Inheritance, variation and evolution

• Mitosis vs meiosis 


  Mitosis is the formation of two genetically identical daughter cells but Meiosis is the formation of four non genetically identical daughter cells

  Meiosis is used in sexual reproduction but mitosis is used in asexual reproduction
• Sexual reproduction 


  Joining of male and female gametes, each containing genetic information from the mother and father

  Gametes are formed by meiosis as they are non identical

  A normal cell has 46 chromosomes. These are two sets of chromosomes. In each pair one chromone is from the mother and the second the father

  Each gamete has 23 chromosomes and they fuse in fertilisation

  The genetic information from each parent is mixed producing variation in the offspring
• Asexual reproduction 


  Involves one parent with no gametes joining

  Happens using the process of mitosis where two identical cells are formed from one cell

  There is no mixing of genetic information

  It leads to clones, which are genetically identical to each other and the parent

  Examples of organisms that reproduce this way are bacteria, some plants and some animals
• Meiosis


  The cell makes copies of its chromosomes so it has double the amount of genetic information

  The cell m copies of its chromosomes so it has double the amount of genetic information

  The cell divides into two cells each with half the amount of chromosomes (46)

  The cell divides again producing four cells each with a quarter the amount of chromosomes (23)

  These are called gametes as they are all genetically different from each other because the chromosomes are shuffled during the process resulting in random chromosomes ending up in each cell
• Meiosis and mitosis 


  These gametes with 23 chromosomes join at fertilisation to produce a cell with 46 chromosomes, the normal number

  This cell divides by mitosis to produce many copies

  More and more cells are produced, and an embryo forms

  The cells begin to take on different roles after this stage (differentiation)
• Advantages of Asexual reproduction (mitosis) 


  Only one parent is needed

  Uses less energy and is faster as organisms do not need to find a mate

  In favourable conditions lots of identical offspring can be produced
• Advantages of sexual reproduction

  Produces variation in offspring

  This means if the environment changes likely an organism in a species will have the characteristics to survive

  Some individuals may die, unlikely whole species become extinct

  Allows us to use selective breeding

  Organisms with desirable characteristics can be bred to produce offspring with desirable characteristics

  Speeds up natural selection
• DNA 


  DNA is a polymer made up of two strands which wrap around each other like a rope - in a structure called a double helix. The DNA in the nucleus is contained in structures called chromosomes

  Between the two strands are the four nitrogenous bases lined up in single rows - these come together to form a series of complementary pairs
• Gene 


  A small section of DNA on a chromosome - a triplet of bases that codes for a specific protein. Each gene codes for a particular sequence of amino acids, together a chain of amino acids can join to make a protein
• Genome 


  all the genes coding for all of the proteins within an organism. The whole human genome has now been studied and this has improved our understanding of the genes linked to different types of diseases, the treatment of inherited disorders has helped in tracing human migration patterns from the past
• Gamete 


  An organisms reproductive cell which has half the number of chromosomes
• Chromosome 


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


  A short section of DNA that codes for a protein, and therefore contributes to a characteristic

  Some Characteristics are controlled by a single gene however most characteristics are the result of many genes interacting
• Alleles 


  The different forms of the gene - humans have two alleles for each gene as they inherit one from each parent
• Dominant allele 


  Only one 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 allels are the same
• Heterozygous 


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


  The combination of alleles an individual has
• Phenotype


  The physical characteristics observed in the individual
• Punnet squares 


  Uppercase letters are used to represent dominant characteristics. Lowercase letter represent recessive characteristics. You can choose any letter but usually A or B is used for simplicity
• Polydactyly


  Having extra fingers or toes

  Caused by dominant allele
• Cystic Fibrosis 


  Disorder of cell membranes, resulting in thick mucus in the airways and pancreas

  Caused by a recessive allele

  Both parents need to either be carriers of have CF and the other is a carrier
• Embryonic screening 


  Allows scientists to see if a child will have a genetic condition or not

  If the embryo is developed in the lab, cells can be taken from it and analysed

  DAN from embryos in the womb can also be extracted
• Arguments for embryonic screening

  Reduces number of people suffering

  Treating disorders is very expensive

  Many regulations to stop it getting out of hand
• Arguments against embryonic screening

  Could encourage people to pick characteristics - creating designer babies

  Expensive to carry out

  May promote prejudice

  Can lead to a miscarriage

  Decisions must be made about terminating a pregnancy
• Sex determination 


  Human bodies have 23 pairs of chromosomes

  22 control characteristics, and the chromosomes in each pair look similar

  23rd has sex-determining genes and they can look different

  Females have two X chromosomes therefore only pass on X

  Males have one X and one Y therefore can pass on only X and Y
• Sex Punnet square

  
• The Phenotype of an organism depends on two things

  Genotype - the genes it inherits

  Environment - the place it lives in
• Sometimes characteristics can result from a combination of genetics and the environment such as weight. Weight depends on the food you eat but also how quickly your body breaks it down and how much it stores as fat depends on your genes
• Genetic mutation is introduced by mutations in the sequence of DNA


  Most have no effect on phenotype

  Some will influence phenotype but it is unlikely that it will bring about a new phenotype

  If the mutation does determine the phenotype and it is advantageous, natural selection will mean it becomes the common phenotype relitavley quickly
• 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
• Theory of evolution 


  All species have evolved from simple forms of life that 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 ago

  The mutation will then be passed on to offspring

  Over many generations, the frequency of the mutation will increase within the population

  This may cause one population of species to become so different they can no longer interbreed to produce fertile offspring, which means they are the new species, this is called speciation
• Selective breeding 


  When humans choose which organisms to breed in order to produce offspring with certain desirable characteristics
• Selective breeding methods 


  Parents with desired characteristics chosen

  Bred together

  Offspring with desired characteristics bred together

  Process repeated many times till all offspring have desired characteristics
• The problem of inbreeding - selective breeding

  Breeding those with similar desirable characteristics means it is likely you are breeding closely related individuals

  This result in reduction of the gene pool as the number of different alleles reduce

  This means if the environment changes, or if there is a new diseases, the species could become extinct as they all have the same genetic makeup

  A small gene pool also leads to a greater chance of genetic defects as recessive characteristics are more likely to be present
• Genetic engineering 


  modifying the genome of an organism by introducing a gene from another organism to give the desired characteristic

  Plant cells have been engineered for disease resist or to have larger fruits

  Bacterial cells have been engineered to produce substances useful to humans, such as human insulin o treat diabetes
• The process of genetic engineering

  1. Genes from chromosomes are 'cut out' using restriction enzymes leaving 'sticky ends'

  2. A virus or bacterial plasmid is cut using the same restriction enzyme to also create sticky ends. this also contains an antibiotic marker geen

  3. The loop and geen stick ends are then joined together by DNAA ligase enzymes

  4. The combined loop is placed ina vector, such as a bacterial cell and then allowed to multiply as it will now contain the modified gene. as the bacteria grow we can see which ones are antibiotic-resistant. THe colonies that are will produce the modified gene

  5. in plants the vector is put into meristematic cells which caan the produce iden copies of the modified plant
• Genetically modified crops 


  These are engineered to be resistant to insects and herbicides

  This will result in increase yields so less crops die