B5

• The genome describes all the genetic information of an organism
• Phenotype


  The physical characteristics that are observed in the individual, e.g. eye colour
• Genotype


  The combination of alleles an individual has, e.g. Aa
• Gamete


  An organism’s reproductive cell (egg in female and sperm in males), which has half the number of chromosomes (23)
• Some characteristics are controlled by a single gene, such as fur colour in mice and red-green colour blindness in humans. However, most characteristics are the result of many different genes interacting
• Chromosome

  A structure found in the nucleus which is made up of a long strand of DNA
• Dominant allele


  Only one (out of the two alleles) is needed for it to be expressed and for the corresponding phenotype to be observed
• Recessive allele


  Two copies are needed for it to be expressed and for the corresponding phenotype to be observed
• Homozygous


  When both inherited alleles are the same (i.e. two dominant alleles or two recessive alleles)
• Studying the human genome has improved understanding of genes linked to different types of disease, the treatment of inherited disorders, and tracing human migration patterns from the past
• Heterozygous


  When one of the inherited alleles is dominant and the other is recessive
• Gene


  A short section of DNA that codes for a protein, and therefore contributes to a characteristic
• If a mutation occurs in a coding region of DNA, it can alter the activity of the protein that the altered area of DNA is meant to code for by changing the protein structure
• The genetic information and its interaction with the environment influence how genetic traits are presented, i.e. the phenotype. This can be with continuous variation, such as height, or in discontinuous variation, such as with eye colour
• If a mutation is in a non-coding region of DNA, it can affect how genes are expressed by stopping transcription of mRNA in the protein synthesis process
• Protein synthesis

  DNA contains the genetic code for making a protein, but it cannot move out of the nucleus as it is too big. 2. The two strands pull apart from each other, and mRNA nucleotides (messenger RNA: a different type of nucleotide) match to their complementary base on the strand. 3. The mRNA nucleotides themselves are then joined together, creating a new strand called the mRNA strand. This is a template of the original DNA. 4. The mRNA then moves out of the nucleus to the cytoplasm and onto structures called ribosomes. 5. At the ribosomes, the bases on the mRNA are read in threes to code for an amino acid (the first three bases code for one amino acid, the second three bases code for another etc). 6. The corresponding amino acids are brought to the ribosomes by carrier molecules. 7. These amino acids connect together to form a protein. 8. When the chain is complete, the protein folds to form a unique 3D structure
• Asexual and sexual reproduction are discussed in Biology (5.1f)
• Allele/variant


  The different forms of the gene - humans have two alleles for each gene as they inherit one from each parent
• All variants (alleles) are caused by mutation. Most mutations have no effect on the phenotype, as most of DNA is non-coding and does not cause a change in any proteins. However, some mutations have a small influence on phenotype and very few can completely change the phenotype if they are in coding regions
• A single gene cross looks at the probability of the offspring of two parents having certain genotypes and phenotypes
• Human body cells have 23 pairs of chromosomes, with the 23rd pair carrying sex determining genes
• Most phenotypic features are the result of multiple different genes interacting rather than a single gene inheritance
• Advantages of sexual reproduction

  • Produces variation in offspring
  • Allows us to use selective breeding
  • Mixes genetic information from two organisms
  • Speeds up natural selection
• Meiosis

  1. The formation of four non-identical cells from one cell

  2. Cells in the reproductive organs divide by meiosis to form gametes

  3. Gametes only have one copy of each chromosome

  4. The cell makes copies of its chromosomes, so it has double the amount of genetic information and is called diploid

  5. The cell divides into two cells, each with half the amount of chromosomes, giving the normal amount of 46 chromosomes

  6. Each cell divides into two again to produce four cells, each with 23 chromosomes, called haploid

  7. These cells are called gametes and they are all genetically different from each other because the chromosomes are shuffled during the process, resulting in random chromosomes ending up in each of the four cells
• There is a 50% chance of babies being born as either a boy or a girl using a Punnett square
• Gregor Mendel worked in the monastery gardens and observed the characteristics passed on to the next generations in plants. He carried out breeding experiments on pea plants
• Uppercase letters represent dominant characteristics, and lowercase letters represent recessive characteristics in Punnett square diagrams
• Advantages of asexual reproduction

  • Only one parent is needed
  • Uses less energy and is faster
  • In favorable conditions, lots of identical offspring can be produced
• Advantages of sexual reproduction are the disadvantages of asexual reproduction and vice versa
• Protein synthesis

  1. Amino acids are brought to the ribosomes by carrier molecules

  2. Amino acids connect together to form a protein

  3. When the chain is complete, the protein folds to form a unique 3D structure
• Females have two X chromosomes and pass on X chromosomes in their eggs, while males have one X chromosome and one Y chromosome and can pass on X or Y chromosomes in their sperm
• Mendel was not recognised till after his death as genes and chromosomes were not yet discovered, so people could not understand his work
• Development of genetics by Mendel

  • Gregor Mendel worked in the monastery gardens and observed the characteristics passed on to the next generations in plants
  • He carried out breeding experiments on pea plants, using smooth peas, wrinkled peas, green peas, and yellow peas and observed the offspring to see which characteristics they had inherited
  • Offspring have some characteristics that their parents have because they inherit ‘hereditary units’ from each
  • One unit is received from each parent
  • Units can be dominant or recessive, and cannot be mixed together
• One population of a species may become so different that they can no longer interbreed to produce fertile offspring. This means they have become a new species. This is called speciation
• In the 20th century, it was understood that chromosomes and units had similar behaviours. It was decided that units (now known as genes) were on the chromosomes
• Natural selection and evolution

  • Evolution is a change in the inherited characteristics of a population over time through a process of natural selection which may result in the formation of a new species
  • All species have evolved from simple life forms that first developed more than three billion years ago
  • Evolution occurs because of natural selection
  • Mutations occur which provide variation between organisms
  • If a mutation provides a survival advantage the organism is more likely to survive to breeding age
  • The mutation will then be passed onto offspring
  • Over many generations, the frequency of the mutation will increase within the population
• Classification systems

  • Artificial classification is when classification is based purely on observations
  • Carl Linnaeus invented the The Linnaean system. Living things were divided into kingdoms, phylum, class, order, family, genus, species
  • Linnaeus’ system was based on human judgement
  • Natural classification is where molecular techniques are used to see similarities between species, such as DNA sequencing to compare protein structures
  • Phylogenetics is the study of how closely related two organisms are, allowing us to see which species originated from what
  • Carl Woese added three large groups called domains above kingdoms by using molecular techniques: Archaea, Bacteria, Eukaryota
  • The binomial system gives each organism a name which is used worldwide. The first part is their genus and the second part is their species
• Within a population, there is usually a large amount of genetic variation between individuals of the same species. This natural variation occurs through small mutations that have occurred throughout time
• Evidence for Evolution
• In the late 19th century chromosomes as a part of cell division were observed