6- Inheritance, Variation and Evolution

• Human cells have 23 pairs of chromosomes 

  • In gametes (sex cells) have 23 single chromosomes- they’re created through meiosis
• Sexual Reproduction

  • Requires fusion of male and female gametes (fertilisation)
  • Mixing of genetic info from both male and female parent & lots of variation (genetically unique) 
  • Involves mitosis 
• Asexual reproduction

  • Only one parent, doesn’t involve gametes 
  • Offsprings are genetically identical (clones)
  • Only involves mitosis 
• Meiosis only takes place in reproductive organ 

  1. All the chromosomes are copied (they double) and then the gamete divides in two 
  2. Then both cells divide one more time, in the gametes the chromosomes are now single (halved the number of original chromosomes) 
  3. Meiosis produces four gametes from one original and they’re all genetically different from each other- they have different alleles  
• In sexual reproduction, two gametes (male and female) join together; this is fertilisation. The cell now has the normal number of chromosomes.

  • After fertilisation; the new cell divides by mitosis producing a clump of identical cells- an embryo. Then they differentiate into other cell types, muscles and nerve cells etc 
• Chromosomes contain the molecule DNA (the genetic material) 

  • Consists of two strands and each is a polymer- joined together through lots of small molecules 
  • The two stands wrap around one another, forming a double helix
• Gene: a small section of DNA on a chromosome  

  • Each gene encodes for a specific sequence of amino acids to make a specific protein 
• Genome: entire genetic material of an organism  

  • Helps to search for gene linked to diseases
  • Help and understand treat inherited disorders 
  • Trace human migration patterns from the past 
• Alleles: versions of a gene 
• Genotype: tells us the alleles present
• Homozygous: two copies of the same allele
• Phenotype: characteristics caused by the allele
• Heterozygous: different alleles for a particular trait
• Recessive: will only show in the phenotype if two copies are present
• Cystic fibrosis: a disorder of the cell membranes- controlled by a single genes which has two alleles 

  • In order to have the disorder the person has to have two copies of the defective allele (have to have defective allele from both parents) 
  • If they have one of the defective alleles they’re just a carrier 
• Polydactyly

  • Have extra fingers and toes and caused by a dominant allele (with one copy of the polydactyly allele you will have it) 
  •  If you have a dominant allele, you will have the characteristic 
• Embryo screening: testing to see if they have alleles for inherited disorders 

  • Expensive: that money should be spent elsewhere in the health services 
  • Large number of embryos are created but a small number implanted: lots of healthy embryos are destroyed and that's unethical 
  • In the future, may be able to screen embryos for desirable features (such as more intelligent/taller) and people see this as unethical 

  In the future: scientist may use this to correct faulty allele and use that to treat inherited disorders (gene therapy) 
• Male sex chromosome:: X/Y, Female sex chromosome: X/X

  
• Variation: the difference is characteristics of individuals in a population 

  • Alleles; genetic causes - natural hair colour, eyes 
  • Environment - language, hair colour  
  • Genes and environment - height, controlled by genes and diet 
• Mutation: random changes to DNA- most have no effect on the phenotype, some may influence phenotype, eg: height- tallness is achieved by diet and allele 

  • Rare that mutation can lead to new phenotype, can be beneficial  if environment changes, this can lead to change in species (eg: resistance to viruses) 
• Life developed on earth more that 3 billion years ago in forms of single cells and everything has evolved from this cell 

  • Evolution by Natural Selection: a process where organisms that are better adapted to an environment will survive and reproduce
• • Evolution: the change in the inherited characteristics of a population over time through the process of natural selection 

  ! two populations of one species can become so different in phenotype that they can no longer interbreed to produce offspring they have now become two separate species
• Example of selective breeding/why

  • dogs=gentle nature
  • food crops= disease resistance
  • cows=more meat/milk
  • plants= large or unusual flowers 
• How is it selective breeding carried out

  1. Choose parents that show the desired characteristics from a mixed population.
  2. Breed these parents. Choose the best offspring, with the desired characteristics, to produce the next generation.
  3. Repeat the process continuously over many generations, until all offspring show the desired characteristics
• Problem with Selective Breeding

  • Breeding together closely related animals/plants is inbreeding; causes some breeds to be prone to disease of inherited defects  
• genetic engineering: Genes from one organism are cut out and transferred to the cell of a different organism.

  • The genome is modified
• Insulin - Genetic Engineering example

  • Insulin is a hormone which regulates blood glucose levels in humans, type 1 diabetes cannot make insulin- so their inject themselves. A bacteria has been modified to produce human insulin
• Genetically Modified Crops 

  • Produces more yield than normal crops, make them resistant to disease or insect accounts, bigger and better fruits, and resistance to herbicides (farmers can spray crops to kill weeds without harming GM crops) 
  • May be harmful to flowers and insects, as well as eating for humans 
• Gene Therapy: the introduction of normal genes into cells in place of missing or defective ones in order to correct genetic disorders.

  • Long term effects are not known 
• Genetic engineering steps: 

  1. Identify gene you want to transfer, use enzymes to isolate the gene 
  2. Transfer the gene into a small circle of DNA called a plasmid (come from bacteria: useful for transferring DNA)  // could use virus too - These are vectors: they transfer the DNA from one organism to another
  3. Desired gene is transferred into the cells of the target organism (animal, plant or microorganism)
• genetic engineering-

  ! always transfer the gene at an early stage in the organism's development (early embryo stage)

  • This makes sure ALL the cells receive the transferred gene so the organism develops the characteristic we want
• Fossils: remains of organisms from millions of years ago which are found in rocks
• How fossils form

  • Parts of organisms have not decayed- when conditions for decay are not present eg: too cold, not enough oxygen, not enough water
  • parts of organisms are replaced by minerals 
  • Preserved traces of organism eg: footprints, plants: spaces where roots were (early forms were) Soft bodied organisms rarely produced fossils- no shell or skeleton  
  • Have been destroyed by changes to rocks in the earth's crust - due to this scientists cannot be certain how life on earth began 
• Extinct: no remaining individuals of a species is still alive 

  • Due to a catastrophic event eg: asteroid colliding wit the earth 
  • Environment changes- changing weather pattern
  • New disease or predator (kills em all)
  • New more successful species evolves and competes with it (for scarce, food or water)
• Bacteria can reproduce every thirty minutes- they evolve rapidly 

  • Antibiotics kill bacteria
• How antibiotic becomes resistance

  • Mutation could occur to bacterium and make it resistant to antibiotics, when an antibiotic is used all bacteria's are killed apart from the resistant one 
  • The antibiotic resistant strain survives and reproduces without any competition from other bacteria and over time the population or resistant strain spreads (people are not immune to it and there's no effective treatment)
• Ways to reduce antibiotic strain

  1. Doctors shouldnt prescribe antibiotics inappropriately (eg on viruses)
  2. Complete the course antibiotics- makes sure all bacterias are killed and none survive to mutate to form a resistant strain
  3. Restrict use of antibiotics in farming 
• New antibiotics take a long time and are expensive, new resistant strains are forming and its unlikely they’ll keep up.
• • Two kingdoms: animal and plant kingdom 

  Named by their Genus & Species (their binomial name)

  
• Classification - Linnaeus system

  1. kingdom
  2. phylum
  3. class
  4. order
  5. family
  6. genus
  7. species