topic 3

• What is an operon

  • A cluster of structural genes that are transcribed together (these code for useful proteins e.g. enzymes)
  • Control elements, including a promoter region (a DNA sequence that RNA polymerase initially binds to) and an operator region (where transcription factors bind)
  • Some operons may include regulatory genes that code for activators or repressors
• Sex chromosomes

  X chromosome carries genes not present on Y chromosome because X chromosome is larger than Y

  males have only one allele of some genes

  if only one allele is inhereited it will be expressed as there is no other gene to mask it
• autosomal chromosome

  Autosomal means that a specific gene is not on a sex chromosome
• autosomal linkage

  Linkage is when genes that are close to one another on a chromosome are likely to be inherited together.
• sister chromatids

  Sister chromatids are identical copies of a single chromosome.
• homologous chromosomes

  homologous chromosomes are composed of two different chromosomes that are not genetically identical despite containing the same sets of genes.
• homologous chromosomes vs sister chromatids

  sister chromatids are genetically identical, whereas homologous chromosomes are composed of two different chromosomes that are not genetically identical despite containing the same sets of genes.
• non-sister chromatids

  contain the same genes but may have different alleles at the same loci.
• linked genes

  the further apart the genes the more likely crossing over will occur and less likely that linkage will occur
• polygenic inheritance

  a characteristic showing continuous variation

  caused by multiple genes at different loci
• variation due to meiosis

  Independent assortment and crossing over produce genetically different gametes

  these gametes can fuse at random during fertilisation with gametes from the other parent

  producing new genetic combinations in zygote