DNA and inheritance

• DNA

  Deoxyribonucleic acid. The material inside the nucleus of cells, carrying the genetic information of a living being.
• Organism

  Living entity, eg animals, plants or micro-organisms.
• Nucleus

  The nucleus controls what happens inside the cell. Chromosomes are structures found in the nucleus of most cells.
• Chromosome

  The structure made of DNA that codes for all the characteristics of an organism.
• Gene

  The basic unit of genetic material inherited from our parents. A gene is a section of DNA which controls part of a cell's chemistry - particularly protein production.
• Allele

  Different versions of the same gene.
• Genotype

  The alleles that an organism has for a particular characteristic, usually written as letters.
• Phenotype

  The visible characteristics of an organism which occur as a result of its genes.
• Dominant allele

  An allele that always expresses itself whether it is partnered by a recessive allele or by another like itself.
• Recessive allele

  Describes the variant of a gene for a particular characteristic which is masked or suppressed in the presence of the dominant variant.
• Homozygous

  This describes a genotype in which the two alleles for the characteristic are identical.
• Heterozygous

  This describes a genotype in which the two alleles for a particular characteristic are different.
• DNA

  Large and complex molecule that carries the genetic code that determines the characteristics of a living organism
• Gene

  Section of DNA that codes for a specific protein, the unit of heredity
• Chromosome

  Long threads of DNA, each made up of many genes, found in the nucleus of most cells
• James Watson and Francis Crick worked out the structure of DNA in 1953
• Double helix


  The shape of the DNA molecule, with two strands twisted together in a spiral
• Base pairs

  Paired chemicals between the two strands of DNA, including thymine, adenine, guanine, and cytosine
• Complementary base pairing

  Thymine pairs with adenine, guanine pairs with cytosine
• Allele

  Different forms of a gene, e.g. for eye colour
• Triplet code

  Sequence of three bases that determines a specific amino acid in a protein
• DNA


  Hereditary molecule responsible for its function
• Genetic cross


  Examination and showing of possible offspring combinations using a Punnett square
• Carrying out a genetic cross

  1. Determine probability of offspring combinations

  2. Calculate probability of offspring combinations
• Monohybrid cross


  Genetic cross of single gene combinations
• Punnett square


  Shows possible offspring combinations and allows calculation of probability of these combinations
• Allele


  Alternative form of a gene
• Dominant allele


  Allele that is expressed when present
• Recessive allele


  Allele that is only expressed when two copies are present
• Genotype


  Genetic makeup of an organism
• Phenotype


  Physical characteristics of an organism
• Homozygous


  Having two identical alleles for a particular trait
• Heterozygous


  Having two different alleles for a particular trait
• Selfing


  Pollen from one flower landing on the stigma of the same flower
• Backcross


  Cross between an F1 individual and one of its parents
• Most phenotypes are controlled by multiple genes
• Probability


  Extent to which something is likely to be the case
• Allele combinations created during fertilisation are a random process
• How to construct Punnett squares

  1. Determine the parental genotypes

  2. Split the alleles for each parental type and add them into your Punnett square around the edges

  3. Work out the new possible genetic combinations inside the Punnett square

  4. Use this to answer the question
• Punnett square


  A diagram used to predict the possible genotypes of offspring