheritance. A monohybrid cross can be used to show how recessive and dominant traits for a single characteristic are inherited
• B3.14 Explain monohybrid inheritance using Punnett squares and family pedigrees.

  Punnet squares are used to work out the theoretical probability of offspring inheriting certain genotypes.

  Punnet squares are used to work out the probabilities of different phenotypes caused by alleles.

  A family pedigree chart shows how genotypes and their resulting phenotypes are inherited in families
• B3.15 Describe how the sex of offspring is determined at fertilisation, using genetic diagrams.

  Men have X and Y chromosomes. Women have two X chromosomes. XX combination allows female characteristics to develop. Equal chances of having boy and girl.
• B3.16 Calculate and analyse outcomes (using probabilities, ratios and percentages) from monohybrid crosses and pedigree analysis or dominant and recessive traits.
• B3.17 Describe the inheritance of the ABO blood groups with reference to codominance and multiple alleles.

  There are four blood types - O, A, B and AB. The gene for blood type in humans has three different alleles - I^O, I^A and I^B.

  A person with the genotype I^AI^B shows the effect of both alleles and has the blood type AB. They are both dominant alleles so I^A and I^B are codominant with each other.

  I^O is recessive so bloody type O only happens when there are two recessive alleles (I^OI^O)
• H B3.18 Explain how sex-linked genetic disorders are inherited - male

  Chromosomes in diploid cells come in pairs. Most pairs, the chromosomes have the same genes. However Y chromosomes miss some genes found in the x chromosome. This means a man (XY) will have only one allele for some genes on the X chromosome (because those genes are missing on the Y chromosome). If the allele for on of these X chromosome genes causes a genetic disorder, the man will develop this.

  If a woman inherits the disorder allele, she may have a healthy allele on her other X chromosome. If the disorder allele is recessive she will not get the disorder. If she inherits two recessive disorder alleles she will develop the disorder. Probability of a woman developing it is lower a man.
• B3.19 State that most phenotypic features are the result of multiple genes rather than single gene inheritance.

  Most phenotypic features are the result of multiple genes rather than single gene inheritance.
• B3.20 Describe the causes of variation that influence phenotype including: (a) genetic variation - different characteristics as a result of mutation and sexual reproduction.

  Genetic variation:

  Genetic variation is caused by the different alleles inherited during sexual reproduction.

  Mutations are changes to the base sequence of DNA. When they occur within a gene they result in an allele, or a different version of a gene.
• Human Genome project

  Researchers managed to map over 20, 000 human genes. The big idea was to find every single human gene.
• B3.21 Discuss the outcomes of the Human Genome Project and its potential applications within medicine.

  Mapping a person's genome can indicate their risk of developing diseases that are caused by different alleles of genes. It can also help identify which medicines might be best to treat a person's illness, because the alleles we have can affect how medicines work in the body.

  -Predict and prevent diseases.

  -testing and treatment of diseases

  -new and better medicines.

  However: increase stress, gene-ism and discrimination
• B3.22 State that there is usually extensive genetic variation within a population of a species and that these arise through mutations

  There is usually extensive genetic variation within a population of a species and that these arise through mutations
• B3.23 State that most genetic mutations have no effect on the phenotype, some mutations have a small effect on the phenotype and, rarely, a single mutation will significantly affect the phenotype.

  Most genetic mutations have no effect on the phenotype.

  Some mutations have a small effect on the phenotype.

  Rarely, a single mutation will significantly affect the phenotype.
• B3.20 Describe the causes of variation that influence phenotype including:

  (b) environmental variation - different characteristics caused by an organism's environment (acquired characteristics)

  Variation caused by the surroundings is called environmental variation.

  Characteristics that are changed by their environment during the life of the individual are called acquired characteristics.
• B3.20 Describe the causes of variation that influence phenotype including: (a) genetic variation - different characteristics as a result of mutation and sexual reproduction (b) environmental variation - different characteristics caused by an organism's environment (acquired characteristics)

  Most variation in a phenotype is determined by a mixture of genetic and environmental factors.