DNA and Inheritance

• DNA


  Chemical that controls the activities of the cell
• How DNA controls the cell

  1. DNA forms a chemical "code" which acts as instructions for the cell to make proteins

  2. The chemical code in DNA controls which amino acids are joined together in which sequence to make different proteins
• All living processes are a series of chemical reactions in cells
• All of these chemical reactions are controlled by enzymes
• All enzymes are proteins
• The chemical reactions that a cell can carry out are therefore determined by the make-up of the DNA in that individual
• DNA can make copies of itself, so that when a cell divides, a copy of the DNA can be put into the nucleus of each of the new cells
• DNA structure


  • Two long chains of alternating sugar and phosphate molecules connected by pairs of bases, twisted to form a 'double helix'
  • Four bases: adenine (A) joins on to thymine (T), and guanine (G) joins on to cytosine (C)
  • The order of these bases along the sugar-phosphate backbone varies in different molecules of DNA
  • The 'code' consists of triplets (groups of three) of bases along the DNA, each triplet codes for an individual amino acid in the protein
• Chromosomes


  In the nucleus of a cell, the long DNA molecules are coiled up into structures called chromosomes
• Gene


  A short length of DNA that codes for one protein
• Genetic profiling


  1. A sample of cells is collected

  2. The DNA is 'cut up' by enzymes, so that it ends up in fragments of different sizes

  3. The fragments are then separated, producing a pattern which is the genetic profile
• Uses of genetic profiling

  • Identify criminals
  • Determine paternity
  • Establish how closely related species are
  • Detect genes associated with diseases
• Genetic terms


  • Gene - a length of DNA that codes for on protein
  • Chromosome - a length of DNA that contains many genes, found in the nucleus and visible during cell division
  • Allele - a variety of gene
• Monohybrid inheritance


  1. Tall is dominant to short, with alleles T and t

  2. Homozygous tall (TT) crossed with homozygous short (tt) produces heterozygous tall (Tt) F1 generation

  3. F1 generation produces gametes with T or t, resulting in 3:1 ratio of tall:short in F2 generation
• Hardly any human characteristic is controlled by a single gene, they are generally controlled by the interaction of many genes
• Gender determination


  • Females have two X chromosomes, males have one X and one Y chromosome
  • At fertilisation, an X chromosome from the egg and either an X or Y chromosome from the sperm determines whether the offspring will be male or female
• Advantages of genetic modification


  • Crops can be designed to survive in difficult farming conditions
  • Crops can be made resistant to herbicides
  • Plants can be modified to produce oils or other substances for biofuels
• Disadvantages of genetic modification


  • Inserted genes can have side effects
  • Pollen from GM crops can spread the modified genes into the natural population
  • GM plants could become established as pests
• Ethical issues with genetic modification


  • Technology is expensive, only richer countries can afford it
  • Large companies can patent GM crops and charge high prices
  • Potential harm to farmers in poorer countries
• Genotype - the genetic make-up of an individual

  Phenotype - the description of the way the genotype ’shows itself’

  Dominant - an allele that shows in the phenotype whenever it is present
• Recessive - an allele that is hidden when a dominant allele is present

  F1/F2 - short for first generation and second generation in a genetic cross

  Homozygous - a homozygote contains two identical alleles for the gene concerned.
• Heterozygous - a heterozygote contains two different alleles for the gene concerned

  Selfing - a technique by which pollen from a plant is used to fertilise ovules in flowers of the same plant