inheritance, variation and evolution

• what does DNA stand for?

  Deoxyribonucleic acid
• DNA is a polymer, its made up of 2 strands coiled together in the shape of a double helix
• what is a gene?

  a small section of DNA found on a

  chromosome
• each gene codes for a particlular amino acid sequence which is a polypeptide

  only 20 amino acids are used
• genes tell cells what order to put the amino acids in
• what is a genome ?

  The complete set of genetic material in an organism.
• what is DNA made up of?

  Nucleotides
• each nucleotide consists of a sugar phosphate group and a base
• the sugar and phosphate groups in the neucleotides form a 'backbone' to the DNA strands

  the sugar and the phosphate groups alternate

  one of 4 different bases (A,T,G,C) join to each sugar
• A always pairs with T

  G always pairs with C

  this is called complementary base pairing
• what is mRNA?


  messenger RNA
• mutations are changes to the genetic code
• mutations change the sequence of the DNA bases in a gene, which produces a genetic variant
• the different types of mutation:

  • insertation
  • deletion
  • substitution
• insertions:

  • a new base is inserted into the DNA base sequence where it shouldn't be
  • an insertion changes the way the groups of 3 bases are read which can change the amino acids that they code for
  • insertions can change more than one amino acid as they have a knock of effect on the bases further on in the sequence
• deletions:

  • when a random base is deleted from the DNA sequence
  • change the way the base sequence is 'read' an have knock on effects further down the sequence
• substitutions:

  • when a random base in the DNA base sequence is changed to a different base
• sexual reproduction produces genetically different cells

  • genetic information from 2 organism is combined to produce offspring which are genetically different to either parent
  • the mother and the faher produce gamtes by meiosis
  • in human each gamete contains 23 chromosomes (half the number of chromoses in a normal cell)
  • the egg and the sperm cell meet then fuse together (fertilisation) to form a cell with the full number of chromosomes
• asexual reproduction produces genetically identical cells

  • only 1 parent
  • occurs by mitosis
  • there's no fusion of gametes, no mixing of chromosomes and no genetic variation between parent and offspring
  • offspring are clones of parent
• What is meiosis?


  Meiosis is a type of cell division that produces gametes (sex cells) with half the number of chromosomes as the parent cell.
• the process of meiosis

  1. mitosis occurs
  2. In the second division the chromosomes line up again in the centre of the cell, the arms of the chromosomes are pulled apart
  3. The result is 4 gametes, each with only a single set of chromosomes in it, each one is genetically different from the others because the chromosomes all get shuffled up during meiosis and each gamete only gets half of them at random
• advantages of sexual reproduction

  • variation in the species (offspring has a mixture of 2 sets of chromosomes, the organism inherits genes from both parents)
  • this increases the chance of a species surviving a change in environment
  • therefore natural selection can occur and the species is more likely to breed successfully and pass these characteristics on
• asexual reproductive positives:

  • only needs one parent
  • uses less energy as organisms dont need to find a mate
  • faster than sexual reproduction
  • many identical offspring can be produced in favourable conditions
• Males have XY chromosomes

  The Y chromosome causes male characteristics
• Females have XX chromosomes

  the XX combination allows female characteristics to develop
• the sperm decides the gender of the embryo as it can have either an X or a Y chromosome
• what are homozygous genes?

  Identical alleles.
• what are heterozygous genes?

  Different alleles
• what is an allele ?

  Genetic variant
• Cystic fibrosis is caused by a recessive allele
• cystic fibrosis is a genetic disorder of the cell membranes, it results in the body producing a lot of thick sticky music in the air passages and in the pancreas
• Cf + the genes that cause it

  • the allele which causes cf is a recessive allele and is carried by about 1 in 25 people
  • as it’s recessive people with only 1 copy of the allele won’t have the disorder - they’re known as carriers
  • for the child to have the disorder both parents must be either carriers or have the disorder themselves
  • there is a 1 in 4 chance of the child having the disorder if both of the parents are carriers
• During IVF the embryo (before being implanted) can be screened for genetic disorders- a cell is removed and the genes are analysed, many disorders can be detected this way including cf
• For embryo screening

  • help stop people suffering
  • treating disorders costs the government a lot of money
  • laws to stop it going too far (parents cannot choose the sex of the child)
• against embryo screening

  • implies people with genetic disorders are undesirable- can create prejudice
  • there may come a point where everyone wants to screen their embryos to pick the most ‘desirable’ one
  • screening is expensive
• Mendels 3 conclusions

  • Characteristics in plants are determined by ‘hereditary units’
  • Hereditary units are passed on to offspring unchanged from both parents, one unit from each parent
  • Hereditary units can be dominant or recessive - if an individual has both the dominant and recessive unit for a characteristic, the dominant characteristic will b expressed
• Mendel’s work creates the starting point to the understanding of genes that we have today:

  • In the late 1800’s, scientists became familiar with chromosomes. They were able to observe how they behaved during cell division
  • In the early 20th century scientists realised similarities between Mendel‘s ‘units‘ and chromosomes, it was proposed that units were found on chromosomes
  • We now know these units as genes
• what is variation (within a species)?

  small differences between animals of the same species (e.g. different hair colour)
• the 2 types of variation;

  • environmental
  • genetic
• what causes genetic variation?

  the combining of genes from the two parents means no two f the species are genetically identical

  (and mutations)