Genetic inheritance: Inheritance, variation and evolution: Biology: GCSE (9:1)

• Gamete


  A male or female organism's reproductive cells such as the sperm or egg, carries half the genetic material of one parent

  
• Chromosomes


  Molecules of tightly-packed DNA that are wrapped up by proteins and contain many genes

  
• Inheritance of chromosomes

  Organisms inherit one copy of each chromosome from their mother and one copy of each chromosome from their father

  
• Number of inherited chromosomes in humans

  23 chromosomes are inherited from a mother's egg cell and 23 chromosomes are inherited from a father's sperm cell, a total of 46 chromosomes or 23 pairs

  
• Gene


  A small section of DNA that acts as a code for a specific characteristic

  
• Alleles


  Different versions or form of the same type of gene, the interaction of two alleles determines the overall displayed characteristic

  
• Two version of each gene

  There are two alleles present for each gene in an human, as humans have two copies of each chromosome

  
• Allele representation

  Individual alleles are written as single letters, if it is a dominant allele it is capitalised or if it is a recessive allele it is written in lowercase

  
• Dominant


  An allele that is always expressed, even if only one copy of the allele is present, for example "A" in AA or Aa

  
• Recessive


  An allele that is only expressed if two copies of the allele are present, for example "a" in aa

  
• Homozygous


  When the two alleles present in an organism are the same, for example AA or aa

  
• Heterozygous


  When the two alleles present in an organism are different, for example Aa

  
• Genotype


  The genetic features of an organism, specifically the combination of alleles that an organism has for a particular characteristic

  
• Phenotype


  The observable properties or characteristics of an organism that is the result of interaction between an organism's genotype and its environment

  
• Genotype affects phenotype


  The combination of alleles operate at a molecular level to develop characteristics that can be expressed as a phenotype

  
• Single-gene expression


  Some characteristics are controlled by a single gene, such as fur colour in mice and red-green colour blindness in humans

  
• Multiple gene interaction


  Most characteristics are a result of multiple genes interacting rather than a single gene
• Punnett square


  A diagram used to predict an offspring's possible genotypes, following the cross of a male parent with a known genotype and a female parent with a known genotype

  
• Genetic cross


  The purposeful mating of two individuals resulting in the combination of genetic material in the offspring

  
• Probability

  Punnett squares are used to work out the probability or chance that an offspring will have a specific combination of alleles
• Ratio, fraction or percentage


  Expressions of probability, for example a 1 in 4 chance could be expressed as 1:3, 1/4 or 25%