class notes

• genetic variation:

  • new combination of alleles
  • different combination of characteristics
  • different expressed genes - protein synthesis
• crossing over:

  • occurs during prophase 1
  • 2 homologous chromosomes lie side by side
  • chromosomes form links, called chiasmata
  • parts of the chromatids can swap places
  • no genes are lost as they have the same loci on both chromosomes
  • alleles are different which gives a new combination of alleles
  • multiple chiasmata can form
• independent assortment:

  • occurs during metaphase 1
  • homologous pairs of chromosomes line up along the equator of the nucleus
  • each pair behaves independently from other pairs
  • random mix of maternal and paternal chromosomes in each gamete
  • different combination of alleles
• hemaphrodites:

  • reproduce sexually by themselves
  • genetically varied offspring
  • still do meiosis, so crossing over and independent assortment
• autosomes - chromosomes that are not sex pairs
• autosomal linkage


  tendency of 2 characteristics to be inherited together because the genes that cause them are close together on the same chromosome due to close loci
• autosomal linkage facts:

  • alleles do not assort independently as the genes are on the same chromosome
  • occurs during meiosis
  • alleles on the same chromosome tend to be inherited together
  • gametes only receive 1 chromosome for each homologous pair
• Autosomal linkage affects the outcome of crosses, as dihybrid crosses predict 4 outcomes, and with heterozygous parents, a ratio of 9:3:3:1, yet in practicality, there are only 2 outcomes, with a 1:1 ratio.
• When there is known autosomal linkage, offspring that have variation are recombinants, as there must have been crossing over for variety.
• haploid nucleus


  contains 1 copy of each chromosome, so half of the genetic material (n)
• diploid nucleus

  contains 2 copies of each chromosome, 1 from each parent (2n)
• Although a diploid nucleus has genetic material of 2n during interphase, it increases to 4n during replication, before mitosis and meiosis.
• plant fertilisation:

  1. pollen tube nucleus makes the pollen tube grow down the style to the ovule, by releasing digestive enzymes
  2. generative nucleus divides by mitosis to form 2 haploid nuclei
  3. pollen tube nucleus dissolves when the pollen tube enters the ovule
  4. 1 of the haploid nuclei fertilises the ovum to form the zygote
  5. other haploid nucleus fertilises the 2 polar nuclei to form the endosperm
• The endosperm surrounds the embryo, and supplies nutrients e.g. amylose, glucose, fats, amino acids. It is a 3n structure, as it has a triploid nucleus.
• Stem cells:

  • cell replacement
  • tissue repair
• Purpose of stem cells:

  • specialised cells divide rarely
  • the more specialised the cell, the less it divides
  • specialisation/differentiation
  • cancer in specialised tissue is rare, as there is less division
• Totipotent - stem cells that are capable of dividing to form any type of specialised animal cell
• Totipotency is present in the cells of a 16 cell embryo. as the embryo develops, totipotence decreases so the stem cells are pluripotent.
• There are more than 200 different types of cells in a human adult.
• In an adult, most tissues contain stem cells. Stem cells are found in bone marrow, so that they can make all types of blood cells.
• Application of stem cells:

  • bone marrow transplant
  • stem cell therapy
• cancer

  • if cells divide when they're not supposed to, a tumor forms
  • uncontrollable cell division
  • mutation in any gene that regulates the cell cycle or controls cell death