Topic 6 - Inheritance, Variation and Evolution

• Sexual reproduction = the fusion of male and female gametes because there are two parents. The offspring contains a mixture of their parents' genes
• Asexual reproduction: there's only one parent involved. There's no fusion of gametes, no mixing of chromosomes and no genetic variation. The offspring is genetically identical to the parent.
• Sex cells:

  • animals - sperm (male) / egg (female)
  • plants - pollen (male) / egg (female)
• Gametes join at fertilisation to restore the normal number of chromosomes
• Sexual reproduction:

  Advantages = the variation increases the chance of a species surviving a change in the environment.

  Disadvantages = it takes longer than asexual reproduction
• Asexual reproduction:

  Advantages = there's only one parent. It uses less energy and it is a lot faster.

  Disadvantages = higher risk for the species, possibly chance they could all die if there's a change in the environment
• Some orgnanisms reproduce using asexual and sexual reproduction depending on the circumstances
• Gene = a section of DNA that codes for a specific protein
• Genome = the entire set of genetic material in an organism
• Why is it important to understand the human genome?

  So it helps to improve medicine in the future
• DNA has 4 bases - A,C,G,T
• DNA polymer = a long chain of nucleotides joined together
• A change in DNA structure results in a change in the protein synthesised.
• Proteins are synthesised on ribosomes.
• When the protein chain is complete, it folds up to form a unique shape which enables the proteins to do their job as enzymes, hormones or forming structure.
• Mutation: a change in the base sequence of DNA

  • they occur continuously and most do not alter the protein.
• Not all parts of DNA code for proteins. Non-coding parts of DNA can switch genes on and off , so variations in these areas of DNA may affect how genes are expressed.
• Genotype - the alleles you have

  Phenotype - the expression of the alleles you have (what you look like)
• What are dominant and recessive alleles?

  Dominant alleles are alleles which are always expressed even if only one copy is present (usually capital letters)

  Recessive alleles are alleles which are only expressed if there's two copies which are present (so there shouldn't be a dominant allele)
• Homozygous - two alleles for the same gene are the same

  Heterozygous - two alleles for a gene are different
• most characteristics are a result of multiple genes interacting
• What's an inherited disorder caused by a dominant allele?

  Polydactyly - this is caused by a dominant allele and it is where you have extra fingers and toes.
• What's an inherited disorder caused by a recessive allele?

  Cystic fibrosis - caused by a single recessive allele, so any homozygous individuals will develop the disease. It's a diseases of the cell membrane and it affects the respiratory and digestive systems by producing abnormally thick mucus
• There are 23 pairs of chromosomes in an ordinary human body (so 46 chromosomes)
• What are the pairs of chromosomes that carries the genes that determine sex?

  Female - XX

  Male - XY
• Single gene inheritance - where each parents passes on one allele to their offspring. The combination of the alleles defines the genotype and phenotype of the offspring
• What is variation?

  Variation is where organisms of the same species have differences. This can be physical differences, behaviour and genes etc.
• Causes of variation:

  • genetic variation - where the variation is controlled entirely by genes.
  • environmental variation - where the variation is caused by the environment and where the species lives
• How does evolution occur through natural selection?

  All living things are different and therefore adapted to different things. The species who are more likely to survive than others (maybe because they are adapted to a habitat or environment) will breed and pass on their characteristics which helped them to survive
• What is selective breeding?

  • it is the process by which humans breed plants and animals for particular genetic characteristics.
• Chosen characteristics for selective breeding include disease crop resistance, crops which produce higher yields etc.
• problems with interbreeding could include a higher risk of disease
• Describe genetic engineering

  • it's a process to modify the genome of an organism by introducing the gene from another organism to give desired characteristics
• Examples of genetic engineering: bacteria has been modified for human insulin for diabetes / genetically modified crops have had their genes modified
• Define a GM crop and give examples

  A genetically modified crop is a crop which has had DNA inserted into the genome of the organism. So, it's given extra genes for new and useful characteristics like:

  • pest resistance
  • frost resistance
  • disease resistance
• Farmers use GM crops to help increase the yield of the crops so there is more produce to sell
• Concerns about GM crops:

  • people aren't sure whether they are beneficial or safe towards the human health.
  • the transplanted genes may get out to the natural environment
  • growing GM crops will affect the number of wild flowers and insects which live around the crops.
• Genetic modification can help overcome some inherited diseases as you could insert working genes into people which have the disease (gene therapy)
• Plant cloning : tissue culture (to preserve rare plant species)

  • tissue sample scraped from parent plant, tissue samples placed in agar growth medium containing nutrients and auxins, samples develop into tiny plantlets then the plantlets are planted into teh compost
• Plant cloning : taking cuttings

  • this is a quick way of cloning plants, mainly used by gardeners.
  • however, it can't produce many plants at once.
  • stem cut from plant, cutting dipped in rooting powder then watered and planted