b6 - inheritance, variation and evolution

• what is sexual reproduction?

  -production of gametes by meiosis

  -gamete from each parent fuses to make a zygote
• what are gametes?

  -sex cells

  -haploid (half the number of chromosomes)
• what is meiosis?

  -forms non-identical cells

  -chromosome number is halved

  -involves 2 divisions (4 daughter cells)
• what must occur prior to meiosis?

  • interphase : copies of genetic material are made during this process
• what happens during the first stage of meiosis?

  • chromosomes pulled apart & so chromosome number is halved
• what happens during the second stage of meiosis?

  • four unique haploid gametes are produced
• why is meiosis important for sexual reproduction?

  • increases genetic variation
  • ensures that zygote formed at fertilisation is diploid
• describe fertilisation & its outcome

  • gametes join together to restore normal no. of chromosomes
  • cell then divides by mitosis
  • as embryo develops, cells differentiate
• describe circumstances in which malarial parasites reproduce sexually & asexually

  • sexual in the mosquito
  • asexual in the human host
• describe circumstances in which fungi reproduce sexually & asexually

  • asexual by spores
  • sexual to give variation
• describe circumstances in which plants reproduce sexually & asexually

  • sexual to produce seeds
  • asexual by runners or bulb division
• what is dna?

  • a double-stranded polymer which forms a double helix
  • genetic material of a cell found in its nucleus
• what is the genome?

  • the entire genetic material of an organism
• why is understanding/mapping the human genome important?

  • searching for genes linked to different diseases
  • understanding & treating inherited disorders
  • tracing human migration patterns
• what is a chromosome?

  • long coiled molecule of DNA that carries genetic information in the form of genes
• what is a gene?

  • a small section of DNA that codes for a specific sequence of amino acids which undergo polymerisation to form a protein
• what are alleles ?

  • different versions of the same gene
• what is a dominant allele?

  • a version of a gene where only one copy is needed for it to be expressed
• what is a recessive allele?

  • a version of a gene where two copies are needed for it to be expressed
• what is meant when an organism is homozygous?

  • when an organism has 2 copies of the same allele (2 recessive or 2 dominant)
• what is meant when an organism is heterozygous?

  • when an organism has 2 different versions of the same gene (one dominant and one recessive)
• what is the genotype?

  • the genes present for a trait
• what is the phenotype?

  • the visible characteristic
• what is the problem with single gene crosses?

  • most characteristics are controlled by multiple alleles rather than just one
• what is an inherited disorder?

  • a disorder caused by the inheritance of certain alleles
• give 2 examples of inherited disorders

  • polydactyly (caused by a dominant allele)
  • cystic fibrosis (caused by a recessive allele)
• what is gene therapy?

  • the insertion of a normal allele into the cells of a person with an inherited disorder to functionally replace the faulty gene
• sex chromosomes that determine genders:

  • males have XY chromosomes
  • females have XX chromosomes
• why are majority of the genes found on the X chromosome rather than the Y chromosome?

  • the X chromosome is bigger than the Y chromosome so more genes are carried on it
• what is variation?

  • differences in the characteristics of individuals in a population is called variation
• what are the causes of variation within a species?

  • genetics
  • environment
  • a mixture of both
• what is genetic variation?

  • variations in the genotypes of organisms due to the presence of different alleles
  • creates differences in phenotypes
• what creates genetic variation in a species?

  • spontaneous mutations
  • sexual reproduction
• what is mutation?

  • a random change to the base sequence of DNA which results in genetic variants
• what is evolution?

  • a gradual change in the inherited traits within a population over time
  • occurs due to natural selection - may result in the formation of a new species
• how do to populations become different species?

  • when their phenotypes become different to the extent that they can no longer inbreed to produce fertile offspring
• what are the main advantages of selective breeding?

  • crops produce a higher yield of grain
  • creates organisms with desirable features
• what are the disadvantages of selective breeding?

  • reduction in the gene pool
  • inbreeding results in genetic disorders
  • development of other physical problems
  • potential to select harmful recessive alleles
• what is genetic engineering?

  • modification of the genome
  • genes from chromosomes can be 'cut-out' & transferred to other cells of organisms
  • enables the formation of an organism with beneficial characteristics
• give an example of uses for genetically modified plants:

  • disease resistance
  • produce larger fruits